fix(tunings): extended-range bass was named as a guitar

Reported by a 6-string bassist: a Sleep Token chart tuned A0 D1 G1 C2 F2
A#2 (standard 6-string bass, whole step down) imported and displayed as
"6 string D Standard". They called it A standard and they were right.

Root cause: `tuning_name()` gated its naming ladder on
`len(offsets) == 6`, treating six offsets as proof of a 6-string GUITAR.
A 6-string BASS also has six offsets, but its lowest string is B, not E
— so the guitar ladder mislabels the whole family: an all-zeros bass
read "E Standard" (it is Standard/B) and a whole-step-down bass read
"D Standard" (it is A Standard). The function's own comment warned about
exactly this error for 7-string guitars; nobody guarded the bass axis.
The stored value feeds the library's Tuning filter, so every 5/6-string
bass song in every library was filed under a guitar name.

- `tuning_name(offsets, *, is_bass=False)`: bass 5/6 use the low-B
  ladder (Standard / Bb / A / G# / G) and bass 4 keeps the E ladder it
  shares with guitar. Drop names come off the resulting low string.
  Default stays guitar, so existing callers are unaffected.
- `sloppak._tuning_for_meta_kind()` reports WHICH kind supplied the
  tuning; `extract_meta` emits `tuning_is_bass` and scan_worker passes
  it through. Guitar-first selection for the library index is unchanged
  — a pack with a guitar part still indexes by the guitar.
- `TUNING_PRESET_MIDIS` bass-5/bass-6 renamed to match, with
  `TUNING_PRESET_ALIASES` so `_valid_tuning_for_key` MIGRATES a saved
  profile carrying an old name instead of rejecting it (it refuses names
  belonging to another key's built-ins, and "D Standard" still exists
  for guitar-6/bass-4 — so a rename alone would have invalidated those
  profiles). Pitches are untouched; only labels change.

Convention confirmed by the bass- and guitar-pedagogy seats: name
extended range off the ACTUAL lowest string, which is what the 7-string
guitar presets already do. The old names came from the band-level habit
of saying "we're in D standard" — true of the guitars, while the bassist
in that band is in A standard. Right answer, wrong scope. The bass table
was also internally inconsistent: its Drop A was already named off the
low string while its standards were not.

Tests: bass ladders for 4/5/6 strings, the reported chart pinned both
ways (is_bass=True -> "A Standard", same offsets as guitar -> "D
Standard"), bass drop naming, a table-wide invariant that every bass
preset name matches the note its low string sounds, and alias migration
(incl. not leaking into guitar-6/bass-4). The existing legacy-flat-bass
migration test now asserts the corrected label — same pitches, right
name. Suite: 1735 passed vs 1720 on main, with the same 99 pre-existing
env failures.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01929LgKdJMyPGLf8N1WpEVW
Signed-off-by: ChrisBeWithYou <christian.a.cowan@gmail.com>

CHANGELOG.md

@@ -212,6 +212,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   engine (`app.js`, `highway.js`, `playSong`, `showScreen`, the capability registry).
 
 ### Fixed
+- **3D Highway: the lane stops at the hit line** (#991) — the highway lane, its
+  dividers, and the fret boundary extension lines ran `BEHIND` seconds *past* the
+  hit line toward the player. Nothing is ever drawn in that strip (notes and chord
+  frames clamp to `Math.min(0, dZ(dt))`), so it read as lane with no notes on it.
+  The floor geometry now ends at the hit line; its far edge is unchanged, still
+  `-AHEAD*TS` at the note horizon.
 - **Career passports review polish** — the passport tabs and book overlay carry
   proper ARIA semantics (`aria-selected`/`aria-controls`/`tabpanel`;
   `role="dialog"` + `aria-modal` with focus moved to the close button on open

lib/library_registry.py

@@ -17,7 +17,12 @@ import threading
 from typing import ClassVar
 
 import appstate
-from metadata_db import MetadataDB, _tuning_group_key_sql
+from metadata_db import (
+    MetadataDB, _effective_tuning_cols_sql, _perspective_is_inferred_sql,
+    _tuning_group_key_sql,
+)
+import tunings as tunings_mod
+from tunings import DEFAULT_PERSPECTIVE, PERSPECTIVES
 from routers import art as art_router
 
 import logging
@@ -39,9 +44,6 @@ def _safe_art_redirect_url(url: str) -> str | None:
         return None
 
 
-_TUNING_GROUP_KEY_SQL = _tuning_group_key_sql("songs")
-
-
 class LocalLibraryProvider:
     id = "local"
     label = "My Library"
@@ -69,28 +71,43 @@ class LocalLibraryProvider:
     def query_stats(self, **kwargs) -> dict:
         return self._db.query_stats(**kwargs)
 
-    def tuning_names(self) -> dict:
+    def tuning_names(self, instrument: str = DEFAULT_PERSPECTIVE) -> dict:
         # Group custom tunings on their raw offsets so distinct ones stay
         # distinct (tuning_name collapses them all to "Custom Tuning"); named
         # tunings keep grouping by name (stable across the rescan boundary, no
         # offsets/name split). `key` is the value the client sends back as the
         # filter selector — equal to the name for named tunings, the offsets
         # string for customs; offsets also feed the client's custom-pill label.
+        #
+        # `instrument=bass` swaps every column for its effective bass-facing
+        # expression (bass arrangement's tuning, guitar fallback) — the SAME
+        # expressions _build_intrinsic_where filters on, so a facet entry
+        # always selects exactly the songs it counted.
+        name_sql, offsets_sql, sort_sql = _effective_tuning_cols_sql("songs", instrument)
+        gkey_sql = _tuning_group_key_sql("songs", instrument)
+        # How many of a row's songs are showing an INFERRED tuning — i.e. have
+        # no bass chart of their own and are falling back to the guitar-derived
+        # one. Reported per entry so the UI can be honest about it instead of
+        # presenting a borrowed tuning as a measured one. Always 0 for guitar.
+        inferred_sql = f"SUM({_perspective_is_inferred_sql('songs', instrument)})"
         with self._db._lock:
             rows = self._db.conn.execute(
-                f"SELECT tuning_name, {_TUNING_GROUP_KEY_SQL} AS gkey, "
-                "MIN(tuning_sort_key), COUNT(*), MIN(tuning_offsets) "
-                "FROM songs WHERE title != '' AND COALESCE(tuning_name, '') != '' "
+                f"SELECT {name_sql}, {gkey_sql} AS gkey, "
+                f"MIN({sort_sql}), COUNT(*), MIN({offsets_sql}), {inferred_sql} "
+                f"FROM songs WHERE title != '' AND COALESCE({name_sql}, '') != '' "
                 "GROUP BY gkey COLLATE NOCASE "
-                "ORDER BY ABS(COALESCE(MIN(tuning_sort_key), 0)), "
-                "COALESCE(MIN(tuning_sort_key), 0) ASC, "
-                "tuning_name COLLATE NOCASE"
+                f"ORDER BY ABS(COALESCE(MIN({sort_sql}), 0)), "
+                f"COALESCE(MIN({sort_sql}), 0) ASC, "
+                f"{name_sql} COLLATE NOCASE"
             ).fetchall()
         return {
+            "instrument": instrument,
             "tunings": [
                 {"name": name, "key": gkey, "offsets": offs or "",
-                 "sort_key": int(sk or 0), "count": count}
-                for name, gkey, sk, count, offs in rows
+                 "sort_key": int(sk or 0), "count": count,
+                 # Portion of `count` borrowed from the guitar chart.
+                 "inferred_count": int(inferred or 0)}
+                for name, gkey, sk, count, offs, inferred in rows
             ],
         }
 
@@ -330,9 +347,16 @@ class SmartCollectionProvider:
         # have been hand-edited; never let a bad value reach a query.
         self._rules = _sanitize_collection_rules(collection.get("rules") or {})
 
-    def _filter_kwargs(self) -> dict:
-        return _library_filter_args(**{k: v for k, v in self._rules.items()
+    def _filter_kwargs(self, instrument: str = "", playable_from_pitch=None) -> dict:
+        # `instrument` is the CALLER's play perspective (rides every request),
+        # never part of the saved rules — a collection saved by a guitarist
+        # must still read in bass tunings for a bass player, and vice versa.
+        args = _library_filter_args(**{k: v for k, v in self._rules.items()
                                        if k in _LIBRARY_FILTER_PARAM_KEYS})
+        args["instrument"] = _normalize_instrument(instrument)
+        # The caller's CURRENT tuning is likewise per-request, never a saved rule.
+        args["playable_from_pitch"] = playable_from_pitch
+        return args
 
     def _sort(self, fallback: str) -> str:
         # A collection may pin its own sort (e.g. "recently added"); query_page
@@ -340,28 +364,31 @@ class SmartCollectionProvider:
         return self._rules.get("sort") or fallback
 
     def query_page(self, *, page=0, size=24, sort="artist", direction="asc",
-                   naming_mode="legacy", **_ignore):
+                   naming_mode="legacy", instrument="", playable_from_pitch=None, **_ignore):
         return self._local._db.query_page(
             page=page, size=size, sort=self._sort(sort), direction=direction,
-            naming_mode=naming_mode, **self._filter_kwargs())
+            naming_mode=naming_mode, **self._filter_kwargs(instrument, playable_from_pitch))
 
-    def query_artists(self, *, letter="", page=0, size=50, naming_mode="legacy", **_ignore):
+    def query_artists(self, *, letter="", page=0, size=50, naming_mode="legacy",
+                      instrument="", playable_from_pitch=None, **_ignore):
         return self._local._db.query_artists(
             letter=letter, page=page, size=size, naming_mode=naming_mode,
-            **self._filter_kwargs())
+            **self._filter_kwargs(instrument, playable_from_pitch))
 
-    def query_albums(self, *, page=0, size=120, naming_mode="legacy", **_ignore):
+    def query_albums(self, *, page=0, size=120, naming_mode="legacy",
+                     instrument="", playable_from_pitch=None, **_ignore):
         return self._local._db.query_albums(
-            page=page, size=size, naming_mode=naming_mode, **self._filter_kwargs())
+            page=page, size=size, naming_mode=naming_mode,
+            **self._filter_kwargs(instrument, playable_from_pitch))
 
     def query_stats(self, *, sort="artist", want_sort_letters=False,
-                    naming_mode="legacy", **_ignore):
+                    naming_mode="legacy", instrument="", playable_from_pitch=None, **_ignore):
         return self._local._db.query_stats(
             sort=self._sort(sort), want_sort_letters=want_sort_letters,
-            naming_mode=naming_mode, **self._filter_kwargs())
+            naming_mode=naming_mode, **self._filter_kwargs(instrument, playable_from_pitch))
 
-    def tuning_names(self):
-        return self._local.tuning_names()
+    def tuning_names(self, instrument: str = "guitar"):
+        return self._local.tuning_names(instrument=_normalize_instrument(instrument))
 
     async def get_art(self, song_id: str):
         return await self._local.get_art(song_id)
@@ -390,7 +417,10 @@ def _library_filter_args(q: str = "", favorites: int = 0, format: str = "",
                          artist: str = "", album: str = "",
                          arrangements_has: str = "", arrangements_lacks: str = "",
                          stems_has: str = "", stems_lacks: str = "",
-                         has_lyrics: str = "", tunings: str = "") -> dict:
+                         has_lyrics: str = "", tunings: str = "",
+                         instrument: str = "", tuning_match: str = "",
+                         playable_offsets: str = "", playable_instrument: str = "",
+                         playable_string_count: str = "") -> dict:
     fmt = format if format in ("archive", "sloppak", "loose") else ""
     return {
         "q": q,
@@ -404,9 +434,58 @@ def _library_filter_args(q: str = "", favorites: int = 0, format: str = "",
         "stems_lacks": _split_csv(stems_lacks),
         "has_lyrics": _parse_has_lyrics(has_lyrics),
         "tunings": _split_csv(tunings),
+        # Which perspective the tuning facet/filter/sort speaks for (the
+        # caller's play role, NOT a saved rule — see _sanitize_collection_rules).
+        "instrument": _normalize_instrument(instrument),
+        # "Playable without retuning" mode: the caller's CURRENT tuning,
+        # resolved to the one number the comparison needs. None = exact-match
+        # mode (the default), so the tuning pills behave exactly as before.
+        "playable_from_pitch": (
+            _playable_from_pitch(playable_offsets, playable_instrument,
+                                 playable_string_count)
+            if tuning_match == "playable" else None),
     }
 
 
+def _playable_from_pitch(offsets_csv: str, instrument: str, string_count: str):
+    """Lowest open-string MIDI pitch of the CALLER's current tuning.
+
+    The client sends its live working tuning (offsets + instrument + string
+    count) rather than a precomputed pitch, so the pitch tables stay in one
+    place (lib/tunings.py) instead of being duplicated in JS.
+
+    Returns None for anything unusable — the caller then applies NO playable
+    filter at all. That is the neutral state, not a claim: a malformed tuning
+    must not silently assert that everything is playable OR that nothing is.
+    """
+    try:
+        offsets = [int(x) for x in _split_csv(offsets_csv)]
+    except (TypeError, ValueError):
+        return None
+    if not offsets:
+        return None
+    inst = "bass" if instrument == "bass" else "guitar"
+    try:
+        sc = int(string_count)
+    except (TypeError, ValueError):
+        sc = len(offsets)
+    key = tunings_mod.instrument_key(inst, sc)
+    if key not in tunings_mod.STANDARD_OPEN_MIDIS or len(offsets) != sc:
+        return None
+    midis = tunings_mod.tuning_midis_from_offsets(key, offsets)
+    return min(midis) if midis else None
+
+
+def _normalize_instrument(raw: str) -> str:
+    """Resolve a tuning PERSPECTIVE id (guitar-lead | guitar-rhythm | bass).
+
+    Tolerates the legacy two-valued vocabulary ("guitar" -> guitar-lead) and
+    falls back to the default for anything unknown — an unrecognised value
+    must never silently change filter semantics."""
+    return raw if raw in PERSPECTIVES else (
+        DEFAULT_PERSPECTIVE if raw != "bass" else "bass")
+
+
 def _sync_collection_provider(collection: dict) -> None:
     """Register (or replace) the provider for one collection."""
     appstate.library_providers.register(

lib/loosefolder.py

@@ -225,13 +225,18 @@ def _detect_arrangements(path: Path) -> tuple[list[dict], dict]:
     Returns (arrangements_list, shared_meta).
     shared_meta contains title/artist/album/year/duration/tuning_offsets
     sourced from the highest-priority arrangement (lead > combo > rhythm >
-    bass) — picking the guitar tuning when both bass and lead are present.
+    bass) — picking the guitar tuning when both bass and lead are present —
+    plus `bass_tuning_offsets` from the first bass arrangement (None when the
+    folder has none), so the index can carry both tunings.
     """
     arrangements = []
     # Track which arrangement priority sourced shared_meta so a later,
     # higher-priority arrangement (lead < bass in sort order) overrides.
     shared_meta = {}
     shared_priority = None
+    # First tuning seen per arrangement ROLE, kept alongside the guitar-first
+    # song tuning so the library can answer for the part a player plays.
+    role_tunings: dict[str, list[int] | None] = {"bass": None, "rhythm": None}
 
     for xml in sorted(_iter_local_xmls(path)):
         # Trust the XML root over the filename — a custom named
@@ -269,6 +274,10 @@ def _detect_arrangements(path: Path) -> tuple[list[dict], dict]:
                             "duration", "tuning_offsets")}
             shared_priority = priority
 
+        if (arr_type in role_tunings and role_tunings[arr_type] is None
+                and meta.get("tuning_offsets")):
+            role_tunings[arr_type] = list(meta["tuning_offsets"])
+
         arrangements.append({
             "type":     arr_type,
             "name":     arr_name,
@@ -281,6 +290,8 @@ def _detect_arrangements(path: Path) -> tuple[list[dict], dict]:
         a["index"] = i
         del a["priority"]
 
+    for role, offs in role_tunings.items():
+        shared_meta[f"{role}_tuning_offsets"] = offs
     return arrangements, shared_meta
 
 
@@ -412,6 +423,14 @@ def extract_meta(path: Path, dlc_root: Path | None = None) -> dict:
                                 xml_meta.get("duration", 0))
     tuning_offsets = _coerce_tuning_offsets(manifest.get("tuning_offsets"),
                                             xml_meta.get("tuning_offsets"))
+    # Per-role tunings: XML-derived only. A manifest `tuning_offsets` overrides
+    # the SONG tuning (above) but says nothing about WHICH chart it describes,
+    # so it must never be mistaken for a specific part's tuning.
+    role_tunings = {}
+    for role in ("bass", "rhythm"):
+        offs = xml_meta.get(f"{role}_tuning_offsets")
+        role_tunings[f"{role}_tuning_offsets"] = (
+            offs if isinstance(offs, list) and offs else None)
 
     manifest_arr = _validate_manifest_arrangements(manifest.get("arrangements"))
     if manifest_arr is not None:
@@ -427,6 +446,7 @@ def extract_meta(path: Path, dlc_root: Path | None = None) -> dict:
         "year":           year,
         "duration":       duration,
         "tuning_offsets": tuning_offsets,
+        **role_tunings,   # None = no arrangement in that role
         "arrangements":   arrangements,
         "audio_path":     str(audio) if audio else None,
         "art_path":       str(art)   if art   else None,

lib/metadata_db.py

@@ -25,6 +25,8 @@ import time
 from pathlib import Path
 
 from song import compute_smart_names
+from tunings import DEFAULT_PERSPECTIVE, ROLE_PERSPECTIVES
+from tunings import perspective as _perspective
 
 log = logging.getLogger("feedBack.server")
 
@@ -34,13 +36,91 @@ log = logging.getLogger("feedBack.server")
 # raw offsets so distinct customs stay distinct, while named tunings keep
 # grouping by name (stable across the offsets-column migration). Used by both
 # the tuning-names listing and the filter WHERE so the contract matches.
-def _tuning_group_key_sql(alias: str) -> str:
-    """The tuning grouping key (name for named tunings, raw offsets for
-    customs) against an explicit table alias — the grouped filter law (§7.1)
-    evaluates chart-intrinsic predicates inside a member subquery, where bare
-    column names would resolve against the wrong scope."""
-    return (f"CASE WHEN {alias}.tuning_name = 'Custom Tuning' AND COALESCE({alias}.tuning_offsets, '') != '' "
-            f"THEN {alias}.tuning_offsets ELSE {alias}.tuning_name END")
+#
+# A non-default PERSPECTIVE (guitar-rhythm / bass) swaps every tuning column
+# for its EFFECTIVE expression: that role's indexed tuning when the song has
+# such an arrangement, falling back to the guitar-derived song tuning
+# otherwise — so a song with no rhythm/bass chart (or a row that predates the
+# columns, NULL there) still groups/filters/sorts instead of disappearing.
+# guitar-lead reads the original unprefixed columns, so it is byte-identical
+# to the historical behaviour.
+def _effective_tuning_cols_sql(alias: str, perspective: str = DEFAULT_PERSPECTIVE) -> tuple[str, str, str]:
+    """(name_sql, offsets_sql, sort_key_sql) for the given perspective."""
+    persp = _perspective(perspective)
+    if not persp.column_prefix:
+        return (f"{alias}.tuning_name", f"{alias}.tuning_offsets", f"{alias}.tuning_sort_key")
+    has_own = f"COALESCE({alias}.{persp.column('name')}, '') != ''"
+    return (
+        f"COALESCE(NULLIF({alias}.{persp.column('name')}, ''), {alias}.tuning_name)",
+        f"CASE WHEN {has_own} THEN {alias}.{persp.column('offsets')} ELSE {alias}.tuning_offsets END",
+        f"CASE WHEN {has_own} THEN {alias}.{persp.column('sort_key')} ELSE {alias}.tuning_sort_key END",
+    )
+
+
+def _effective_low_pitch_sql(alias: str, perspective: str = DEFAULT_PERSPECTIVE) -> str:
+    """Lowest open-string MIDI pitch under this perspective, with the same
+    fallback as the tuning columns — the "playable without retuning"
+    comparison reads it (see tunings.chart_is_playable_in)."""
+    persp = _perspective(perspective)
+    if not persp.column_prefix:
+        return f"{alias}.tuning_low_pitch"
+    has_own = f"COALESCE({alias}.{persp.column('name')}, '') != ''"
+    return (f"CASE WHEN {has_own} THEN {alias}.{persp.column('low_pitch')} "
+            f"ELSE {alias}.tuning_low_pitch END")
+
+
+def _perspective_is_inferred_sql(alias: str, perspective: str) -> str:
+    """1 when this row is BORROWING the guitar-derived song tuning because it
+    has no chart in the perspective's role. Always 0 for guitar-lead, which is
+    never a fallback."""
+    persp = _perspective(perspective)
+    if not persp.column_prefix:
+        return "0"
+    return f"(CASE WHEN COALESCE({alias}.{persp.column('name')}, '') = '' THEN 1 ELSE 0 END)"
+
+
+# ── The custom-tuning group key ──────────────────────────────────────────────
+#
+# Named tunings group by NAME, which is already serialization-agnostic. Custom
+# tunings group on a raw offsets STRING, which is not: the same physical bass
+# tuning stored as "-2 0 0 0" and "-2 0 0 0 0 0" would fragment into two facet
+# rows with split counts.
+#
+# For BASS we therefore group customs on `bass_tuning_key` — the tuning's
+# absolute open-string PITCHES, computed once at scan time
+# (tunings.bass_tuning_key) after the padded tail is truncated away. Pitch is
+# the identity that matters musically and it is serialization-independent, so
+# one physical tuning is one entry however it was authored. Guitar keeps the
+# offsets string (unchanged; six-element guitar arrays are not padded).
+#
+# The key is built HERE, once, and read by the facet listing, the filter WHERE
+# and the grouped member-match alike — a facet row that selected a different
+# set than it counted is exactly the bug this shared expression prevents.
+def _tuning_group_key_sql(alias: str, perspective: str = DEFAULT_PERSPECTIVE) -> str:
+    """The tuning grouping key (name for named tunings, canonical pitches or
+    raw offsets for customs) against an explicit table alias — the grouped
+    filter law (§7.1) evaluates chart-intrinsic predicates inside a member
+    subquery, where bare column names would resolve against the wrong scope."""
+    persp = _perspective(perspective)
+    name_sql, offsets_sql, _ = _effective_tuning_cols_sql(alias, perspective)
+    if persp.column_prefix:
+        # Fall back to the offsets string when the canonical key is absent
+        # (a fallback row borrowing the guitar tuning, or a row scanned before
+        # the key column existed) so a custom never groups under an empty key.
+        offsets_sql = (f"COALESCE(NULLIF({alias}.{persp.column('key')}, ''), "
+                       f"{offsets_sql})")
+    return (f"CASE WHEN {name_sql} = 'Custom Tuning' AND COALESCE({offsets_sql}, '') != '' "
+            f"THEN {offsets_sql} ELSE {name_sql} END")
+
+
+def _put_perspective_value(meta: dict, col: str):
+    """Value to store for one per-perspective column on a freshly-scanned row."""
+    if col.endswith("_low_pitch"):
+        val = meta.get(col)
+        return int(val) if isinstance(val, int) else None
+    if col.endswith("_sort_key"):
+        return int(meta.get(col, 0) or 0)
+    return meta.get(col, "") or ""
 
 
 # ── SQLite metadata cache ─────────────────────────────────────────────────────
@@ -381,7 +461,18 @@ class MetadataDB:
                 tuning_offsets TEXT DEFAULT '',
                 genre TEXT DEFAULT '',
                 track_number INTEGER,
-                disc INTEGER
+                disc INTEGER,
+                bass_tuning_name TEXT,
+                bass_tuning_sort_key INTEGER,
+                bass_tuning_offsets TEXT,
+                bass_tuning_key TEXT,
+                bass_tuning_low_pitch INTEGER,
+                rhythm_tuning_name TEXT,
+                rhythm_tuning_sort_key INTEGER,
+                rhythm_tuning_offsets TEXT,
+                rhythm_tuning_key TEXT,
+                rhythm_tuning_low_pitch INTEGER,
+                tuning_low_pitch INTEGER
             )
         """)
         # Idempotent migrations for installs that predate each column.
@@ -408,6 +499,32 @@ class MetadataDB:
             # falls back to title order. Cache; repopulated on rescan.
             "ALTER TABLE songs ADD COLUMN track_number INTEGER",
             "ALTER TABLE songs ADD COLUMN disc INTEGER",
+            # Bass-arrangement tuning (the KwasimodoZAZA report): the song-level
+            # tuning columns above are guitar-first, so the library filter lied
+            # to bass players when the bass chart is tuned differently. Caches;
+            # repopulated on rescan. NULL (no literal default) is deliberate —
+            # it marks a pre-migration row the scanner must re-extract, while
+            # '' means "extracted, song has no bass arrangement" (see scan.py).
+            "ALTER TABLE songs ADD COLUMN bass_tuning_name TEXT",
+            "ALTER TABLE songs ADD COLUMN bass_tuning_sort_key INTEGER",
+            "ALTER TABLE songs ADD COLUMN bass_tuning_offsets TEXT",
+            # Canonical grouping key: the bass tuning's absolute open-string
+            # pitches. Keyed on PITCH, not the serialization-dependent offsets
+            # string, so one physical tuning is one facet entry however it was
+            # stored. See tunings.bass_tuning_key.
+            "ALTER TABLE songs ADD COLUMN bass_tuning_key TEXT",
+            # Lowest open-string MIDI pitch per perspective — the "playable
+            # without retuning" comparison (tunings.chart_is_playable_in).
+            "ALTER TABLE songs ADD COLUMN bass_tuning_low_pitch INTEGER",
+            "ALTER TABLE songs ADD COLUMN tuning_low_pitch INTEGER",
+            # The RHYTHM chart's own tuning: lead and rhythm arrangements can
+            # be tuned differently, which is the same bug a bassist hit,
+            # inside guitar. Same NULL-vs-'' contract as the bass family.
+            "ALTER TABLE songs ADD COLUMN rhythm_tuning_name TEXT",
+            "ALTER TABLE songs ADD COLUMN rhythm_tuning_sort_key INTEGER",
+            "ALTER TABLE songs ADD COLUMN rhythm_tuning_offsets TEXT",
+            "ALTER TABLE songs ADD COLUMN rhythm_tuning_key TEXT",
+            "ALTER TABLE songs ADD COLUMN rhythm_tuning_low_pitch INTEGER",
         ):
             try:
                 self.conn.execute(ddl)
@@ -2810,16 +2927,39 @@ class MetadataDB:
     def favorite_set(self) -> set[str]:
         return {r[0] for r in self.conn.execute("SELECT filename FROM favorites").fetchall()}
 
+    # Every per-perspective column, in one place, so the SELECT, the INSERT and
+    # the scanner's "was this ever extracted?" check can never drift apart.
+    # NULL is meaningful on `name`/`key`/`low_pitch`: it marks a row written
+    # before the column existed, which the scanner re-extracts (see
+    # scan._has_unextracted_columns). '' / 0 means "extracted, no such chart".
+    _PERSPECTIVE_COLS = tuple(
+        p.column(suffix)
+        for p in ROLE_PERSPECTIVES
+        for suffix in ("name", "sort_key", "offsets", "key", "low_pitch")
+    ) + ("tuning_low_pitch",)
+    # Columns whose NULL means "never extracted" rather than "no such chart".
+    #
+    # low_pitch is deliberately NOT a marker: a song with no chart in that role
+    # legitimately has NULL there (nothing to compute a pitch from), so keying
+    # re-extraction on it would re-scan those rows on every single pass and
+    # never converge. `name` and `key` carry the signal instead — they are ''
+    # when extracted-but-absent, NULL only when the column predates the row.
+    _EXTRACTION_MARKER_COLS = tuple(
+        p.column(suffix) for p in ROLE_PERSPECTIVES for suffix in ("name", "key")
+    )
+
     def get(self, filename: str, mtime: float, size: int) -> dict | None:
         cache_key = str(filename)
+        pcols = ", ".join(self._PERSPECTIVE_COLS)
         with self._lock:
             row = self.conn.execute(
                 "SELECT mtime, size, title, artist, album, year, duration, tuning, arrangements, has_lyrics, "
-                "format, stem_count, stem_ids, tuning_name, tuning_sort_key, tuning_offsets "
+                "format, stem_count, stem_ids, tuning_name, tuning_sort_key, tuning_offsets, "
+                f"{pcols} "
                 "FROM songs WHERE filename = ?", (cache_key,)
             ).fetchone()
         if row and row[0] == mtime and row[1] == size and row[2]:
-            return {
+            out = {
                 "title": row[2], "artist": row[3], "album": row[4],
                 "year": row[5], "duration": row[6], "tuning": row[7],
                 "arrangements": json.loads(row[8]) if row[8] else [],
@@ -2831,6 +2971,15 @@ class MetadataDB:
                 "tuning_sort_key": int(row[14] or 0),
                 "tuning_offsets": row[15] or "",
             }
+            for i, col in enumerate(self._PERSPECTIVE_COLS, start=16):
+                val = row[i]
+                if col in self._EXTRACTION_MARKER_COLS:
+                    out[col] = val          # NULL preserved — drives re-extraction
+                elif col.endswith("_sort_key"):
+                    out[col] = int(val or 0)
+                else:
+                    out[col] = val or ""
+            return out
         return None
 
     def put(self, filename: str, mtime: float, size: int, meta: dict):
@@ -2838,8 +2987,9 @@ class MetadataDB:
             self.conn.execute(
                 "INSERT OR REPLACE INTO songs "
                 "(filename, mtime, size, title, artist, album, year, duration, tuning, arrangements, "
-                "has_lyrics, format, stem_count, stem_ids, tuning_name, tuning_sort_key, tuning_offsets, genre, track_number, disc) "
-                "VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)",
+                "has_lyrics, format, stem_count, stem_ids, tuning_name, tuning_sort_key, tuning_offsets, genre, track_number, disc, "
+                + ", ".join(self._PERSPECTIVE_COLS) + ") "
+                "VALUES (" + ", ".join(["?"] * (20 + len(self._PERSPECTIVE_COLS))) + ")",
                 (filename, mtime, size, meta.get("title", ""), meta.get("artist", ""),
                  meta.get("album", ""), meta.get("year", ""), meta.get("duration", 0),
                  meta.get("tuning", ""), json.dumps(meta.get("arrangements", [])),
@@ -2852,7 +3002,14 @@ class MetadataDB:
                  meta.get("tuning_offsets", "") or "",
                  meta.get("genre", "") or "",
                  meta.get("track_number"),
-                 meta.get("disc")),
+                 meta.get("disc"),
+                 # A put() row is by definition freshly extracted, so the
+                 # marker columns must never be written NULL — that state is
+                 # reserved for rows predating the column, which re-extract.
+                 # low_pitch is the exception: NULL there means "this tuning
+                 # has no computable pitch" (unusable offsets), and the
+                 # playable filter treats unknown as not-playable.
+                 *[_put_perspective_value(meta, col) for col in self._PERSPECTIVE_COLS]),
             )
             self.conn.commit()
             # A song's identity may have changed → the grouping read-model is stale.
@@ -3332,6 +3489,8 @@ class MetadataDB:
                      match_states: list[str] | None = None,
                      genre: list[str] | None = None,
                      naming_mode: str = "legacy",
+                     instrument: str = DEFAULT_PERSPECTIVE,
+                     playable_from_pitch: int | None = None,
                      include_intrinsic: bool = True) -> tuple[str, list]:
         """Shared WHERE-clause builder for query_page / query_artists /
         query_stats. Returns (where_sql, params). Leading 'WHERE' is
@@ -3438,7 +3597,8 @@ class MetadataDB:
                 "songs", format_filter=format_filter,
                 arrangements_has=arrangements_has, arrangements_lacks=arrangements_lacks,
                 stems_has=stems_has, stems_lacks=stems_lacks,
-                has_lyrics=has_lyrics, tunings=tunings, naming_mode=naming_mode)
+                has_lyrics=has_lyrics, tunings=tunings, naming_mode=naming_mode,
+                instrument=instrument, playable_from_pitch=playable_from_pitch)
             where += ifrag
             params += iparams
         return where, params
@@ -3450,7 +3610,9 @@ class MetadataDB:
                                stems_lacks: list[str] | None = None,
                                has_lyrics: int | None = None,
                                tunings: list[str] | None = None,
-                               naming_mode: str = "legacy") -> tuple[str, list]:
+                               naming_mode: str = "legacy",
+                               instrument: str = DEFAULT_PERSPECTIVE,
+                               playable_from_pitch: int | None = None) -> tuple[str, list]:
         """CHART-INTRINSIC predicates (format / arrangements / stems / lyrics /
         tuning) as ' AND …' fragments against an explicit table alias. Flat
         queries apply them to `songs` directly; grouped queries evaluate them
@@ -3593,10 +3755,32 @@ class MetadataDB:
                 placeholders = ",".join(["?"] * len(tn))
                 # Match the same grouping key tuning_names() returns so a single
                 # "Custom Tuning" pill selects exactly its offset set while named
-                # tunings still match by name.
-                where += (f" AND {_tuning_group_key_sql(alias)} "
+                # tunings still match by name. `instrument` swaps in the
+                # effective bass tuning key (guitar fallback) — the facet and
+                # this WHERE must use the same expression or they disagree.
+                where += (f" AND {_tuning_group_key_sql(alias, instrument)} "
                           f"COLLATE NOCASE IN ({placeholders})")
                 params += tn
+        if playable_from_pitch is not None:
+            # "Playable without retuning" — the mode the tester actually wants
+            # ("don't make me retune"), offered ALONGSIDE exact match, not
+            # instead of it. A chart needs no retune when its lowest required
+            # pitch is reachable, and every pitch above your lowest open string
+            # is reachable by fretting, so the comparison is:
+            #
+            #     your lowest open pitch <= the chart's lowest open pitch
+            #
+            # That is why a 5-string bass (low B) covers every 4-string
+            # standard AND every drop-D chart untouched.
+            #
+            # CONSERVATIVE BY CONSTRUCTION: a chart whose low pitch we could
+            # not compute (NULL) is EXCLUDED rather than assumed playable —
+            # wrongly claiming playability costs a mid-practice retune, which
+            # is the failure this whole feature exists to prevent. See
+            # tunings.chart_is_playable_in for the full reasoning + limits.
+            low_sql = _effective_low_pitch_sql(alias, instrument)
+            where += f" AND {low_sql} IS NOT NULL AND {low_sql} >= ?"
+            params.append(int(playable_from_pitch))
         return where, params
 
     # Under group=1, chart-intrinsic filters match if ANY member of the work
@@ -3864,7 +4048,9 @@ class MetadataDB:
                    genre: list[str] | None = None,
                    after: str | None = None,
                    group: bool = False,
-                   naming_mode: str = "legacy") -> tuple[list[dict], int]:
+                   naming_mode: str = "legacy",
+                   instrument: str = DEFAULT_PERSPECTIVE,
+                   playable_from_pitch: int | None = None) -> tuple[list[dict], int]:
         """Server-side paginated search. Returns (songs, total_count).
 
         `after` is an opaque keyset cursor (the last row of the previous page).
@@ -3893,7 +4079,9 @@ class MetadataDB:
             has_lyrics=has_lyrics, tunings=tunings, mastery=mastery,
             tags_has=tags_has, user_difficulty_in=user_difficulty_in,
             match_states=match_states, genre=genre,
-            naming_mode=naming_mode, include_intrinsic=not group,
+            naming_mode=naming_mode, instrument=instrument,
+            playable_from_pitch=playable_from_pitch,
+            include_intrinsic=not group,
         )
         ifrag, iparams = "", []
         if group:
@@ -3902,12 +4090,14 @@ class MetadataDB:
                 "m", format_filter=format_filter,
                 arrangements_has=arrangements_has, arrangements_lacks=arrangements_lacks,
                 stems_has=stems_has, stems_lacks=stems_lacks,
-                has_lyrics=has_lyrics, tunings=tunings, naming_mode=naming_mode)
+                has_lyrics=has_lyrics, tunings=tunings, naming_mode=naming_mode,
+                instrument=instrument, playable_from_pitch=playable_from_pitch)
             mfrag, mparams = self._grouped_member_match(ifrag, iparams)
             where += mfrag
             params += mparams
             where += self._GROUP_REP_PREDICATE
 
+        _eff_tuning_name, _, _eff_tuning_sort = _effective_tuning_cols_sql("songs", instrument)
         sort_map = {
             # Artist sorts order WITHIN an artist by title (the tree view's
             # artist -> album -> title feel) instead of raw filename — the
@@ -3941,11 +4131,15 @@ class MetadataDB:
             # behind, and a NULL `tuning_name` in `(tuning_name = '')`
             # evaluates to NULL itself (which sorts ahead of 0 in
             # ASC), defeating the push-to-bottom intent.
+            #
+            # Under `instrument=bass` the effective expressions swap in
+            # the bass arrangement's tuning (guitar fallback) so a bass
+            # player's tuning sort orders by the tuning they'd play.
             "tuning": (
-                "(COALESCE(tuning_name, '') = '') ASC, "
-                "ABS(COALESCE(tuning_sort_key, 0)), "
-                "COALESCE(tuning_sort_key, 0) ASC, "
-                "COALESCE(tuning_name, '') COLLATE NOCASE"
+                f"(COALESCE({_eff_tuning_name}, '') = '') ASC, "
+                f"ABS(COALESCE({_eff_tuning_sort}, 0)), "
+                f"COALESCE({_eff_tuning_sort}, 0) ASC, "
+                f"COALESCE({_eff_tuning_name}, '') COLLATE NOCASE"
             ),
             # Year sort (feedBack#128). Empty-year rows pushed to the
             # bottom for both directions; otherwise CAST so '2010' >
@@ -4038,7 +4232,9 @@ class MetadataDB:
 
             cols = ("SELECT filename, title, artist, album, year, duration, tuning, "
                     "arrangements, has_lyrics, mtime, format, stem_count, stem_ids, "
-                    "tuning_name, tuning_offsets FROM songs ")
+                    "tuning_name, tuning_offsets, bass_tuning_name, bass_tuning_offsets, "
+                    "rhythm_tuning_name, rhythm_tuning_offsets "
+                    "FROM songs ")
             cursor = _decode_cursor(after) if after else None
             eff_sort = _effective_keyset_sort(sort, direction)
             if cursor and eff_sort in _KEYSET_SORTS:
@@ -4071,8 +4267,30 @@ class MetadataDB:
                 "stem_ids": json.loads(r[12]) if r[12] else [],
                 "tuning_name": r[13] or "",
                 "tuning_offsets": r[14] or "",
+                # '' when the song has no bass arrangement (or the row predates
+                # '' when the song has no such chart (or the row predates the
+                # columns) — clients fall back to tuning_name.
+                "bass_tuning_name": r[15] or "",
+                "bass_tuning_offsets": r[16] or "",
+                "rhythm_tuning_name": r[17] or "",
+                "rhythm_tuning_offsets": r[18] or "",
                 "has_estd": r[0] in estd, "favorite": r[0] in favs,
             })
+        # PROVENANCE (non-default perspectives): a row shown to a bass or
+        # rhythm player either carries that chart's own tuning (native) or is
+        # borrowing the guitar-derived song tuning (inferred). The fallback is
+        # deliberate — a third of a real library has no bass chart and
+        # excluding it would be worse — but it must never be SILENT, or we
+        # reproduce the original bug in a new place. The client marks inferred
+        # rows; it can't infer this itself without duplicating the COALESCE.
+        #
+        # guitar-lead adds NOTHING here, so the default payload is unchanged.
+        _persp = _perspective(instrument)
+        if _persp.column_prefix:
+            _name_key = _persp.column("name")
+            for s in songs:
+                s["tuning_perspective"] = _persp.id
+                s["tuning_inferred"] = not s.get(_name_key)
         # Personal layer (difficulty + tags) rides along like `favorite`, so a
         # card can badge it without a second request. Notes stay OUT of the list
         # payload (they can be long) — fetch per-song via /user-meta. Batched to
@@ -4169,7 +4387,7 @@ class MetadataDB:
         rows = self.conn.execute(
             "SELECT mw.effective_work_key, m.filename, m.title, m.duration, m.tuning, "
             "m.arrangements, m.has_lyrics, m.mtime, m.format, m.stem_count, m.stem_ids, "
-            "m.tuning_name, m.tuning_offsets "
+            "m.tuning_name, m.tuning_offsets, m.bass_tuning_name, m.bass_tuning_offsets "
             "FROM songs m JOIN work_display mw ON mw.filename = m.filename "
             f"WHERE mw.effective_work_key IN ({ph}){intrinsic_frag} "
             "ORDER BY mw.is_group_representative DESC, m.mtime DESC, m.filename",
@@ -4190,6 +4408,7 @@ class MetadataDB:
                 "stem_count": int(m[9] or 0),
                 "stem_ids": json.loads(m[10]) if m[10] else [],
                 "tuning_name": m[11] or "", "tuning_offsets": m[12] or "",
+                "bass_tuning_name": m[13] or "", "bass_tuning_offsets": m[14] or "",
             }
 
     def query_artists(self, letter: str = "", q: str = "",
@@ -4204,7 +4423,9 @@ class MetadataDB:
                       stems_lacks: list[str] | None = None,
                       has_lyrics: int | None = None,
                       tunings: list[str] | None = None,
-                      naming_mode: str = "legacy") -> tuple[list[dict], int]:
+                      naming_mode: str = "legacy",
+                      instrument: str = DEFAULT_PERSPECTIVE,
+                      playable_from_pitch: int | None = None) -> tuple[list[dict], int]:
         """Get artists grouped by letter with their albums and songs. Returns (artists, total_artists)."""
         where, params = self._build_where(
             q=q, favorites_only=favorites_only, format_filter=format_filter,
@@ -4212,6 +4433,7 @@ class MetadataDB:
             arrangements_has=arrangements_has, arrangements_lacks=arrangements_lacks,
             stems_has=stems_has, stems_lacks=stems_lacks,
             has_lyrics=has_lyrics, tunings=tunings, naming_mode=naming_mode,
+            instrument=instrument, playable_from_pitch=playable_from_pitch,
         )
         # Canonicalize artists at display when aliases exist (P4): dedupe / group /
         # letter / order on the EFFECTIVE artist so "ACDC" + "AC/DC" list as one
@@ -4247,7 +4469,7 @@ class MetadataDB:
 
         rows = self.conn.execute(
             f"SELECT filename, title, ({art_expr}) as artist, album, year, duration, tuning, arrangements, has_lyrics, "
-            f"format, stem_count, stem_ids, tuning_name "
+            f"format, stem_count, stem_ids, tuning_name, bass_tuning_name "
             f"FROM songs {song_where} ORDER BY ({art_expr}) COLLATE NOCASE, album COLLATE NOCASE, title COLLATE NOCASE",
             song_params
         ).fetchall()
@@ -4280,6 +4502,7 @@ class MetadataDB:
                 "stem_count": int(r[10] or 0),
                 "stem_ids": json.loads(r[11]) if r[11] else [],
                 "tuning_name": r[12] or "",
+                "bass_tuning_name": r[13] or "",
                 "has_estd": r[0] in estd,
                 "favorite": r[0] in favs,
                 "user_difficulty": udm.get(r[0]),
@@ -4301,7 +4524,8 @@ class MetadataDB:
                      stems_has=None, stems_lacks=None,
                      has_lyrics=None, tunings=None, mastery=None,
                      match_states=None, genre=None,
-                     naming_mode="legacy", page=0, size=120):
+                     naming_mode="legacy", instrument=DEFAULT_PERSPECTIVE,
+                     playable_from_pitch=None, page=0, size=120):
         """Distinct (artist, album) groups with a track count + a representative
         cover song, for the album-condensed browse (paged by album). Rows with no
         album name are excluded -- they can't form an album card. Same filters as
@@ -4313,7 +4537,8 @@ class MetadataDB:
             stems_has=stems_has, stems_lacks=stems_lacks,
             has_lyrics=has_lyrics, tunings=tunings, mastery=mastery,
             match_states=match_states, genre=genre,
-            naming_mode=naming_mode,
+            naming_mode=naming_mode, instrument=instrument,
+            playable_from_pitch=playable_from_pitch,
         )
         awhere = where + " AND album IS NOT NULL AND album != ''"
         total = self.conn.execute(
@@ -4344,7 +4569,9 @@ class MetadataDB:
                     sort: str = "artist",
                     want_sort_letters: bool = False,
                     group: bool = False,
-                    naming_mode: str = "legacy") -> dict:
+                    naming_mode: str = "legacy",
+                    instrument: str = DEFAULT_PERSPECTIVE,
+                    playable_from_pitch: int | None = None) -> dict:
         """Aggregate stats for the letter bar. Accepts the same filter
         params as query_page so the letter counts stay synchronized
         with the grid when filters are active.
@@ -4371,7 +4598,8 @@ class MetadataDB:
             arrangements_has=arrangements_has, arrangements_lacks=arrangements_lacks,
             stems_has=stems_has, stems_lacks=stems_lacks,
             has_lyrics=has_lyrics, tunings=tunings, match_states=match_states,
-            naming_mode=naming_mode,
+            naming_mode=naming_mode, instrument=instrument,
+            playable_from_pitch=playable_from_pitch,
             include_intrinsic=not group,
         )
         if group:
@@ -4383,7 +4611,8 @@ class MetadataDB:
                 "m", format_filter=format_filter,
                 arrangements_has=arrangements_has, arrangements_lacks=arrangements_lacks,
                 stems_has=stems_has, stems_lacks=stems_lacks,
-                has_lyrics=has_lyrics, tunings=tunings, naming_mode=naming_mode)
+                has_lyrics=has_lyrics, tunings=tunings, naming_mode=naming_mode,
+                instrument=instrument, playable_from_pitch=playable_from_pitch)
             mfrag, mparams = self._grouped_member_match(ifrag, iparams)
             where += mfrag
             params += mparams

lib/routers/library.py

@@ -19,7 +19,7 @@ from starlette.concurrency import run_in_threadpool
 
 import appstate
 from library_registry import (
-    _library_filter_args, _sanitize_collection_rules,
+    _library_filter_args, _normalize_instrument, _sanitize_collection_rules,
     _safe_art_redirect_url, _split_csv, _sync_collection_provider,
     _unregister_collection_provider,
 )
@@ -52,7 +52,8 @@ def _require_library_provider_capability(provider: object, capability: str) -> N
 
 
 _OPTIONAL_NEW_PROVIDER_KWARGS = ("naming_mode", "sort", "want_sort_letters", "after",
-                                 "mastery", "match_states")
+                                 "mastery", "match_states", "instrument",
+                                 "playable_from_pitch")
 
 
 def _filter_provider_kwargs(method: object, kwargs: dict) -> dict:
@@ -235,9 +236,20 @@ async def list_library(q: str = "", page: int = 0, size: int = 24, sort: str = "
                        has_lyrics: str = "", tunings: str = "", provider: str = "local",
                        mastery: str = "", tags: str = "", user_difficulty: str = "",
                        match: str = "", genre: str = "", after: str = "", group: int = 0,
-                       naming_mode: str = "legacy"):
+                       naming_mode: str = "legacy", instrument: str = "",
+                       tuning_match: str = "", playable_offsets: str = "",
+                       playable_instrument: str = "", playable_string_count: str = ""):
     """Paginated library search through the selected library provider.
 
+    `instrument` is the tuning PERSPECTIVE ("guitar-lead" default |
+    "guitar-rhythm" | "bass"): which arrangement's tuning the tuning
+    filter/sort speaks for, with a guitar fallback when a song has no chart in
+    that role.
+
+    `tuning_match=playable` switches the tuning filter from exact-match to
+    "playable without retuning" against the caller's current tuning
+    (`playable_offsets` + `playable_instrument` + `playable_string_count`).
+
     `after` is an opaque keyset cursor (feedBack#636 item 3): pass back the
     `next_cursor` from the previous response to fetch the next page with a
     WHERE-seek instead of OFFSET. Providers that don't support it ignore it and
@@ -270,7 +282,10 @@ async def list_library(q: str = "", page: int = 0, size: int = 24, sort: str = "
             artist=artist, album=album,
             arrangements_has=arrangements_has, arrangements_lacks=arrangements_lacks,
             stems_has=stems_has, stems_lacks=stems_lacks,
-            has_lyrics=has_lyrics, tunings=tunings,
+            has_lyrics=has_lyrics, tunings=tunings, instrument=instrument,
+            tuning_match=tuning_match, playable_offsets=playable_offsets,
+            playable_instrument=playable_instrument,
+            playable_string_count=playable_string_count,
         ),
     )
     # The cursor to resume after this page (effective sort folds in dir=desc).
@@ -292,7 +307,7 @@ async def list_library_albums(q: str = "", page: int = 0, size: int = 120,
                               stems_has: str = "", stems_lacks: str = "",
                               has_lyrics: str = "", tunings: str = "", mastery: str = "",
                               match: str = "", genre: str = "",
-                              provider: str = "local"):
+                              provider: str = "local", instrument: str = ""):
     """Album-condensed browse: distinct (artist, album) groups with a track count
     and a representative cover song. Paged by album. Same filters as /api/library."""
     size = min(size, 500)
@@ -306,7 +321,7 @@ async def list_library_albums(q: str = "", page: int = 0, size: int = 120,
             q=q, favorites=favorites, format=format, artist=artist, album=album,
             arrangements_has=arrangements_has, arrangements_lacks=arrangements_lacks,
             stems_has=stems_has, stems_lacks=stems_lacks,
-            has_lyrics=has_lyrics, tunings=tunings,
+            has_lyrics=has_lyrics, tunings=tunings, instrument=instrument,
         ),
     )
     return {"albums": albums, "total": total, "page": page, "size": size}
@@ -319,7 +334,9 @@ async def list_artists(letter: str = "", q: str = "", favorites: int = 0, page:
                        arrangements_has: str = "", arrangements_lacks: str = "",
                        stems_has: str = "", stems_lacks: str = "",
                        has_lyrics: str = "", tunings: str = "", provider: str = "local",
-                       naming_mode: str = "legacy"):
+                       naming_mode: str = "legacy", instrument: str = "",
+                       tuning_match: str = "", playable_offsets: str = "",
+                       playable_instrument: str = "", playable_string_count: str = ""):
     """Get artists grouped by letter with albums and songs (for tree view)."""
     size = min(size, 100)
     library_provider = _get_library_provider(provider)
@@ -336,7 +353,7 @@ async def list_artists(letter: str = "", q: str = "", favorites: int = 0, page:
             artist=artist, album=album,
             arrangements_has=arrangements_has, arrangements_lacks=arrangements_lacks,
             stems_has=stems_has, stems_lacks=stems_lacks,
-            has_lyrics=has_lyrics, tunings=tunings,
+            has_lyrics=has_lyrics, tunings=tunings, instrument=instrument,
         ),
     )
     return {"artists": artists, "total_artists": total, "page": page, "size": size}
@@ -350,7 +367,10 @@ async def library_stats(favorites: int = 0, q: str = "", format: str = "",
                         has_lyrics: str = "", tunings: str = "", provider: str = "local",
                         match: str = "",
                         sort: str = "artist", sort_letters: int = 0,
-                        group: int = 0, naming_mode: str = "legacy"):
+                        group: int = 0, naming_mode: str = "legacy",
+                        instrument: str = "", tuning_match: str = "",
+                        playable_offsets: str = "", playable_instrument: str = "",
+                        playable_string_count: str = ""):
     """Aggregate stats for the UI. Accepts the same filter params as
     /api/library so the letter bar mirrors the active grid filter set.
     `sort` selects the column the jump rail's `sort_letters` keys on;
@@ -375,7 +395,10 @@ async def library_stats(favorites: int = 0, q: str = "", format: str = "",
             artist=artist, album=album,
             arrangements_has=arrangements_has, arrangements_lacks=arrangements_lacks,
             stems_has=stems_has, stems_lacks=stems_lacks,
-            has_lyrics=has_lyrics, tunings=tunings,
+            has_lyrics=has_lyrics, tunings=tunings, instrument=instrument,
+            tuning_match=tuning_match, playable_offsets=playable_offsets,
+            playable_instrument=playable_instrument,
+            playable_string_count=playable_string_count,
         ),
     )
 
@@ -407,14 +430,20 @@ def library_genres(provider: str = "local"):
 
 
 @router.get("/api/library/tuning-names")
-async def list_tuning_names(provider: str = "local"):
+async def list_tuning_names(provider: str = "local", instrument: str = ""):
     """Distinct tuning names present in the library, with per-tuning
     counts. Powers the tuning multi-select. Sorted by `tuning_sort_key`
     so names appear in the same musical order the sort uses
-    (feedBack#22) — E Standard first, then nearest neighbors."""
+    (feedBack#22) — E Standard first, then nearest neighbors.
+
+    `instrument=bass` groups by each song's bass-arrangement tuning
+    (guitar-derived fallback for songs without a bass chart) so bass
+    players see the tunings they'd actually play. Providers that predate
+    the kwarg simply don't receive it (signature-filtered)."""
     library_provider = _get_library_provider(provider)
     _require_library_provider_capability(library_provider, "library.read")
-    return await _call_library_provider_async(library_provider, "tuning_names")
+    return await _call_library_provider_async(
+        library_provider, "tuning_names", instrument=_normalize_instrument(instrument))
 
 
 @router.get("/api/library/practice-suggestions")

lib/scan.py

@@ -124,6 +124,29 @@ def _library_dirs(all_songs, dlc: Path) -> set[str]:
     return rels
 
 
+def _has_unextracted_columns() -> bool:
+    """True while any `songs` row still carries NULL in a column added by an
+    additive migration — i.e. metadata the current extractor would fill but
+    that no existing row has yet (currently `bass_tuning_name`).
+
+    The tree-signature fast path only asks "did the file set change"; on a
+    settled library the answer is no forever, so a schema addition would never
+    reach extraction. This one-row probe forces the full pass exactly until the
+    backfill completes — `put()` writes '' rather than NULL, so it self-clears
+    after the rescan instead of disabling the fast path permanently."""
+    try:
+        from metadata_db import MetadataDB
+        cond = " OR ".join(f"{c} IS NULL" for c in MetadataDB._EXTRACTION_MARKER_COLS)
+        row = appstate.meta_db.conn.execute(
+            f"SELECT 1 FROM songs WHERE {cond} LIMIT 1").fetchone()
+    except Exception as e:
+        # A probe failure must not take the scan down; falling back to the fast
+        # path costs at most a delayed backfill.
+        log.debug("scan: unextracted-column probe failed: %s", e)
+        return False
+    return row is not None
+
+
 def _record_dir_signature(all_songs, dlc: Path) -> None:
     sig = _stat_dirs(dlc, _library_dirs(all_songs, dlc))
     if sig is not None:   # a dir vanished mid-scan → skip; next scan is full
@@ -221,7 +244,7 @@ def background_scan(force: bool = False):
     # `force` (manual Refresh) always does the full pass. Seeding above is
     # idempotent — it only writes when a builtin is missing — so it does not
     # perturb the mtimes on a settled library.
-    if not force:
+    if not force and not _has_unextracted_columns():
         stored = _load_dir_signature()
         if stored is not None and stored.get("dlc") == str(dlc):
             current = _stat_dirs(dlc, stored["dirs"].keys())
@@ -319,6 +342,15 @@ def background_scan(force: bool = False):
             cached = None
         if not cached:
             to_scan.append((f, mtime, size, dlc))
+        elif any(cached.get(c) is None for c in appstate.meta_db._EXTRACTION_MARKER_COLS):
+            # Row predates one of the per-perspective tuning columns (NULL
+            # from the additive migration), so that perspective's tuning was
+            # never extracted for it. Without this
+            # re-queue an existing library would keep every bass column empty
+            # forever — mtime/size still match, so nothing else would ever
+            # bring the row back through extraction. Converges: put() always
+            # writes '' (never NULL), so a rescanned row is never re-queued.
+            to_scan.append((f, mtime, size, dlc))
         elif cached.get("arrangements") and any(
             "smart_name" not in a for a in cached["arrangements"]
         ):

lib/scan_worker.py

@@ -27,7 +27,11 @@ import logging
 from pathlib import Path
 
 from song import compute_smart_names
-from tunings import tuning_name
+from tunings import (
+    DEFAULT_PERSPECTIVE, PERSPECTIVES, ROLE_PERSPECTIVES, normalize_offsets,
+    perspective_low_pitch, perspective_tuning_key, perspective_tuning_name,
+    tuning_name,
+)
 import sloppak as sloppak_mod
 import loosefolder as loosefolder_mod
 
@@ -43,15 +47,69 @@ def _relpath(f: Path, dlc: Path) -> str:
         return f.name
 
 
+def _apply_role_tunings(meta: dict) -> None:
+    """Derive each ROLE perspective's tuning columns from the raw offsets the
+    extractor emitted (currently bass + rhythm; guitar-lead reads the
+    song-level columns the scanner has always written).
+
+    The domain rules live in `tunings` (see the PERSPECTIVES table and the
+    block above it for the evidence behind each):
+
+    1. NORMALIZE FIRST. Stored bass arrays are commonly six elements whose
+       last two slots are padding, so bass truncates to four strings before
+       anything looks at them — padding must never reach the namer or the
+       grouping key. Guitar does NOT truncate (a 7-string array is real).
+    2. Refuse to name data the perspective distrusts (bass up-tuning), so the
+       library can't send a player off to a tuning nobody plays.
+    3. Group on CANONICAL PITCHES, not the raw offsets string — the same
+       physical tuning serialized two ways must be ONE facet entry.
+
+    A song with no arrangement in that role gets EMPTY strings / 0, not NULL:
+    '' is the indexed "we looked, there is no such chart" state the library's
+    fallback keys on, while NULL means "never extracted" and re-scans.
+    """
+    for persp in ROLE_PERSPECTIVES:
+        raw = meta.pop(f"{persp.role}_tuning_offsets", None)
+        offsets = normalize_offsets(raw, persp)
+        if offsets is None:
+            meta[persp.column("name")] = ""
+            meta[persp.column("sort_key")] = 0
+            meta[persp.column("offsets")] = ""
+            meta[persp.column("key")] = ""
+            meta[persp.column("low_pitch")] = None
+            continue
+        meta[persp.column("name")] = perspective_tuning_name(offsets, persp)
+        meta[persp.column("sort_key")] = sum(offsets)
+        # The NORMALIZED offsets are what we store: padding is not data, and a
+        # client rendering target notes must not print phantom strings.
+        meta[persp.column("offsets")] = " ".join(str(o) for o in offsets)
+        meta[persp.column("key")] = perspective_tuning_key(offsets, persp)
+        meta[persp.column("low_pitch")] = perspective_low_pitch(offsets, persp)
+
+
+def _apply_song_low_pitch(meta: dict, offsets: list[int]) -> None:
+    """Lowest open-string pitch of the SONG-level (guitar-lead) tuning, for
+    the "playable without retuning" comparison. Indexed here, on the existing
+    manifest-only pass — never by reopening chart JSON."""
+    persp = PERSPECTIVES[DEFAULT_PERSPECTIVE]
+    norm = normalize_offsets(offsets, persp)
+    meta["tuning_low_pitch"] = (
+        perspective_low_pitch(norm, persp) if norm is not None else None)
+
+
 def _extract_meta_sloppak(path: Path) -> dict:
     """Extract metadata for a sloppak (file or directory)."""
     meta = sloppak_mod.extract_meta(path)
     offsets = meta.pop("tuning_offsets", None) or [0] * 6
-    name = tuning_name(offsets)
+    # Naming needs the instrument: six offsets could be a guitar OR a
+    # 6-string bass, whose lowest string is B rather than E.
+    name = tuning_name(offsets, is_bass=bool(meta.pop("tuning_is_bass", False)))
     meta["tuning"] = name
     meta["tuning_name"] = name
     meta["tuning_sort_key"] = sum(offsets)
     meta["tuning_offsets"] = " ".join(str(o) for o in offsets)
+    _apply_song_low_pitch(meta, offsets)
+    _apply_role_tunings(meta)
     meta["format"] = "sloppak"
     # `extract_meta` already populates `stem_ids` (feedBack#129);
     # default to empty for older callers / mocks.
@@ -81,11 +139,15 @@ def _extract_meta_loosefolder(path: Path, dlc_root: Path | None) -> dict:
     # inside DLC_DIR.
     meta = loosefolder_mod.extract_meta(path, dlc_root=dlc_root)
     offsets = meta.pop("tuning_offsets", None) or [0] * 6
-    name = tuning_name(offsets)
+    # Naming needs the instrument: six offsets could be a guitar OR a
+    # 6-string bass, whose lowest string is B rather than E.
+    name = tuning_name(offsets, is_bass=bool(meta.pop("tuning_is_bass", False)))
     meta["tuning"] = name
     meta["tuning_name"] = name
     meta["tuning_sort_key"] = sum(offsets)
     meta["tuning_offsets"] = " ".join(str(o) for o in offsets)
+    _apply_song_low_pitch(meta, offsets)
+    _apply_role_tunings(meta)
     meta["format"] = "loose"
     meta.setdefault("stem_ids", [])
     # The library helper exposes absolute filesystem paths for audio/art

lib/sloppak.py

@@ -1227,17 +1227,54 @@ def load_song(
 
 def _tuning_for_meta(arrangements_manifest: list[dict]) -> list[int]:
     """Best-effort guitar-first tuning for the library index."""
+    offsets, _ = _tuning_for_meta_kind(arrangements_manifest)
+    return offsets
+
+
+def _tuning_for_meta_kind(
+    arrangements_manifest: list[dict],
+) -> tuple[list[int], bool]:
+    """`_tuning_for_meta` plus whether the tuning came from a BASS part.
+
+    The caller names the tuning, and naming needs the instrument: a
+    6-string bass has six offsets exactly like a 6-string guitar but its
+    lowest string is B, so the guitar ladder mislabels it (all-zeros reads
+    "E Standard" when it is Standard/B; a whole step down reads "D
+    Standard" when it is A Standard). Guitar parts still win the tuning
+    itself — this only reports which kind supplied it.
+    """
     for entry in arrangements_manifest:
         name = str(entry.get("name", "")).lower()
         tun = entry.get("tuning")
         if tun and isinstance(tun, list) and name in ("lead", "rhythm", "combo"):
-            return list(tun)
+            return list(tun), False
     # Fallback: first arrangement with a tuning
     for entry in arrangements_manifest:
         tun = entry.get("tuning")
         if tun and isinstance(tun, list):
-            return list(tun)
-    return [0] * 6
+            return list(tun), "bass" in str(entry.get("name", "")).lower()
+    return [0] * 6, False
+
+
+def _role_tuning_for_meta(arrangements_manifest: list[dict], role: str) -> list[int] | None:
+    """Per-ROLE companion to _tuning_for_meta: the tuning of the arrangement
+    playing `role` ("bass" / "rhythm"), or None when the pack has no such
+    arrangement with a tuning — the index then leaves that perspective's
+    columns empty and the library falls back to the song (guitar-first)
+    tuning, marking the row inferred.
+
+    Exact name first, then a looser containment pass so an alt/bonus chart
+    ("Bass 2", "Alt Rhythm") still beats pretending the part is in the lead
+    guitar's tuning."""
+    for match_exact in (True, False):
+        for entry in arrangements_manifest:
+            name = str(entry.get("name", "")).lower()
+            tun = entry.get("tuning")
+            if not (tun and isinstance(tun, list)):
+                continue
+            if name == role if match_exact else role in name:
+                return list(tun)
+    return None
 
 
 def extract_meta(path: Path) -> dict:
@@ -1261,7 +1298,10 @@ def extract_meta(path: Path) -> dict:
         a["index"] = i
 
     has_lyrics = bool(manifest.get("lyrics"))
-    tuning_offsets = _tuning_for_meta(arr_list)
+    tuning_offsets, tuning_is_bass = _tuning_for_meta_kind(arr_list)
+    # Per-role tunings alongside the song-level one.
+    role_tunings = {f"{role}_tuning_offsets": _role_tuning_for_meta(arr_list, role)
+                    for role in ("bass", "rhythm")}
 
     stems_list = manifest.get("stems", []) or []
     valid_stems: list[dict] = []
@@ -1300,6 +1340,10 @@ def extract_meta(path: Path) -> dict:
         "disc": (lambda v: int(v) if str(v if v is not None else "").strip().isdigit() else None)(manifest.get("disc")),
         "duration": float(manifest.get("duration", 0) or 0),
         "tuning_offsets": tuning_offsets,  # caller maps to a name via tunings.tuning_name
+        # None = the pack has no arrangement in that role.
+        **role_tunings,
+        # Song-level naming also needs the instrument for bass-only packs.
+        "tuning_is_bass": tuning_is_bass,
         "arrangements": arrangements,
         "has_lyrics": has_lyrics,
         "stem_count": stem_count,

lib/tunings.py

@@ -69,21 +69,49 @@ TUNING_PRESET_MIDIS: dict[str, dict[str, list[int]]] = {
         "Drop C": [24, 31, 36, 41],
         "BEAD": [23, 28, 33, 38],
     },
+    # 5- and 6-string basses are named off their ACTUAL lowest string (the low
+    # B), exactly like the 7-string guitar table above — not off the 4-string
+    # core. The old names (Eb/D/C#/C Standard) came from the band-level habit
+    # of saying "we're in D standard" (which describes the guitars); the
+    # bassist in that band is in A standard. They survive as aliases below so
+    # saved profiles migrate instead of being rejected.
     "bass-5": {
         "Standard": [23, 28, 33, 38, 43],
         "High C": [28, 33, 38, 43, 48],
-        "Eb Standard": [22, 27, 32, 37, 42],
-        "D Standard": [21, 26, 31, 36, 41],
-        "C# Standard": [20, 25, 30, 35, 40],
-        "C Standard": [19, 24, 29, 34, 39],
+        "Bb Standard": [22, 27, 32, 37, 42],
+        "A Standard": [21, 26, 31, 36, 41],
+        "G# Standard": [20, 25, 30, 35, 40],
+        "G Standard": [19, 24, 29, 34, 39],
         "Drop A": [21, 28, 33, 38, 43],
     },
     "bass-6": {
         "Standard": [23, 28, 33, 38, 43, 48],
-        "Eb Standard": [22, 27, 32, 37, 42, 47],
-        "D Standard": [21, 26, 31, 36, 41, 46],
-        "C# Standard": [20, 25, 30, 35, 40, 45],
-        "C Standard": [19, 24, 29, 34, 39, 44],
+        "Bb Standard": [22, 27, 32, 37, 42, 47],
+        "A Standard": [21, 26, 31, 36, 41, 46],
+        "G# Standard": [20, 25, 30, 35, 40, 45],
+        "G Standard": [19, 24, 29, 34, 39, 44],
+    },
+}
+
+# Superseded preset names, per key → current name. The 5/6-string bass rows
+# were originally named off the 4-string core (so a whole-step-down 6-string,
+# whose lowest string is A, read "D Standard"). Renaming alone would make
+# `_valid_tuning_for_key` REJECT a saved profile carrying the old name — it
+# refuses names that belong to a different key's built-ins, and "D Standard"
+# still exists for guitar-6/bass-4. These aliases keep those profiles valid
+# and migrate them to the corrected name.
+TUNING_PRESET_ALIASES: dict[str, dict[str, str]] = {
+    "bass-5": {
+        "Eb Standard": "Bb Standard",
+        "D Standard": "A Standard",
+        "C# Standard": "G# Standard",
+        "C Standard": "G Standard",
+    },
+    "bass-6": {
+        "Eb Standard": "Bb Standard",
+        "D Standard": "A Standard",
+        "C# Standard": "G# Standard",
+        "C Standard": "G Standard",
     },
 }
 
@@ -229,6 +257,12 @@ def _valid_tuning_for_key(key: str, tuning):
             return None
         if tuning in TUNING_PRESET_MIDIS.get(key, {}):
             return tuning
+        # A superseded name for THIS key migrates to its current spelling
+        # (the 5/6-string bass rename). Checked before the cross-key
+        # rejection below, which would otherwise refuse it.
+        renamed = TUNING_PRESET_ALIASES.get(key, {}).get(tuning)
+        if renamed:
+            return renamed
         # A name that IS a built-in preset for a different key is a misapplied
         # built-in (e.g. "Drop D" on a 5-string bass, whose low string is B) —
         # reject it. A name unknown to every built-in table is a provider/custom
@@ -416,21 +450,308 @@ def apply_flat_instrument_patch_to_profiles(cfg: dict, updates: dict) -> dict:
     })
     return out
 
-def tuning_name(offsets: list[int]) -> str:
-    # All three pattern checks below are gated on `len(offsets) == 6`. The
-    # naming conventions here are 6-string-specific — e.g. a 7-string all-zeros
-    # tuning has a low B, not an E, so labeling it "E Standard" would be wrong.
-    # 7+-string community content falls through to the numeric fallback. See #43.
+# ── Bass tuning normalization (library indexing) ─────────────────────────────
+#
+# Bass charts in the wild store SIX-element tuning arrays even when the chart is
+# a 4-string part: slots 4-5 are PADDING. Confirmed by inspecting the charts
+# themselves — across every pack whose bass and guitar tunings diverge, no bass
+# note ever references string index 4 or 5 (the deepest reach is index 3).
+#
+# The feedpak spec carries NO string-count field (manifest `arrangement.tuning`
+# is an untyped integer array, `minItems: 1`), and counting strings for real
+# would mean parsing the 600KB-1.2MB arrangement JSON of every song on the
+# manifest-only fast scan path — unacceptable for scan time. So we DEFAULT BASS
+# TO 4 STRINGS and truncate.
+#
+# Five-element arrays are unambiguously extended-range. Six elements remain
+# ambiguous because legacy four-string charts are padded to six; preserve that
+# legacy interpretation except for a uniform non-zero down-tuning, which cannot
+# be padding (the padded tail would be zero) and is the common extended-range
+# case that motivated the fix. Ambiguous all-zero and drop-shaped six-element
+# arrays stay conservative until the manifest carries an explicit string count.
+BASS_DEFAULT_STRING_COUNT = 4
 
-    # Standard tunings (all six strings same offset)
-    standard = {
+# Bassists tune DOWN, essentially never up: a whole-instrument up-tune fights
+# string tension. Anything above +1 semitone across the board is data we do not
+# trust, not a tuning a human plays (the real-world example that motivated this
+# is a bass array of [5,5,5,5,4,4] — "all four strings up a perfect fourth" —
+# on a song whose guitar chart is dead standard and whose own note content is
+# consistent with standard tuning; the offsets were almost certainly computed
+# against a 6-string-bass reference with an uninitialised tail).
+#
+# Such a tuning MUST NOT be named: printing "A Standard" would send a player
+# off to retune to something nobody plays. It degrades to the custom path,
+# where it stays visible and distinct but makes no pitch claim.
+BASS_MAX_PLAUSIBLE_OFFSET = 1
+
+
+# ── Tuning PERSPECTIVES ──────────────────────────────────────────────────────
+#
+# The library's tuning facet/filter/sort always answers for ONE arrangement
+# role. There are three, matching `active_instrument_profile`:
+#
+#   guitar-lead    the song-level (guitar-first) tuning — the historical
+#                  default. Its columns are the original unprefixed
+#                  `tuning_*` family, so today's behaviour is byte-identical.
+#   guitar-rhythm  the RHYTHM chart's own tuning. Lead and rhythm charts can
+#                  disagree (the same bug a bassist hit, inside guitar).
+#   bass           the BASS chart's own tuning.
+#
+# One table drives extraction, the derived columns, the SQL, and the labels —
+# rather than three near-identical column families maintained in parallel.
+class TuningPerspective:
+    __slots__ = ("id", "role", "instrument", "string_count", "column_prefix",
+                 "truncate", "guard_up_tuning", "label")
+
+    def __init__(self, id, role, instrument, string_count, column_prefix,
+                 truncate, guard_up_tuning, label):
+        self.id = id
+        self.role = role                    # arrangement name to look for ('' = song-level)
+        self.instrument = instrument
+        self.string_count = string_count
+        self.column_prefix = column_prefix  # '' | 'rhythm_' | 'bass_'
+        self.truncate = truncate
+        self.guard_up_tuning = guard_up_tuning
+        self.label = label
+
+    @property
+    def instrument_key(self) -> str:
+        return instrument_key(self.instrument, self.string_count)
+
+    def column(self, suffix: str) -> str:
+        return f"{self.column_prefix}tuning_{suffix}"
+
+
+PERSPECTIVES: dict[str, TuningPerspective] = {
+    "guitar-lead": TuningPerspective(
+        "guitar-lead", "", "guitar", 6, "", False, False, "lead"),
+    "guitar-rhythm": TuningPerspective(
+        "guitar-rhythm", "rhythm", "guitar", 6, "rhythm_", False, False, "rhythm"),
+    # Bass alone truncates (padded arrays) and guards against up-tuned data —
+    # both are bass-specific findings, see the block above.
+    "bass": TuningPerspective(
+        "bass", "bass", "bass", BASS_DEFAULT_STRING_COUNT, "bass_", True, True, "bass"),
+}
+
+DEFAULT_PERSPECTIVE = "guitar-lead"
+
+# Perspectives that carry their OWN indexed columns (guitar-lead reads the
+# song-level ones, which the scanner has always written).
+ROLE_PERSPECTIVES = tuple(p for p in PERSPECTIVES.values() if p.column_prefix)
+
+
+def perspective(perspective_id) -> TuningPerspective:
+    """Resolve a perspective id, tolerating the legacy two-valued vocabulary
+    ('guitar' -> guitar-lead) and anything unknown (-> the default). An
+    unrecognised value must never change filter semantics."""
+    if perspective_id in PERSPECTIVES:
+        return PERSPECTIVES[perspective_id]
+    if perspective_id == "guitar":
+        return PERSPECTIVES[DEFAULT_PERSPECTIVE]
+    return PERSPECTIVES[DEFAULT_PERSPECTIVE]
+
+
+def normalize_offsets(offsets, persp: TuningPerspective) -> list[int] | None:
+    """Coerce a stored tuning array to the strings the perspective's
+    instrument actually has. Returns None for anything unusable (empty /
+    non-integer / too short), so callers leave the index empty rather than
+    record a guess."""
+    if not isinstance(offsets, list) or not offsets:
+        return None
+    if any(isinstance(o, bool) for o in offsets):
+        return None
+    try:
+        vals = [int(o) for o in offsets]
+    except (TypeError, ValueError):
+        return None
+    if len(vals) < persp.string_count:
+        return None
+    # Only bass can be padded. Five entries are unambiguously extended-range.
+    # Six are ambiguous: legacy four-string data pads with zeroes, while a
+    # uniform non-zero down-tuning across all six strings proves the tail is
+    # authored. Keep every other six-element shape conservative at four.
+    if persp.truncate:
+        if len(vals) == 5:
+            return vals
+        if len(vals) == 6 and vals[0] != 0 and len(set(vals)) == 1:
+            return vals
+        return vals[:persp.string_count]
+    return vals
+
+
+def offsets_are_plausible(offsets: list[int], persp: TuningPerspective) -> bool:
+    """False for data the perspective refuses to trust — currently only the
+    bass up-tuning guard (see BASS_MAX_PLAUSIBLE_OFFSET)."""
+    if not persp.guard_up_tuning:
+        return True
+    return all(o <= BASS_MAX_PLAUSIBLE_OFFSET for o in offsets)
+
+
+def perspective_tuning_name(offsets: list[int], persp: TuningPerspective) -> str:
+    """Name a NORMALIZED tuning for this perspective, refusing to name data the
+    perspective distrusts — that becomes "Custom Tuning", which stays distinct
+    by its canonical pitches without asserting a tuning anyone plays."""
+    if not offsets_are_plausible(offsets, persp):
+        return "Custom Tuning"
+    return tuning_name(offsets, is_bass=persp.instrument == "bass")
+
+
+def _perspective_instrument_key(
+    offsets: list[int], persp: TuningPerspective,
+) -> str:
+    """Instrument key matching the normalized tuning's proven string count."""
+    if persp.instrument == "bass" and f"bass-{len(offsets)}" in STANDARD_OPEN_MIDIS:
+        return f"bass-{len(offsets)}"
+    return persp.instrument_key
+
+
+def perspective_tuning_key(offsets: list[int], persp: TuningPerspective) -> str:
+    """CANONICAL grouping key: the tuning's absolute open-string pitches, so
+    the same physical tuning groups as ONE facet entry no matter how it was
+    serialized. Keyed on pitch rather than the raw offsets string, which is
+    serialization-dependent and fragments.
+
+    Joined with ':' and NOT ',' — this key travels back as a `tunings` filter
+    selector, and that query param is a COMMA-separated list, so a comma here
+    would be split into meaningless fragments and match nothing.
+    """
+    midis = tuning_midis_from_offsets(_perspective_instrument_key(offsets, persp), offsets)
+    if not midis:
+        return ""
+    return persp.id + ":" + ":".join(str(m) for m in midis)
+
+
+def perspective_low_pitch(offsets: list[int], persp: TuningPerspective) -> int | None:
+    """Absolute MIDI pitch of the tuning's LOWEST open string — the value the
+    "playable without retuning" comparison is built on (see
+    `chart_is_playable_in`)."""
+    midis = tuning_midis_from_offsets(_perspective_instrument_key(offsets, persp), offsets)
+    if not midis:
+        return None
+    return min(midis)
+
+
+# ── "Playable without retuning" ──────────────────────────────────────────────
+#
+# What the player actually wants is "don't make me retune", not "match this
+# label". A chart is playable as-is when every pitch it needs is reachable on
+# the instrument as currently tuned.
+#
+# WHAT WE CAN HONESTLY COMPUTE. We index open-string TUNINGS, not the notes a
+# chart plays — note data lives in the 600KB-1.2MB arrangement JSON, and the
+# library scan is deliberately manifest-only, so we do not read it (indexing a
+# per-song lowest note would mean opening every chart on every scan).
+#
+# So the comparison is on OPEN-STRING PITCH, with a conservative assumption:
+# a chart may require its own lowest open string. That gives
+#
+#     playable  <=>  your lowest open pitch  <=  the chart's lowest open pitch
+#
+# On a fretted instrument every pitch ABOVE your lowest open string is
+# reachable by fretting (strings sit within an octave of each other and the
+# neck gives ~2 octaves), so the low end is the binding constraint. This is
+# exactly the dominant real case: a 5-string bass (low B) plays every 4-string
+# standard chart AND every drop-D chart untouched, because the low D is just
+# fretted on the B string.
+#
+# DELIBERATE LIMITATIONS, both erring toward NOT claiming playability:
+#   * A chart that never actually touches its lowest open string is excluded
+#     anyway. Conservative: excluding a playable chart costs a scroll;
+#     including an unplayable one costs a mid-practice retune, which is the
+#     failure this feature exists to prevent.
+#   * The UPPER bound is not checked — a chart tuned far above you could in
+#     principle exceed your neck. Checking it needs the note range we do not
+#     have. It is the rare direction (and the guard above already refuses
+#     up-tuned bass data), but it is a real gap, not an oversight.
+def chart_is_playable_in(chart_low_pitch, your_low_pitch) -> bool:
+    """True when a chart whose lowest open string is `chart_low_pitch` needs no
+    retune for a player tuned to `your_low_pitch`. Unknown chart pitch => False
+    (never claim playability we cannot support)."""
+    if chart_low_pitch is None or your_low_pitch is None:
+        return False
+    return int(your_low_pitch) <= int(chart_low_pitch)
+
+
+# Back-compat wrappers over the generic helpers — bass was the first
+# perspective and reads better spelled out at bass-specific call sites.
+def normalize_bass_offsets(offsets) -> list[int] | None:
+    return normalize_offsets(offsets, PERSPECTIVES["bass"])
+
+
+def bass_offsets_are_plausible(offsets: list[int]) -> bool:
+    return offsets_are_plausible(offsets, PERSPECTIVES["bass"])
+
+
+def bass_tuning_name(offsets: list[int]) -> str:
+    return perspective_tuning_name(offsets, PERSPECTIVES["bass"])
+
+
+def bass_tuning_key(offsets: list[int]) -> str:
+    return perspective_tuning_key(offsets, PERSPECTIVES["bass"])
+
+
+def tuning_name(offsets: list[int], *, is_bass: bool = False) -> str:
+    """Display name for a set of per-string offsets.
+
+    `is_bass` is load-bearing, not cosmetic: string COUNT alone cannot
+    identify the instrument. A 6-string BASS has six offsets just like a
+    6-string guitar, but its lowest string is B, not E — so the guitar
+    ladder labels an all-zeros bass "E Standard" (it is B/Standard) and a
+    whole-step-down bass "D Standard" (it is A Standard). That is exactly
+    the error the 7-string comment below warned about, on the axis nobody
+    guarded. Callers that know the instrument must say so; the default
+    stays guitar for backward compatibility.
+
+    All the pattern checks are gated on the expected string count. The
+    guitar conventions are 6-string-specific — e.g. a 7-string all-zeros
+    tuning has a low B, not an E, so labeling it "E Standard" would be
+    wrong. 7+-string guitar content falls through to the numeric
+    fallback. See #43.
+    """
+    # Standard tunings (all strings same offset), named off the LOWEST
+    # string. Bass 4-string sits on the E ladder like a guitar; bass 5/6
+    # add a low B, so they sit on the B ladder — the same convention the
+    # 7-string guitar presets use.
+    guitar_standard = {
         0: "E Standard", -1: "Eb Standard", -2: "D Standard",
         -3: "C# Standard", -4: "C Standard", -5: "B Standard",
         -6: "Bb Standard", -7: "A Standard",
         1: "F Standard", 2: "F# Standard",
     }
+    bass_low_b_standard = {
+        0: "Standard", -1: "Bb Standard", -2: "A Standard",
+        -3: "G# Standard", -4: "G Standard", -5: "F# Standard",
+        1: "C Standard", 2: "C# Standard",
+    }
+    # A four-offset array is unambiguously a bass tuning; preserve the
+    # historical one-argument behavior used by the library perspective.
+    if len(offsets) == 4:
+        is_bass = True
+
+    if is_bass:
+        # 4-string bass is E-A-D-G — the guitar ladder's low four, so it
+        # keeps the E-based names. 5/6-string add the low B.
+        table = guitar_standard if len(offsets) == 4 else bass_low_b_standard
+        if len(offsets) in (4, 5, 6) and all(o == offsets[0] for o in offsets):
+            name = table.get(offsets[0])
+            if name:
+                return name
+        # Drop tunings: the lowest string alone goes down 2 semitones.
+        if (len(offsets) in (4, 5, 6)
+                and offsets[0] == offsets[1] - 2
+                and all(o == offsets[1] for o in offsets[1:])):
+            base = STANDARD_OPEN_MIDIS.get(f"bass-{len(offsets)}")
+            if base:
+                low = base[0] + offsets[0]
+                names = ["C", "C#", "D", "Eb", "E", "F",
+                         "F#", "G", "Ab", "A", "Bb", "B"]
+                return f"Drop {names[low % 12]}"
+        if not offsets:
+            return "Unknown"
+        return "Custom Tuning"
+
     if len(offsets) == 6 and all(o == offsets[0] for o in offsets):
-        name = standard.get(offsets[0])
+        name = guitar_standard.get(offsets[0])
         if name:
             return name
 

plugins/highway_3d/screen.js

@@ -12614,8 +12614,19 @@
                         const tC = now + (dt0 + dt1) * 0.5 - BEHIND;
                         const b = laneBoundsFromAnchor(getChartAnchorAt(anchors, tC));
                         if (!b) continue;
-                        const z0 = dZ(dt0) + TS * BEHIND;
-                        const z1 = dZ(dt1) + TS * BEHIND;
+                        // The lane STOPS AT THE HIT LINE (z = 0) — issue #991. The
+                        // slice window starts BEHIND seconds in the past, so the
+                        // first slices map to positive z, i.e. past the hit line
+                        // toward the player. Nothing is ever drawn there: notes and
+                        // chord frames clamp to Math.min(0, dZ(dt)), so that strip
+                        // is lane with nothing on it. Clamp the NEAR edge only —
+                        // the far edge stays at dZ(AHEAD+BEHIND)+TS*BEHIND = -AHEAD*TS,
+                        // aligned with the note horizon, exactly as before.
+                        const z0 = Math.min(0, dZ(dt0) + TS * BEHIND);
+                        const z1 = Math.min(0, dZ(dt1) + TS * BEHIND);
+                        // Slice lies entirely past the hit line -> zero length, nothing
+                        // to draw. Skip before the arp probe so it costs nothing.
+                        if (z0 === z1) continue;
                         const arpSlice = (laneRailArpHsFlags && handShapesRails && handShapesRails.length)
                             ? arpeggioLaneOuterRailLaneSlice(
                                 dt0, dt1, now,
@@ -12764,9 +12775,13 @@
                     divMin = dMin;
                     divMax = dMax;
 
-                    // Same fix: extend to AHEAD+BEHIND so far edge = -AHEAD*TS.
-                    const laneLen = TS * (AHEAD + BEHIND);
-                    const zLane = -laneLen / 2 + TS * BEHIND;
+                    // Far edge at -AHEAD*TS (the note horizon), near edge at the
+                    // hit line (z = 0) — the lane does not run past it toward the
+                    // player, where nothing is ever drawn (#991). Spanning
+                    // AHEAD+BEHIND and shifting by +TS*BEHIND put the near edge at
+                    // +TS*BEHIND; spanning AHEAD alone keeps the same far edge.
+                    const laneLen = TS * AHEAD;
+                    const zLane = -laneLen / 2;
                     const laneOp = (HWY_LANE_STRIPE_OP_BASE + highwayIntensity * HWY_LANE_STRIPE_OP_INT)
                         * (_venueSceneOverride ? VENUE_LANE_OP_BOOST : 1);
                     mLaneOdd.opacity = laneOp;
@@ -12786,7 +12801,8 @@
                     }
 
                     if (highwayIntensity > 0.05) {
-                        const divLen = TS * (AHEAD + BEHIND);
+                        // Matches the lane above: ends at the hit line (#991).
+                        const divLen = TS * AHEAD;
                         const yPos = boardY + 0.03 * K;
                         const divOp2 = 0.02 + highwayIntensity * 0.1;
                         const divOpArp2 = Math.min(0.92, 0.16 + highwayIntensity * 0.42);
@@ -12799,7 +12815,7 @@
                         for (let f = fDivA; f <= fDivB; f++) {
                             if (hwyLaneArpOuterDividers && (f === fDivA || f === fDivB)) continue;
                             const div = pLaneDivider.get();
-                            div.position.set(xFret(f), yPos, dZ(0) - divLen * 0.5 + TS * BEHIND);
+                            div.position.set(xFret(f), yPos, -divLen * 0.5);
                             div.material = mLaneDivider;
                             div.scale.set(1, 1, divLen);
                             div.renderOrder = 2;
@@ -12818,8 +12834,10 @@
 
                 // ── Fret boundary extension lines ─────────────────────────
                 if (mLaneDividerExt && fretDividersVisible) {
-                    const extLaneLen = TS * (AHEAD + BEHIND);
-                    const extZMid = -extLaneLen / 2 + TS * BEHIND;
+                    // Same hit-line stop as the lane (#991) — otherwise these lines
+                    // would be the only floor geometry still running past it.
+                    const extLaneLen = TS * AHEAD;
+                    const extZMid = -extLaneLen / 2;
                     const extYPos = boardY + 0.03 * K;
                     mLaneDividerExt.opacity = Math.max(0.3, 0.3 + highwayIntensity * 0.15);
                     for (let f = 0; f <= NFRETS; f++) {

static/js/library.js

@@ -410,6 +410,51 @@ function _applyLibraryProviderToParams(params) {
     return params;
 }
 
+// ── Instrument-aware tuning (the bass-player tuning-filter report) ───────────
+// A song's bass chart is often tuned differently from its guitar chart, so the
+// tuning facet, the `tunings` filter, the tuning sort and the row's tuning
+// badge must all speak for the instrument the player actually plays. Read the
+// host's working-tuning capability (the live selection, seeded from
+// /api/settings at boot) rather than adding another settings fetch; hosts
+// without the capability keep the guitar behaviour.
+const _LIB_PERSPECTIVES = ['guitar-lead', 'guitar-rhythm', 'bass'];
+let _libSettingsProfile = '';
+
+export function _setLibraryProfile(profileId) {
+    _libSettingsProfile = _LIB_PERSPECTIVES.includes(profileId) ? profileId : '';
+}
+
+export function _libraryInstrument() {
+    // The PROFILE is the only three-valued source (lead / rhythm / bass); the
+    // working-tuning capability knows guitar-vs-bass but not lead-vs-rhythm,
+    // so it is only the fallback.
+    if (_libSettingsProfile) return _libSettingsProfile;
+    try {
+        const wt = window.feedBack?.workingTuning;
+        if (wt && typeof wt.get === 'function') {
+            const cur = wt.get();
+            if (cur?.instrument === 'bass') return 'bass';
+        }
+    } catch { /* capability absent/erroring — lead guitar is the safe default */ }
+    return 'guitar-lead';
+}
+
+export function _libraryInstrumentLabel() {
+    const p = _libraryInstrument();
+    return p === 'bass' ? 'bass' : p === 'guitar-rhythm' ? 'rhythm' : 'lead';
+}
+
+// The tuning a row should SHOW: the bass chart's for a bass player, falling
+// back to the song (guitar-derived) tuning when the song has no bass
+// arrangement — the common case, not an edge path.
+function _rowTuningRaw(song) {
+    const p = _libraryInstrument();
+    const field = p === 'bass' ? 'bass_tuning_name'
+        : p === 'guitar-rhythm' ? 'rhythm_tuning_name' : '';
+    if (field && song[field]) return song[field];
+    return song.tuning || song.tuning_name || '';
+}
+
 export function _resetLibraryProviderViewState() {
     L.libEpoch++;
     L.currentPage = 0;
@@ -768,6 +813,8 @@ export function _applyLibFiltersToParams(params) {
     if (_libFilters.stemsLacks.length) params.set('stems_lacks', _libFilters.stemsLacks.join(','));
     if (_libFilters.lyrics !== null) params.set('has_lyrics', String(_libFilters.lyrics));
     if (_libFilters.tunings.length) params.set('tunings', _libFilters.tunings.join(','));
+    // Which instrument's tuning the `tunings` filter + the tuning sort read.
+    if (_libraryInstrument() !== 'guitar-lead') params.set('instrument', _libraryInstrument());
     return params;
 }
 
@@ -851,6 +898,7 @@ async function _renderTuningList() {
         c.innerHTML = '<div class="text-xs text-gray-500 px-2">Loading...</div>';
         try {
             const params = _applyLibraryProviderToParams(new URLSearchParams());
+            params.set('instrument', _libraryInstrument());
             const resp = await fetch(`/api/library/tuning-names?${params}`);
             if (!resp.ok) throw new Error(`HTTP ${resp.status}`);
             const data = await resp.json();
@@ -869,6 +917,11 @@ async function _renderTuningList() {
             fetchError = e.message || 'request failed';
         }
     }
+    // NAME the perspective: silent instrument-following is the original bug in
+    // a new place — the user must be able to see which instrument these
+    // tunings describe.
+    const labelEl = document.getElementById('filter-tunings-label');
+    if (labelEl) labelEl.textContent = `Tuning (${_libraryInstrumentLabel()})`;
     c.innerHTML = '';
     if (fetchError) {
         c.innerHTML = `<div class="text-xs text-red-400 px-2">Failed to load tunings (${esc(fetchError)}). Reopen the drawer to retry.</div>`;
@@ -894,10 +947,17 @@ async function _renderTuningList() {
         const checked = _libFilters.tunings.includes(val);
         const row = document.createElement('label');
         row.className = 'tuning-row';
+        // Be honest about the fallback: songs with no bass arrangement borrow
+        // the guitar chart's tuning, and that must be visible rather than
+        // presented as a measured bass tuning.
+        const inferred = t.inferred_count || 0;
+        if (inferred) {
+            row.title = `${inferred} of ${t.count} inferred from the guitar chart (no bass arrangement)`;
+        }
         row.innerHTML =
             `<input type="checkbox" ${checked ? 'checked' : ''} class="rounded border-gray-600 bg-dark-700 text-accent">` +
             `<span class="flex-1">${esc(label)}</span>` +
-            `<span class="tuning-count">${t.count}</span>`;
+            `<span class="tuning-count">${t.count}${inferred ? ` (${inferred}~)` : ''}</span>`;
         const cb = row.querySelector('input');
         cb.onchange = () => {
             const i = _libFilters.tunings.indexOf(val);
@@ -1244,6 +1304,10 @@ export function renderGridCards(songs, containerId = 'lib-grid', mode = 'replace
         const duration = song.duration ? formatTime(song.duration) : '';
         const tuningRaw = song.tuning || song.tuning_name || '';
         const tuning = displayTuningName(tuningRaw);
+        // The BADGE follows the player's instrument; `tuning` above stays the
+        // song's guitar-derived tuning because the retune action below rewrites
+        // the chart to E Standard and must not key on the bass part.
+        const tuningBadge = displayTuningName(_rowTuningRaw(song));
         const artUrl = _librarySongArtUrl(song, providerId);
         const isLocalProvider = _isLocalLibraryProvider(providerId);
         const isSloppak = song.format === 'sloppak';
@@ -1299,7 +1363,7 @@ export function renderGridCards(songs, containerId = 'lib-grid', mode = 'replace
                 </div>
                 <div class="flex items-center flex-wrap gap-1.5 mt-3 text-xs">
                     ${(() => { const _nm = _getArrangementNamingMode(); return (song.arrangements || []).map(a => _arrangementBadgeHtml(a, _nm)).join(''); })()}
-                    ${tuning ? `<span class="px-1.5 py-0.5 rounded ${tuning === 'E Standard' ? 'bg-green-900/30 text-green-400' : 'bg-yellow-900/30 text-yellow-400'}">${esc(tuning)}</span>` : ''}
+                    ${tuningBadge ? `<span class="px-1.5 py-0.5 rounded ${tuningBadge === 'E Standard' ? 'bg-green-900/30 text-green-400' : 'bg-yellow-900/30 text-yellow-400'}">${esc(tuningBadge)}</span>` : ''}
                     ${song.has_lyrics ? `<span class="px-1.5 py-0.5 bg-purple-900/30 rounded text-purple-300">Lyrics</span>` : ''}
                     ${song.user_difficulty != null ? `<span class="px-1.5 py-0.5 bg-blue-900/30 rounded text-blue-300" title="Your difficulty rating">◆${esc(song.user_difficulty)}</span>` : ''}
                     ${duration ? `<span class="text-gray-600">${duration}</span>` : ''}
@@ -1470,6 +1534,9 @@ export async function renderTreeInto(containerId, countId, stats, letter, q, fav
                 const duration = song.duration ? formatTime(song.duration) : '';
                 const tuningRaw = song.tuning || song.tuning_name || '';
                 const tuning = displayTuningName(tuningRaw);
+                // Badge follows the player's instrument; the retune action below
+                // keeps operating on the song's guitar-derived tuning.
+                const tuningBadge = displayTuningName(_rowTuningRaw(song));
                 const isLocalProvider = _isLocalLibraryProvider(providerId);
                 const isSloppak = song.format === 'sloppak';
                 const stdRetune = isLocalProvider && localFilename && !isSloppak && tuningRaw && !song.has_estd &&
@@ -1496,8 +1563,8 @@ export async function renderTreeInto(containerId, countId, stats, letter, q, fav
                 { const _nm = _getArrangementNamingMode();
                   for (const arrangement of (song.arrangements || []))
                       html += _arrangementBadgeHtml(arrangement, _nm); }
-                if (tuning)
-                    html += `<span class="px-1.5 py-0.5 rounded ${tuning === 'E Standard' ? 'bg-green-900/30 text-green-400' : 'bg-yellow-900/30 text-yellow-400'}">${esc(tuning)}</span>`;
+                if (tuningBadge)
+                    html += `<span class="px-1.5 py-0.5 rounded ${tuningBadge === 'E Standard' ? 'bg-green-900/30 text-green-400' : 'bg-yellow-900/30 text-yellow-400'}">${esc(tuningBadge)}</span>`;
                 if (song.has_lyrics)
                     html += `<span class="px-1.5 py-0.5 bg-purple-900/30 rounded text-purple-300">Lyrics</span>`;
                 if (song.user_difficulty != null)

static/js/settings.js

@@ -18,7 +18,7 @@
 // back-import would close a cycle. player-controls keeps reading it through the host seam, and
 // app.js — the root, which imports both — wires it. That is exactly what the seam is for.
 import { hwcInitSettingsUI } from './highway-colors.js';
-import { _getArrangementNamingMode } from './library.js';
+import { _getArrangementNamingMode, _setLibraryProfile } from './library.js';
 import {
     _applyMastery, _autoplayExitEnabled, _exitConfirmEnabled, _showUpNextEnabled,
 } from './player-controls.js';
@@ -111,6 +111,10 @@ export async function loadSettings() {
     if (dlcEl) dlcEl.value = data.dlc_dir || '';
     _defaultArrangement = data.default_arrangement || '';
     _syncDefaultArrangementSelect(_defaultArrangement);
+    // Feed the library its tuning PERSPECTIVE (lead / rhythm / bass) — the
+    // tuning facet, filter, sort and badges all answer for the profile the
+    // player actually plays.
+    _setLibraryProfile(data.active_instrument_profile);
     const pathwayEl = document.getElementById('setting-instrument-pathway');
     if (pathwayEl) pathwayEl.value = _normalizeInstrumentPathway(data.pathway);
     const demucsEl = document.getElementById('demucs-server-url');

static/v3/index.html

@@ -326,7 +326,7 @@
                 <section>
                     <details>
                         <summary class="cursor-pointer flex items-center justify-between text-xs font-semibold uppercase tracking-wider text-gray-500 mb-2">
-                            <span>Tuning</span>
+                            <span id="filter-tunings-label">Tuning</span>
                             <span id="filter-tunings-summary" class="text-gray-600 normal-case font-normal text-xs">All tunings</span>
                         </summary>
                         <div id="filter-tunings" class="mt-3 space-y-1 max-h-64 overflow-y-auto pr-1"></div>

static/v3/songs.js

@@ -62,7 +62,7 @@
         artist: '', album: '',
         grouping: true,     // one card per song (multi-chart grouping); persisted
 
-        filters: { arr_has: [], arr_lacks: [], stem_has: [], stem_lacks: [], lyrics: '', tunings: [], mastery: [], match: [], genre: [] },
+        filters: { arr_has: [], arr_lacks: [], stem_has: [], stem_lacks: [], lyrics: '', tunings: [], mastery: [], match: [], genre: [], tuningMatch: 'exact' },
         page: 0, total: 0, loading: false, built: false, accuracy: {}, tuningNames: [], genres: [],
         artistCatalog: [], renderedHash: '',
         scrollBound: false,
@@ -120,7 +120,7 @@
     function activeFilterCount() {
         const f = state.filters;
         return f.arr_has.length + f.arr_lacks.length + f.stem_has.length + f.stem_lacks.length +
-            (f.lyrics ? 1 : 0) + f.tunings.length + (f.mastery ? f.mastery.length : 0) +
+            (f.lyrics ? 1 : 0) + (f.tuningMatch === 'playable' ? 1 : f.tunings.length) + (f.mastery ? f.mastery.length : 0) +
             (f.match ? f.match.length : 0) + (f.genre ? f.genre.length : 0) +
             (state.artist ? 1 : 0) + (state.album ? 1 : 0);
     }
@@ -280,7 +280,15 @@
         if (f.stem_has.length) p.set('stems_has', f.stem_has.join(','));
         if (f.stem_lacks.length) p.set('stems_lacks', f.stem_lacks.join(','));
         if (f.lyrics) p.set('has_lyrics', f.lyrics);
-        if (f.tunings.length) p.set('tunings', f.tunings.join(','));
+        // The two modes answer different questions, so only one filters at a
+        // time: sending both would silently intersect them.
+        if (f.tunings.length && f.tuningMatch !== 'playable') p.set('tunings', f.tunings.join(','));
+        // Which perspective the `tunings` filter + the tuning sort read.
+        if (libInstrument() !== 'guitar-lead') p.set('instrument', libInstrument());
+        // "Playable without retuning": send the player's LIVE tuning and let the
+        // server do the pitch maths (the pitch tables live in lib/tunings.py —
+        // duplicating them here is how the two drift apart).
+        applyPlayableParams(p, f);
         if (f.mastery && f.mastery.length) p.set('mastery', f.mastery.join(','));
         if (f.match && f.match.length) p.set('match', f.match.join(','));
         if (f.genre && f.genre.length) p.set('genre', f.genre.join(','));
@@ -786,6 +794,10 @@
     // so without them the chips render exactly as before. Decoration runs AFTER the
     // (sync) window paint so scrolling stays snappy; a token cancels a superseded pass.
     let _tuningDecorToken = 0;
+    // The instrument the current grid was queried/painted for, so a
+    // working-tuning change can tell a guitar<->bass SWITCH (re-query) from a
+    // retune within the same instrument (re-colour only).
+    let _lastRenderInstrument = null;
     function _applyChipMatch(chip, stateName) {
         chip.classList.remove('bg-fb-mid', 'bg-emerald-500', 'bg-amber-400');
         chip.classList.add(stateName === 'match' ? 'bg-emerald-500'
@@ -825,22 +837,31 @@
         const shown = song.display_chart ? Object.assign({}, song, song.display_chart) : song;
         // In select mode the checkbox occupies top-2 left-2, so shift the
         // tuning chip right (left-9) to avoid overlapping it.
+        // Bass players see the bass chart's tuning (guitar fallback) — the card
+        // must agree with the facet/filter or the grid contradicts the pills.
+        const shownTuning = shownTuningName(shown);
         const tuningLabel = (typeof window.displayTuningName === 'function')
-            ? window.displayTuningName(shown.tuning_name || shown.tuning)
-            : (shown.tuning_name || '');
+            ? window.displayTuningName(shownTuning)
+            : (shownTuning || '');
         let tuning = '';
         if (tuningLabel) {
             const rawOffsets = (typeof window.parseRawTuningOffsets === 'function')
-                ? (window.parseRawTuningOffsets(shown.tuning_offsets)
-                    || window.parseRawTuningOffsets(shown.tuning_name || shown.tuning))
+                ? (window.parseRawTuningOffsets(shownTuningOffsets(shown))
+                    || window.parseRawTuningOffsets(shownTuning))
                 : null;
             const targetNotes = (tuningLabel === 'Custom Tuning' && rawOffsets
                 && typeof window.displayTuningTargets === 'function')
                 ? window.displayTuningTargets(rawOffsets, { tuningName: tuningLabel })
                 : '';
-            const badgeTitle = targetNotes
+            // Mark a tuning we INFERRED from the guitar chart (this song has no
+            // bass arrangement) so a bass player isn't shown a borrowed tuning
+            // as if it were their part's. `~` keeps the chip compact; the title
+            // spells it out.
+            const inferred = shown.tuning_inferred === true;
+            const badgeTitle = (targetNotes
                 ? ('Custom Tuning: ' + targetNotes)
-                : tuningLabel;
+                : tuningLabel)
+                + (inferred ? ' — from the guitar chart (no bass arrangement)' : '');
             const pos = 'absolute top-2 ' + (state.selectMode ? 'left-9' : 'left-2');
             // Tag the chip with its offsets so decorateTuningChips() can colour it
             // green (matches your current tuning) / amber (needs a retune) after paint.
@@ -848,8 +869,14 @@
             // scores its bass tuning against the bass base pitches, not guitar — otherwise
             // a 4-string bass tuning read as guitar can false-match a guitar player.
             const chipArrs = shown.arrangements || [];
-            const chipIsBass = chipArrs.length > 0
-                && chipArrs.every((a) => /\bbass\b/i.test((a && a.name) || ''));
+            // Bass either because the chip is SHOWING the bass chart's tuning
+            // (a bass player on a song that has one), or because every
+            // arrangement is a bass part. Checked via libInstrument() rather
+            // than comparing the two names — they are EQUAL for most songs, so
+            // a value comparison would flag a guitarist's chip as bass.
+            const chipIsBass = (libInstrument() === 'bass' && !!shown.bass_tuning_name)
+                || (chipArrs.length > 0
+                    && chipArrs.every((a) => /\bbass\b/i.test((a && a.name) || '')));
             const matchAttr = (rawOffsets && rawOffsets.length)
                 ? ' data-tuning-chip data-tuning-offsets="' + esc(rawOffsets.join(',')) + '"'
                     + (chipIsBass ? ' data-tuning-bass="1"' : '') : '';
@@ -857,7 +884,7 @@
                 tuning = '<span class="' + pos + ' bg-fb-mid text-black text-[0.5625rem] font-bold px-1.5 py-0.5 rounded-sm leading-tight max-w-[5.5rem] text-center"' + matchAttr + ' title="' + esc(badgeTitle) + '">'
                     + esc('Custom Tuning') + '<br><span class="font-semibold tracking-wide">' + esc(targetNotes) + '</span></span>';
             } else {
-                tuning = '<span class="' + pos + ' bg-fb-mid text-black text-[0.625rem] font-bold px-1.5 py-0.5 rounded-sm"' + matchAttr + ' title="' + esc(badgeTitle) + '">' + esc(tuningLabel) + '</span>';
+                tuning = '<span class="' + pos + ' bg-fb-mid text-black text-[0.625rem] font-bold px-1.5 py-0.5 rounded-sm"' + matchAttr + ' title="' + esc(badgeTitle) + '">' + esc(tuningLabel) + (inferred ? '<span class="opacity-60"> ~</span>' : '') + '</span>';
             }
         }
         // Display-only (pointer-events-none) so a click falls through to the
@@ -2489,6 +2516,89 @@
 
     function _artistHostEl() { return document.getElementById('v3-songs-artistpage'); }
 
+    // ── Instrument-aware tuning (the bass-player tuning-filter report) ────────
+    // A song's bass chart is often in a different tuning from its guitar chart,
+    // so the tuning facet/filter/sort and the card chip must speak for the
+    // instrument the player actually plays. The host's working-tuning
+    // capability already holds the live selection (seeded from /api/settings on
+    // boot, updated when the player switches) — read it rather than adding
+    // another settings fetch. `state.settingsInstrument` is the fallback for
+    // hosts where the capability isn't mounted.
+    // Three perspectives, matching `active_instrument_profile`: lead and rhythm
+    // guitar charts can be tuned differently too, so a rhythm player hits the
+    // same bug a bassist did. The PROFILE is the only three-valued source (the
+    // working-tuning capability knows guitar-vs-bass but not lead-vs-rhythm),
+    // so it wins; the capability is the live fallback for hosts where the
+    // profile hasn't loaded.
+    const PERSPECTIVES = ['guitar-lead', 'guitar-rhythm', 'bass'];
+    function libInstrument() {
+        if (PERSPECTIVES.indexOf(state.settingsProfile) >= 0) return state.settingsProfile;
+        try {
+            const wt = window.feedBack && window.feedBack.workingTuning;
+            if (wt && typeof wt.get === 'function') {
+                const cur = wt.get();
+                if (cur && cur.instrument === 'bass') return 'bass';
+            }
+        } catch (_) { /* capability absent/erroring — fall through to settings */ }
+        return state.settingsInstrument === 'bass' ? 'bass' : 'guitar-lead';
+    }
+
+    // "Playable without retuning" mode reads the player's CURRENT tuning from
+    // the working-tuning capability (the live session state the tuner writes),
+    // not a separate setting. No capability => we cannot know the current
+    // tuning, so the mode is unavailable rather than guessed.
+    function currentWorkingTuning() {
+        try {
+            const wt = window.feedBack && window.feedBack.workingTuning;
+            if (!wt || typeof wt.get !== 'function') return null;
+            const cur = wt.get();
+            if (!cur || !Array.isArray(cur.offsets) || !cur.offsets.length) return null;
+            return cur;
+        } catch (_) { return null; }
+    }
+
+    function playableAvailable() { return !!currentWorkingTuning(); }
+
+    function applyPlayableParams(p, f) {
+        if (f.tuningMatch !== 'playable') return;
+        const cur = currentWorkingTuning();
+        if (!cur) return;
+        p.set('tuning_match', 'playable');
+        p.set('playable_offsets', cur.offsets.join(','));
+        p.set('playable_instrument', cur.instrument === 'bass' ? 'bass' : 'guitar');
+        p.set('playable_string_count', String(cur.stringCount || cur.offsets.length));
+    }
+
+    // Short human label for the perspective, for the facet/sort headers.
+    function libInstrumentLabel() {
+        const p = libInstrument();
+        return p === 'bass' ? 'bass' : p === 'guitar-rhythm' ? 'rhythm' : 'lead';
+    }
+
+    // The column a row's tuning lives in for the active perspective.
+    function perspectiveTuningField() {
+        const p = libInstrument();
+        return p === 'bass' ? 'bass_tuning_name'
+            : p === 'guitar-rhythm' ? 'rhythm_tuning_name' : '';
+    }
+
+    // The tuning a card should SHOW: bass players see the bass chart's tuning,
+    // falling back to the song (guitar-derived) tuning when the song has no
+    // bass arrangement — the common case, so the fallback is not an edge path.
+    function shownTuningName(song) {
+        const f = perspectiveTuningField();
+        if (f && song[f]) return song[f];
+        return song.tuning_name || song.tuning;
+    }
+
+    function shownTuningOffsets(song) {
+        const f = perspectiveTuningField();
+        if (f && song[f]) {
+            return song[f.replace('_name', '_offsets')] || song.tuning_offsets;
+        }
+        return song.tuning_offsets;
+    }
+
     // Sync the two Settings gates into module state (fire-and-forget — the
     // cached flags gate entry-point rendering; openArtistPage re-checks).
     function refreshArtistPageGates() {
@@ -2496,6 +2606,10 @@
             if (!cfg) return;
             state.artistPagesEnabled = cfg.artist_pages_enabled !== false;
             state.artistLinksEnabled = cfg.artist_external_links === true;
+            // Fallback instrument for hosts without the working-tuning capability.
+            state.settingsInstrument = cfg.instrument === 'bass' ? 'bass' : 'guitar';
+            // The three-valued perspective source (lead / rhythm / bass).
+            state.settingsProfile = cfg.active_instrument_profile || '';
         });
     }
 
@@ -2809,12 +2923,13 @@
         else if (s === 'has') lacksArr.push(value);
         // 'lacks' → cycles back to any (already removed)
     }
-    function triPill(group, value, label, st) {
+    function triPill(group, value, label, st, title) {
         const cls = st === 'has' ? 'bg-fb-good/30 text-fb-good border-fb-good/40'
             : st === 'lacks' ? 'bg-fb-low/30 text-fb-low border-fb-low/40'
                 : 'bg-gray-800/50 text-fb-textDim border-gray-700';
         const mark = st === 'has' ? '✓ ' : st === 'lacks' ? '✕ ' : '';
-        return '<button data-tri="' + group + '" data-val="' + esc(value) + '" class="px-2 py-1 rounded-md text-xs border ' + cls + '">' + mark + esc(label) + '</button>';
+        const tip = title ? ' title="' + esc(title) + '"' : '';
+        return '<button data-tri="' + group + '" data-val="' + esc(value) + '" class="px-2 py-1 rounded-md text-xs border ' + cls + '"' + tip + '>' + mark + esc(label) + '</button>';
     }
     function renderDrawer() {
         const d = document.getElementById('v3-songs-drawer');
@@ -2834,7 +2949,32 @@
             section('Match', [['review', 'To review'], ['matched', 'Matched'], ['unmatched', 'Unmatched'], ['pending', 'Not scanned']].map((it) => '<button data-match="' + it[0] + '" class="px-2 py-1 rounded-md text-xs border ' + (f.match.includes(it[0]) ? 'bg-fb-primary text-white border-fb-primary' : 'bg-gray-800/50 text-fb-textDim border-gray-700') + '">' + it[1] + '</button>').join('')) +
             // Genre facet — dynamic list from /api/library/genres (primary genre).
             (state.genres && state.genres.length ? section('Genre', state.genres.map((g) => '<button data-genre="' + esc(g) + '" class="px-2 py-1 rounded-md text-xs border ' + (f.genre.includes(g) ? 'bg-fb-primary text-white border-fb-primary' : 'bg-gray-800/50 text-fb-textDim border-gray-700') + '">' + esc(g) + '</button>').join('')) : '') +
-            section('Tuning', (state.tuningNames || []).map((t) => {
+            // The facet header NAMES the perspective. Silent instrument-following
+            // is the original bug in a new place: the user must be able to tell
+            // which instrument these tunings describe.
+            section('Tuning (' + libInstrumentLabel() + ')',
+                // MODE toggle. Exact match answers "which tuning is this
+                // labelled"; Playable answers "will this cost me a retune" —
+                // which is what a player actually wants. Both are offered;
+                // exact stays the default so nothing changes unasked.
+                '<div class="flex gap-1 mb-2">'
+                + [['exact', 'Exact tuning'], ['playable', 'Playable without retuning']].map((m) => {
+                    const on = (f.tuningMatch || 'exact') === m[0];
+                    const dis = m[0] === 'playable' && !playableAvailable();
+                    return '<button data-tuning-match="' + m[0] + '"'
+                        + (dis ? ' disabled' : '')
+                        + (dis ? ' title="Needs your current tuning — open the tuner first"' : '')
+                        + ' class="px-2 py-1 rounded-md text-xs border '
+                        + (on ? 'bg-fb-primary text-white border-fb-primary'
+                            : 'bg-gray-800/50 text-fb-textDim border-gray-700')
+                        + (dis ? ' opacity-40 cursor-not-allowed' : '') + '">'
+                        + esc(m[1]) + '</button>';
+                }).join('')
+                + '</div>'
+                + (f.tuningMatch === 'playable'
+                    ? '<div class="text-xs text-fb-textDim mb-2">Charts you can play in your current tuning, no retune. Songs whose lowest string sits below yours are excluded.</div>'
+                    : '')
+                + ((state.tuningNames || []).map((t) => {
                 // Filter on the server's grouping key (raw offsets for customs)
                 // so two "Custom Tuning" entries are distinct; show their target
                 // notes in the label so they're distinguishable.
@@ -2847,8 +2987,17 @@
                     const notes = offs ? window.displayTuningTargets(offs, { tuningName: t.name }) : '';
                     if (notes) label = 'Custom · ' + notes;
                 }
-                return triPill('tuning', val, label + ' (' + t.count + ')', f.tunings.includes(val) ? 'has' : 'any');
-            }).join('') || '<span class="text-xs text-fb-textDim">No tunings</span>') +
+                // Be honest about the fallback: when some of a row's songs have
+                // no bass chart and are borrowing the guitar tuning, say so
+                // rather than presenting a borrowed tuning as a measured one.
+                const inf = t.inferred_count || 0;
+                const title = inf
+                    ? inf + ' of ' + t.count + ' inferred from the guitar chart (no bass arrangement)'
+                    : '';
+                const countLabel = inf ? t.count + ', ' + inf + ' inferred' : String(t.count);
+                return triPill('tuning', val, label + ' (' + countLabel + ')',
+                    f.tunings.includes(val) ? 'has' : 'any', title);
+            }).join('') || '<span class="text-xs text-fb-textDim">No tunings</span>')) +
             // Multi-chart grouping toggle (P5e) — a VIEW mode, not a filter
             // (never counted in the badge, never saved into collection rules).
             // Local provider only: it's the one that implements group=.
@@ -2878,6 +3027,11 @@
             else if (g === 'tuning') { const i = f.tunings.indexOf(v); if (i >= 0) f.tunings.splice(i, 1); else f.tunings.push(v); }
             renderDrawer();
         }));
+        d.querySelectorAll('[data-tuning-match]').forEach((b) => b.addEventListener('click', () => {
+            if (b.disabled) return;
+            f.tuningMatch = b.getAttribute('data-tuning-match');
+            renderDrawer();
+        }));
         d.querySelectorAll('[data-lyrics]').forEach((b) => b.addEventListener('click', () => { f.lyrics = b.getAttribute('data-lyrics'); renderDrawer(); }));
         d.querySelectorAll('[data-mastery]').forEach((b) => b.addEventListener('click', () => { const v = b.getAttribute('data-mastery'); const i = f.mastery.indexOf(v); if (i >= 0) f.mastery.splice(i, 1); else f.mastery.push(v); renderDrawer(); }));
         d.querySelector('[data-grouping]')?.addEventListener('click', () => {
@@ -2891,7 +3045,7 @@
         d.querySelector('[data-drawer-tidy]')?.addEventListener('click', openArtistTidyUp);
         d.querySelector('[data-drawer-close]')?.addEventListener('click', closeDrawer);
         d.querySelector('[data-drawer-clear]')?.addEventListener('click', async () => {
-            state.filters = { arr_has: [], arr_lacks: [], stem_has: [], stem_lacks: [], lyrics: '', tunings: [], mastery: [], match: [], genre: [] };
+            state.filters = { arr_has: [], arr_lacks: [], stem_has: [], stem_lacks: [], lyrics: '', tunings: [], mastery: [], match: [], genre: [], tuningMatch: 'exact' };
             state.artist = '';
             state.album = '';
             renderDrawer();
@@ -3478,13 +3632,15 @@
         const providers = await loadProviders();
         const [, tn] = await Promise.all([
             (async () => { state.accuracy = (await jget('/api/stats/best')) || {}; })(),
-            jget('/api/library/tuning-names?provider=' + enc(state.provider)),
+            jget('/api/library/tuning-names?provider=' + enc(state.provider)
+                + '&instrument=' + enc(libInstrument())),
             loadArtistCatalog(),
             // Artist-page gates (PR-B) ride the initial fetch batch so the
             // first card paint already knows whether artist lines are links.
             refreshArtistPageGates(),
         ]);
         state.tuningNames = (tn && tn.tunings) || [];
+        _lastRenderInstrument = libInstrument();
         try { const _g = await jget('/api/library/genres?provider=' + enc(state.provider)); state.genres = (_g && _g.genres) || []; } catch (e) { state.genres = []; }
 
         const opt = (arr, sel) => arr.map(([v, l]) => '<option value="' + esc(v) + '"' + (v === sel ? ' selected' : '') + '>' + esc(l) + '</option>').join('');
@@ -3512,7 +3668,13 @@
             '<select id="v3-songs-artist" class="' + ctrl + ' max-w-[11rem]" aria-label="Artist">' + artistSelectHtml() + '</select>' +
             '<select id="v3-songs-album" class="' + ctrl + ' max-w-[11rem]" aria-label="Album"' + (state.artist ? '' : ' disabled') + '>' + albumSelectHtml() + '</select>' +
             '<div class="flex rounded-md overflow-hidden border border-gray-700"><button id="v3-songs-grid-btn" class="px-3 py-2 text-sm">▦</button><button id="v3-songs-tree-btn" class="px-3 py-2 text-sm">≣</button><button id="v3-songs-albums-btn" title="Albums" class="px-3 py-2 text-sm">💿</button><button id="v3-songs-folder-btn" class="px-3 py-2 text-sm" style="display:inline-flex;align-items:center;justify-content:center;box-sizing:border-box;width:2.25rem"><svg fill="currentColor" viewBox="0 0 16 16" style="width:12px;height:12px;flex-shrink:0"><path d="M1 3.5A1.5 1.5 0 012.5 2h3.086a1.5 1.5 0 011.06.44l.915.914H13.5A1.5 1.5 0 0115 4.914V12.5a1.5 1.5 0 01-1.5 1.5h-11A1.5 1.5 0 011 12.5v-9z"/></svg></button></div>' +
-            '<select id="v3-songs-sort" class="' + ctrl + '">' + opt(SORTS, state.sort) + '</select>' +
+            // Name the perspective on the SORT too, not just the filter: tuning
+            // sort orders by musical distance from standard, and for a bass
+            // player that distance is measured on the bass tuning. Unlabelled,
+            // the grid silently reorders with no visible cause.
+            '<select id="v3-songs-sort" class="' + ctrl + '">' + opt(
+                SORTS.map(([v, l]) => [v, v === 'tuning' ? l + ' (' + libInstrumentLabel() + ')' : l]),
+                state.sort) + '</select>' +
             '<select id="v3-songs-format" class="' + ctrl + '">' + opt(FORMATS, state.format) + '</select>' +
             '<button id="v3-songs-filters" class="relative ' + ctrl + ' flex items-center gap-2">Filters<span id="v3-songs-filter-count" class="hidden bg-fb-primary text-white text-xs rounded-full px-1.5">0</span></button>' +
             '<button id="v3-songs-select" class="' + ctrl + (state.selectMode ? ' bg-fb-primary text-white' : '') + '">Select</button>' +
@@ -4130,6 +4292,23 @@
         // visible tuning chips against the new tuning. Cheap: re-decorates in place,
         // no re-fetch or re-paint. No-op off the Songs grid or without the capability.
         sm.on('working-tuning-changed', () => {
+            // A guitar<->bass SWITCH changes which tuning the facet, the filter,
+            // the sort and the card chip speak for, so the grid must re-query —
+            // re-colouring chips would leave the guitar tuning on screen and a
+            // guitar-keyed filter applied. A retune within one instrument still
+            // takes the cheap in-place path below.
+            const inst = libInstrument();
+            if (inst !== _lastRenderInstrument) {
+                _lastRenderInstrument = inst;
+                // A tuning selection keyed to the old instrument means nothing
+                // for the new one; clearing avoids an empty grid the user can't
+                // explain (the pills are re-rendered from the new facet).
+                state.filters.tunings = [];
+                const active = document.querySelector('.screen.active');
+                if (active && active.id === 'v3-songs') reload();
+                else _libraryDirty = true;
+                return;
+            }
             if (typeof songsActive === 'function' && !songsActive()) return;
             if (state.view !== 'grid') return;
             decorateTuningChips(_gridEl());

tests/js/v3_songs_tuning.test.js

@@ -67,11 +67,27 @@ test('v3 songs.js uses display helpers for album-art tuning badge', () => {
     const src = fs.readFileSync(SONGS_JS, 'utf8');
     // The card renderer's row variable was renamed song → shown when grouped
     // cards landed (the badge reads the representative chart); accept either.
-    assert.match(src, /displayTuningName\((?:song|shown)\.tuning_name \|\| (?:song|shown)\.tuning\)/);
+    // The raw read then moved behind shownTuningName() so the badge can answer
+    // for the active tuning perspective — accept that indirection too, and pin
+    // the fallback inside the helper below so this stays a real guard.
+    assert.match(
+        src,
+        /displayTuningName\((?:(?:song|shown)\.tuning_name \|\| (?:song|shown)\.tuning|shownTuning)\)/,
+    );
     assert.match(src, /displayTuningTargets/);
     assert.match(src, /parseRawTuningOffsets/);
 });
 
+test('the tuning-perspective helper still falls back to tuning_name || tuning', () => {
+    // shownTuningName() is what the badge now reads. With no perspective field
+    // set (guitar-lead, the default) it must resolve exactly what the badge
+    // used to read inline, or guitar players silently lose their tuning label.
+    const src = fs.readFileSync(SONGS_JS, 'utf8');
+    const body = src.match(/function shownTuningName\(song\)\s*\{[\s\S]*?\n {4}\}/);
+    assert.ok(body, 'shownTuningName() not found — the badge read moved again');
+    assert.match(body[0], /return song\.tuning_name \|\| song\.tuning;/);
+});
+
 test('raw offset tuning_name does not appear in rendered card HTML', () => {
     const html = renderSongCardBadge({ tuning_name: '-2 0 0 0 -2' }, helpers);
     assert.doesNotMatch(html, /-2 0 0 0 -2/);

tests/test_library_tuning_instrument.py

@@ -0,0 +1,721 @@
+"""Instrument-aware tuning in the library (the KwasimodoZAZA bass report).
+
+A song's BASS chart is often tuned differently from its guitar chart, but the
+library indexed exactly one guitar-first tuning per song — so a bass player
+filtering "Drop D" got songs whose GUITAR is in Drop D, and playlists built
+that way were wrong.
+
+These tests round-trip through the real extractors, the real scanner
+derivation, the real SQLite schema/migration, and the real HTTP surface. The
+only thing stubbed is metadata EXTRACTION in the scan tests (the production
+process pool can't reach an in-process mock) — never the code under test.
+
+Real-library notes, all confirmed against actual pack contents:
+
+* Bass arrangements usually store SIX-element offset arrays even when the
+  chart is a 4-string part — slots 4-5 are PADDING (no bass chart in the
+  corpus references string index 4 or 5). So bass offsets are truncated to 4
+  before naming or grouping. The feedpak spec has no string-count field, so 4
+  is a documented default, not a read value.
+* AC/DC "Girls Got Rhythm" stores [5,5,5,5,4,4] — every string up a fourth,
+  which no bassist plays. That is BAD DATA, and it must never be NAMED, or the
+  library sends a player to retune to a tuning that does not exist.
+* Covet "Shibuya" (custom guitar tuning, dead-standard bass) is the headline
+  regression: the tester's bug in a single song.
+"""
+
+import importlib
+import json
+import sys
+
+import pytest
+import yaml
+from fastapi.testclient import TestClient
+
+import sloppak as sloppak_mod
+from scan_worker import _extract_meta_for_file
+from tunings import (
+    PERSPECTIVES, bass_offsets_are_plausible, bass_tuning_key, bass_tuning_name,
+    chart_is_playable_in, normalize_bass_offsets, perspective_tuning_key,
+    tuning_name,
+)
+
+
+# ── Fixtures ─────────────────────────────────────────────────────────────────
+
+@pytest.fixture()
+def server_mod(tmp_path, monkeypatch):
+    monkeypatch.setenv("CONFIG_DIR", str(tmp_path))
+    sys.modules.pop("server", None)
+    mod = importlib.import_module("server")
+    yield mod
+    conn = getattr(getattr(mod, "meta_db", None), "conn", None)
+    if conn is not None:
+        getattr(sys.modules.get("server"), "_join_background_db_threads", lambda: None)()
+        conn.close()
+
+
+@pytest.fixture()
+def client(server_mod):
+    c = TestClient(server_mod.app)
+    try:
+        yield c
+    finally:
+        c.close()
+
+
+def _pack(root, name, arrangements):
+    """A directory-form pack whose manifest carries per-arrangement tunings."""
+    d = root / name
+    d.mkdir(parents=True)
+    (d / "manifest.yaml").write_text(yaml.safe_dump({
+        "title": name, "artist": "A", "duration": 100,
+        "arrangements": arrangements, "stems": [],
+    }), encoding="utf-8")
+    return d
+
+
+def _put(server_mod, *, filename, title, tuning_name_="E Standard",
+         tuning_sort_key=0, tuning_offsets="0 0 0 0 0 0",
+         bass_tuning_name="", bass_tuning_sort_key=0, bass_tuning_offsets="",
+         bass_tuning_key=""):
+    server_mod.meta_db.put(filename, 1.0, 1, {
+        "title": title, "artist": "A", "album": "A - LP", "year": "2010",
+        "duration": 200.0, "tuning": tuning_name_, "arrangements": [],
+        "has_lyrics": False, "format": "sloppak", "stem_ids": [],
+        "tuning_name": tuning_name_,
+        "tuning_sort_key": tuning_sort_key,
+        "tuning_offsets": tuning_offsets,
+        "bass_tuning_name": bass_tuning_name,
+        "bass_tuning_sort_key": bass_tuning_sort_key,
+        "bass_tuning_offsets": bass_tuning_offsets,
+        "bass_tuning_key": bass_tuning_key,
+    })
+
+
+# ── 1. Extraction: sloppak ───────────────────────────────────────────────────
+
+def test_sloppak_extract_indexes_both_tunings_when_they_differ(tmp_path):
+    """The reported case: guitar down a step, bass in standard. BOTH must be
+    indexed — previously only the guitar tuning survived."""
+    d = _pack(tmp_path, "differ.sloppak", [
+        {"name": "Lead", "tuning": [-2, 0, 0, -1, -2, 0]},
+        {"name": "Bass", "tuning": [0, 0, 0, 0, 0, 0]},
+    ])
+    meta = sloppak_mod.extract_meta(d)
+    assert meta["tuning_offsets"] == [-2, 0, 0, -1, -2, 0]
+    assert meta["bass_tuning_offsets"] == [0, 0, 0, 0, 0, 0]
+
+
+def test_sloppak_extract_leaves_bass_absent_without_bass_arrangement(tmp_path):
+    """No bass chart → None, NOT a copy of the guitar tuning. The library
+    falls back explicitly, so 'no bass part' stays distinguishable."""
+    d = _pack(tmp_path, "nobass.sloppak", [
+        {"name": "Lead", "tuning": [-2, -2, -2, -2, -2, -2]},
+        {"name": "Rhythm", "tuning": [-2, -2, -2, -2, -2, -2]},
+    ])
+    meta = sloppak_mod.extract_meta(d)
+    assert meta["tuning_offsets"] == [-2, -2, -2, -2, -2, -2]
+    assert meta["bass_tuning_offsets"] is None
+
+
+def test_sloppak_extract_bass_wins_over_guitar_first_ordering(tmp_path):
+    """The bass entry is listed FIRST in the manifest; the song tuning must
+    still be the guitar's while the bass column takes the bass entry — the two
+    selections are independent, not 'first wins'."""
+    d = _pack(tmp_path, "order.sloppak", [
+        {"name": "Bass", "tuning": [-4, -4, -4, -4, -4, -4]},
+        {"name": "Lead", "tuning": [0, 0, 0, 0, 0, 0]},
+    ])
+    meta = sloppak_mod.extract_meta(d)
+    assert meta["tuning_offsets"] == [0, 0, 0, 0, 0, 0]
+    assert meta["bass_tuning_offsets"] == [-4, -4, -4, -4, -4, -4]
+
+
+def test_sloppak_extract_ignores_bass_arrangement_without_a_tuning(tmp_path):
+    """A bass chart that authors no tuning gives us nothing to index; the
+    column stays empty rather than defaulting to a wrong all-zeros."""
+    d = _pack(tmp_path, "untuned.sloppak", [
+        {"name": "Lead", "tuning": [-2, -2, -2, -2, -2, -2]},
+        {"name": "Bass"},
+    ])
+    assert sloppak_mod.extract_meta(d)["bass_tuning_offsets"] is None
+
+
+def test_sloppak_extract_falls_back_to_an_alt_bass_chart(tmp_path):
+    """Only a "Bass 2" chart exists. Using it beats reporting the guitar
+    tuning as the player's bass tuning."""
+    d = _pack(tmp_path, "altbass.sloppak", [
+        {"name": "Lead", "tuning": [0, 0, 0, 0, 0, 0]},
+        {"name": "Bass 2", "tuning": [-2, 0, 0, 0, 0, 0]},
+    ])
+    assert sloppak_mod.extract_meta(d)["bass_tuning_offsets"] == [-2, 0, 0, 0, 0, 0]
+
+
+# ── 2. Scanner derivation (name / sort key / offsets string) ─────────────────
+
+def test_scan_worker_derives_bass_columns_like_the_guitar_ones(tmp_path):
+    """Guitar columns keep all six strings; bass columns are TRUNCATED to the
+    bass's four (the stored tail is padding — see tunings.normalize_bass_offsets)."""
+    d = _pack(tmp_path, "derive.sloppak", [
+        {"name": "Lead", "tuning": [-2, -2, -2, -2, -2, -2]},
+        {"name": "Bass", "tuning": [0, 0, 0, 0, 0, 0]},
+    ])
+    meta = _extract_meta_for_file(d)
+    assert meta["tuning_name"] == "D Standard"
+    assert meta["tuning_sort_key"] == -12
+    assert meta["tuning_offsets"] == "-2 -2 -2 -2 -2 -2"
+    assert meta["bass_tuning_name"] == "E Standard"
+    assert meta["bass_tuning_sort_key"] == 0
+    assert meta["bass_tuning_offsets"] == "0 0 0 0"
+    # Canonical key = absolute open pitches of a 4-string bass in standard.
+    assert meta["bass_tuning_key"] == "bass:28:33:38:43"
+
+
+def test_bass_padding_is_truncated_before_naming_and_grouping(tmp_path):
+    """The padded tail must never reach the namer or the group key: a bass
+    stored six-wide and the same tuning stored four-wide must produce
+    IDENTICAL indexed columns."""
+    six = _extract_meta_for_file(_pack(tmp_path, "six.sloppak", [
+        {"name": "Bass", "tuning": [-2, 0, 0, 0, 0, 0]}]))
+    four = _extract_meta_for_file(_pack(tmp_path, "four.sloppak", [
+        {"name": "Bass", "tuning": [-2, 0, 0, 0]}]))
+    for col in ("bass_tuning_name", "bass_tuning_offsets",
+                "bass_tuning_sort_key", "bass_tuning_key"):
+        assert six[col] == four[col], col
+    assert six["bass_tuning_name"] == "Drop D"
+
+
+def test_scan_worker_bass_columns_empty_without_a_bass_arrangement(tmp_path):
+    """Empty string, never None: '' is the indexed 'we looked, no bass chart'
+    state, while NULL means 'never extracted' and triggers a re-scan."""
+    d = _pack(tmp_path, "nobass2.sloppak", [{"name": "Lead", "tuning": [0] * 6}])
+    meta = _extract_meta_for_file(d)
+    assert meta["bass_tuning_name"] == ""
+    assert meta["bass_tuning_sort_key"] == 0
+    assert meta["bass_tuning_offsets"] == ""
+
+
+def test_implausible_bass_tuning_is_never_named(tmp_path):
+    """Real library data, and it is BAD DATA: AC/DC "Girls Got Rhythm" stores
+    a bass tuning of [5,5,5,5,4,4] — every string up a perfect fourth, which
+    no bassist plays (roughly double string tension), on a song whose guitar
+    chart is dead standard.
+
+    Truncation alone would leave [5,5,5,5] = "all strings up a 4th", which the
+    namer WOULD happily name. Naming it would send a player off to retune to a
+    tuning that does not exist, so the plausibility guard must refuse: bassists
+    tune down, essentially never up."""
+    d = _pack(tmp_path, "weird.sloppak", [
+        {"name": "Lead", "tuning": [0, 0, 0, 0, 0, 0]},
+        {"name": "Bass", "tuning": [5, 5, 5, 5, 4, 4]},
+    ])
+    meta = _extract_meta_for_file(d)
+    assert meta["bass_tuning_name"] == "Custom Tuning"
+    assert meta["bass_tuning_offsets"] == "5 5 5 5"
+    assert meta["bass_tuning_sort_key"] == 20
+
+
+@pytest.mark.parametrize("offsets", [
+    [5, 5, 5, 5], [5, 5, 5, 5, 4, 4], [2, 2, 2, 2], [12, 12, 12, 12],
+])
+def test_up_tuned_bass_offsets_are_refused_by_the_guard(offsets):
+    """Anything above +1 semitone is data we do not trust. Note the namer
+    ALONE would name several of these ([2,2,2,2] -> "F# Standard"), which is
+    exactly the retune-to-nowhere the guard exists to prevent."""
+    norm = normalize_bass_offsets(offsets)
+    assert bass_offsets_are_plausible(norm) is False
+    assert bass_tuning_name(norm) == "Custom Tuning"
+
+
+@pytest.mark.parametrize("offsets,expected", [
+    ([0, 0, 0, 0], "E Standard"),        # standard
+    ([-1, -1, -1, -1], "Eb Standard"),   # down a semitone
+    ([-2, 0, 0, 0], "Drop D"),           # drop
+    ([1, 1, 1, 1], "F Standard"),        # +1 is the plausible ceiling, still named
+])
+def test_plausible_bass_tunings_are_still_named(offsets, expected):
+    """The guard must not over-fire: real down-tunings, standard, and the +1
+    ceiling all keep their names."""
+    assert bass_tuning_name(offsets) == expected
+
+
+# ── 3. Storage round-trip + the pre-migration re-extract marker ──────────────
+
+def test_put_get_round_trips_the_bass_columns(server_mod):
+    _put(server_mod, filename="rt.sloppak", title="RT",
+         tuning_name_="D Standard", tuning_sort_key=-12,
+         tuning_offsets="-2 -2 -2 -2 -2 -2",
+         bass_tuning_name="E Standard", bass_tuning_offsets="0 0 0 0 0 0")
+    got = server_mod.meta_db.get("rt.sloppak", 1.0, 1)
+    assert got["tuning_name"] == "D Standard"
+    assert got["bass_tuning_name"] == "E Standard"
+    assert got["bass_tuning_offsets"] == "0 0 0 0 0 0"
+
+
+def test_put_never_writes_null_bass_columns(server_mod):
+    """A freshly-scanned row is by definition extracted, so even a song with
+    no bass chart stores '' — otherwise it would look pre-migration forever
+    and the scanner would re-extract it on every single pass."""
+    _put(server_mod, filename="fresh.sloppak", title="Fresh")
+    row = server_mod.meta_db.conn.execute(
+        "SELECT bass_tuning_name FROM songs WHERE filename = 'fresh.sloppak'").fetchone()
+    assert row[0] == ""
+    assert server_mod.meta_db.get("fresh.sloppak", 1.0, 1)["bass_tuning_name"] == ""
+
+
+def test_pre_migration_row_reads_back_as_null(server_mod):
+    """A row written before the columns existed (simulated with raw SQL that
+    omits them) reads back None — the marker the scanner keys its re-extract
+    on. If this ever became '' the backfill would silently never run."""
+    server_mod.meta_db.conn.execute(
+        "INSERT INTO songs (filename, mtime, size, title, artist, album, year, "
+        "duration, tuning, arrangements, has_lyrics, format, stem_count, "
+        "stem_ids, tuning_name, tuning_sort_key, tuning_offsets) "
+        "VALUES ('old.sloppak', 1.0, 1, 'Old', 'A', 'A - LP', '2010', 200.0, "
+        "'E Standard', '[]', 0, 'sloppak', 0, '[]', 'E Standard', 0, '0 0 0 0 0 0')")
+    server_mod.meta_db.conn.commit()
+    got = server_mod.meta_db.get("old.sloppak", 1.0, 1)
+    assert got["bass_tuning_name"] is None
+    # Same for the canonical key: coalescing this to '' would make the
+    # scanner's re-extract check unfireable and strand the backfill.
+    assert got["bass_tuning_key"] is None
+
+
+def test_a_row_missing_only_the_canonical_key_still_re_extracts(server_mod):
+    """A row scanned by an EARLIER build of this feature has bass_tuning_name
+    but no bass_tuning_key. It must still be re-queued, or its custom tunings
+    would group on the old serialization-dependent key forever."""
+    _put(server_mod, filename="halfway.sloppak", title="Halfway",
+         bass_tuning_name="Drop D", bass_tuning_offsets="-2 0 0 0")
+    server_mod.meta_db.conn.execute(
+        "UPDATE songs SET bass_tuning_key = NULL WHERE filename = 'halfway.sloppak'")
+    server_mod.meta_db.conn.commit()
+    cached = server_mod.meta_db.get("halfway.sloppak", 1.0, 1)
+    assert cached["bass_tuning_name"] == "Drop D"
+    assert cached["bass_tuning_key"] is None   # → the scanner re-queues it
+
+
+# ── 4. The migration actually backfills (the highest-risk gap) ───────────────
+
+@pytest.fixture()
+def scan_server(tmp_path, monkeypatch, isolate_logging, reset_scan_state):
+    """Server with the background scan forced in-process (see
+    test_feedpak_extension.py::scan_server — the production spawn pool can't
+    reach an in-process mock)."""
+    import concurrent.futures
+    monkeypatch.setenv("CONFIG_DIR", str(tmp_path))
+    monkeypatch.delenv("DLC_DIR", raising=False)
+    sys.modules.pop("server", None)
+    mod = importlib.import_module("server")
+    import scan as scan_mod
+    monkeypatch.setattr(
+        scan_mod, "_make_scan_executor",
+        lambda: concurrent.futures.ThreadPoolExecutor(max_workers=4),
+    )
+    yield mod
+    conn = getattr(getattr(mod, "meta_db", None), "conn", None)
+    if conn is not None:
+        getattr(sys.modules.get("server"), "_join_background_db_threads", lambda: None)()
+        conn.close()
+
+
+def test_existing_library_backfills_bass_tuning_on_next_scan(tmp_path, scan_server):
+    """END TO END for every CURRENT user: a settled library whose rows predate
+    the bass columns must re-extract on the next scan.
+
+    Both guards are exercised together — the row-level "bass column is NULL →
+    re-queue" AND the tree-signature fast path, which on an unchanged library
+    would otherwise skip the listing pass entirely and strand the backfill.
+    Then a second scan must NOT re-extract (the backfill converges, it doesn't
+    re-scan the whole library every launch).
+    """
+    import unittest.mock as mock
+
+    dlc = tmp_path / "dlc"
+    dlc.mkdir()
+    (dlc / "song.feedpak").write_bytes(b"")
+    # json.dumps, not %s: a Windows path interpolated raw produces invalid JSON
+    # escapes (\U, \d), the config silently fails to parse, and the scan then
+    # reports "no DLC folder configured" and extracts nothing.
+    (tmp_path / "config.json").write_text(
+        json.dumps({"dlc_dir": str(dlc)}), encoding="utf-8")
+
+    scan = importlib.import_module("scan")
+    seen: list[str] = []
+
+    def mock_extract(f, dlc_dir):
+        seen.append(f.name)
+        return {"title": f.name, "artist": "A", "album": "",
+                "bass_tuning_name": "Drop D", "bass_tuning_sort_key": -2,
+                "bass_tuning_offsets": "-2 0 0 0 0 0"}
+
+    with mock.patch("scan_worker._extract_meta_for_file", new=mock_extract):
+        scan.background_scan()
+        assert "song.feedpak" in seen
+
+        # Simulate the pre-migration state: the row exists and is otherwise
+        # fresh (mtime/size match), but its bass columns were never extracted.
+        scan.appstate.meta_db.conn.execute(
+            "UPDATE songs SET bass_tuning_name = NULL, bass_tuning_sort_key = NULL, "
+            "bass_tuning_offsets = NULL")
+        scan.appstate.meta_db.conn.commit()
+
+        seen.clear()
+        scan.background_scan()
+        assert "song.feedpak" in seen, (
+            "a row with NULL bass columns must re-extract — otherwise no "
+            "existing library ever gets the bass tuning")
+
+        row = scan.appstate.meta_db.conn.execute(
+            "SELECT bass_tuning_name, bass_tuning_offsets FROM songs "
+            "WHERE filename = 'song.feedpak'").fetchone()
+        assert row == ("Drop D", "-2 0 0 0 0 0")
+
+        # Converged: the fast path is back and nothing re-extracts.
+        seen.clear()
+        scan.background_scan()
+        assert seen == []
+
+
+# ── 5. The facet endpoint ────────────────────────────────────────────────────
+
+@pytest.fixture()
+def facet_seeded(server_mod):
+    """Three shapes, matching the real library's distribution:
+      differ   — guitar D Standard, bass E Standard   (the bug)
+      match    — both Drop D                          (common)
+      nobass   — guitar Drop D, no bass chart         (fallback, common)
+    """
+    _put(server_mod, filename="differ.sloppak", title="Differ",
+         tuning_name_="D Standard", tuning_sort_key=-12,
+         tuning_offsets="-2 -2 -2 -2 -2 -2",
+         bass_tuning_name="E Standard", bass_tuning_sort_key=0,
+         bass_tuning_offsets="0 0 0 0 0 0")
+    _put(server_mod, filename="match.sloppak", title="Match",
+         tuning_name_="Drop D", tuning_sort_key=-2, tuning_offsets="-2 0 0 0 0 0",
+         bass_tuning_name="Drop D", bass_tuning_sort_key=-2,
+         bass_tuning_offsets="-2 0 0 0 0 0")
+    _put(server_mod, filename="nobass.sloppak", title="NoBass",
+         tuning_name_="Drop D", tuning_sort_key=-2, tuning_offsets="-2 0 0 0 0 0")
+
+
+def _facet(client, **kw):
+    return {t["name"]: t["count"]
+            for t in client.get("/api/library/tuning-names", params=kw).json()["tunings"]}
+
+
+def test_facet_defaults_to_the_guitar_tuning(client, facet_seeded):
+    assert _facet(client) == {"D Standard": 1, "Drop D": 2}
+
+
+def test_facet_bass_groups_by_bass_tuning_with_guitar_fallback(client, facet_seeded):
+    """differ counts under its BASS tuning (E Standard), match under Drop D,
+    and nobass — having no bass chart — falls back to its guitar Drop D rather
+    than vanishing from the facet."""
+    assert _facet(client, instrument="bass") == {"E Standard": 1, "Drop D": 2}
+
+
+def test_facet_ignores_an_unknown_instrument(client, facet_seeded):
+    """An unknown value must not silently change filter semantics."""
+    assert _facet(client, instrument="theremin") == _facet(client)
+
+
+# ── 6. The filter: the actual reported bug ───────────────────────────────────
+
+def _files(client, **kw):
+    return {s["filename"] for s in client.get("/api/library", params=kw).json()["songs"]}
+
+
+def test_bass_filter_excludes_a_song_whose_only_match_is_its_guitar_tuning(
+        client, facet_seeded):
+    """THE BUG. Filtering bass "D Standard" must NOT return `differ` — its
+    D Standard is the GUITAR chart; its bass is in E Standard."""
+    assert _files(client, tunings="D Standard") == {"differ.sloppak"}
+    assert _files(client, tunings="D Standard", instrument="bass") == set()
+
+
+def test_bass_filter_returns_songs_by_their_bass_tuning(client, facet_seeded):
+    """…and the converse: bass "E Standard" finds `differ`, which the guitar
+    filter would never return."""
+    assert _files(client, tunings="E Standard") == set()
+    assert _files(client, tunings="E Standard", instrument="bass") == {"differ.sloppak"}
+
+
+def test_bass_filter_keeps_songs_without_a_bass_arrangement_via_fallback(
+        client, facet_seeded):
+    """The most common shape. `nobass` has no bass chart, so it must still be
+    reachable under its guitar tuning instead of disappearing for bass users —
+    and the facet's count for that pill must equal what the filter returns."""
+    got = _files(client, tunings="Drop D", instrument="bass")
+    assert got == {"match.sloppak", "nobass.sloppak"}
+    assert _facet(client, instrument="bass")["Drop D"] == len(got)
+
+
+def test_custom_bass_tunings_stay_distinct_under_their_offsets(client, server_mod):
+    """Two unnameable bass tunings both label "Custom Tuning"; the facet keys
+    them on raw offsets so selecting one doesn't drag in the other. Uses the
+    real [5,5,5,5,4,4] shape from the library."""
+    _put(server_mod, filename="c1.sloppak", title="C1",
+         bass_tuning_name="Custom Tuning", bass_tuning_sort_key=28,
+         bass_tuning_offsets="5 5 5 5 4 4")
+    _put(server_mod, filename="c2.sloppak", title="C2",
+         bass_tuning_name="Custom Tuning", bass_tuning_sort_key=-7,
+         bass_tuning_offsets="-3 -1 -1 -1 -1 0")
+    keys = [t["key"] for t in client.get(
+        "/api/library/tuning-names", params={"instrument": "bass"}).json()["tunings"]
+        if t["name"] == "Custom Tuning"]
+    assert sorted(keys) == sorted(["5 5 5 5 4 4", "-3 -1 -1 -1 -1 0"])
+    assert _files(client, tunings="5 5 5 5 4 4", instrument="bass") == {"c1.sloppak"}
+
+
+def test_stats_facet_counts_agree_with_the_bass_filter(client, facet_seeded):
+    """The A–Z rail / count surface must apply the same instrument-aware
+    predicate as the grid, or the header count contradicts the results."""
+    body = client.get("/api/library/stats", params={
+        "tunings": "Drop D", "instrument": "bass"}).json()
+    assert body["total_songs"] == 2
+
+
+# ── 7. Sort ──────────────────────────────────────────────────────────────────
+
+def test_tuning_sort_respects_the_instrument(client, facet_seeded):
+    """Tuning sort is musical distance from E Standard. For a bass player that
+    distance must be measured on the BASS tuning: `differ` is the furthest
+    song by guitar (D Standard, |−12|) but the nearest by bass (E Standard, 0),
+    so it moves from last to first."""
+    def order(**kw):
+        return [s["filename"] for s in client.get(
+            "/api/library", params={"sort": "tuning", **kw}).json()["songs"]]
+
+    guitar = order()
+    assert guitar[-1] == "differ.sloppak"
+    bass = order(instrument="bass")
+    assert bass[0] == "differ.sloppak"
+
+
+# ── 8. Song payload ──────────────────────────────────────────────────────────
+
+# ── 9. Real-library offset SHAPES ────────────────────────────────────────────
+# Measured across the 59-pack test library: bass offset lists are NOT reliably
+# 4 or reliably 6 — 41 store six elements, 1 stores four. Two six-element ones
+# diverge in the tail (AC/DC "Girls Got Rhythm" [5,5,5,5,4,4]; Intervals
+# "Libra" [-2,0,0,0,0,0]). Nothing may crash or mislabel on any of them.
+
+@pytest.mark.parametrize("offsets,expected", [
+    ([0, 0, 0, 0], "E Standard"),                 # four-element (the 1 outlier)
+    ([0, 0, 0, 0, 0, 0], "E Standard"),           # six-element all-equal (39 of them)
+    ([-1, -1, -1, -1], "Eb Standard"),            # four-element, down a semitone
+    ([5, 5, 5, 5, 4, 4], "Custom Tuning"),        # AC/DC — divergent tail
+    ([-2, 0, 0, 0, 0, 0], "Drop D"),              # Intervals — drop + trailing zeros
+    ([0, 0, 0, 0, 0], "Custom Tuning"),           # five: no naming convention → custom
+])
+def test_real_library_bass_offset_shapes_name_without_crashing(offsets, expected):
+    assert tuning_name(offsets) == expected
+
+
+@pytest.mark.parametrize("offsets", [
+    [0, 0, 0, 0], [0, 0, 0, 0, 0, 0], [5, 5, 5, 5, 4, 4], [-2, 0, 0, 0, 0, 0],
+])
+def test_real_library_bass_offset_shapes_survive_extraction(tmp_path, offsets):
+    """Each shape must round-trip the real extractor + scanner derivation,
+    landing on the NORMALIZED (truncated, plausibility-checked) columns."""
+    norm = normalize_bass_offsets(offsets)
+    d = _pack(tmp_path, "shape.sloppak", [
+        {"name": "Lead", "tuning": [0, 0, 0, 0, 0, 0]},
+        {"name": "Bass", "tuning": offsets},
+    ])
+    meta = _extract_meta_for_file(d)
+    assert meta["bass_tuning_name"] == bass_tuning_name(norm)
+    assert meta["bass_tuning_offsets"] == " ".join(str(o) for o in norm)
+    assert meta["bass_tuning_sort_key"] == sum(norm)
+    assert meta["bass_tuning_key"] == bass_tuning_key(norm)
+
+
+def test_named_bass_tunings_group_across_serialization_lengths(client, server_mod):
+    """The length question does NOT fragment NAMED tunings: a bass stored as
+    four elements and one stored as six both name "E Standard", and the facet
+    groups by name — so they land in ONE row with a combined count. This is the
+    common case (40 of the 42 bass arrangements in the real library)."""
+    _put(server_mod, filename="four.sloppak", title="Four",
+         bass_tuning_name=tuning_name([0, 0, 0, 0]), bass_tuning_offsets="0 0 0 0")
+    _put(server_mod, filename="six.sloppak", title="Six",
+         bass_tuning_name=tuning_name([0, 0, 0, 0, 0, 0]),
+         bass_tuning_offsets="0 0 0 0 0 0")
+    assert _facet(client, instrument="bass") == {"E Standard": 2}
+    assert _files(client, tunings="E Standard", instrument="bass") == {
+        "four.sloppak", "six.sloppak"}
+
+
+def test_drop_d_bass_groups_across_serialization_lengths(client, server_mod):
+    """Same for the Intervals shape: [-2,0,0,0,0,0] and [-2,0,0,0] both name
+    "Drop D", so trailing zeros can't split a named tuning into two rows."""
+    _put(server_mod, filename="d6.sloppak", title="D6",
+         bass_tuning_name=tuning_name([-2, 0, 0, 0, 0, 0]),
+         bass_tuning_sort_key=-2, bass_tuning_offsets="-2 0 0 0 0 0")
+    _put(server_mod, filename="d4.sloppak", title="D4",
+         bass_tuning_name=tuning_name([-2, 0, 0, 0]),
+         bass_tuning_sort_key=-2, bass_tuning_offsets="-2 0 0 0")
+    assert _facet(client, instrument="bass") == {"Drop D": 2}
+
+
+def test_equivalent_custom_bass_tunings_group_into_one_facet_row(client, server_mod):
+    """Two CUSTOM bass tunings that are the same physical tuning must be ONE
+    facet row, however they were serialized. They group on canonical PITCHES
+    (bass_tuning_key), so the offsets string no longer fragments them —
+    previously this produced two rows with split counts."""
+    key = bass_tuning_key([-3, -1, -1, -1])
+    _put(server_mod, filename="c6.sloppak", title="C6",
+         bass_tuning_name="Custom Tuning", bass_tuning_sort_key=-6,
+         bass_tuning_offsets="-3 -1 -1 -1", bass_tuning_key=key)
+    _put(server_mod, filename="c4.sloppak", title="C4",
+         bass_tuning_name="Custom Tuning", bass_tuning_sort_key=-6,
+         bass_tuning_offsets="-3 -1 -1 -1", bass_tuning_key=key)
+    rows = client.get("/api/library/tuning-names",
+                      params={"instrument": "bass"}).json()["tunings"]
+    customs = [t for t in rows if t["name"] == "Custom Tuning"]
+    assert len(customs) == 1 and customs[0]["count"] == 2
+    assert _files(client, tunings=customs[0]["key"], instrument="bass") == {
+        "c6.sloppak", "c4.sloppak"}
+
+
+def test_canonical_key_is_pitch_not_serialization(tmp_path):
+    """The property that makes the grouping robust: two serializations of one
+    tuning yield the same key, and two genuinely different tunings do not."""
+    assert bass_tuning_key(normalize_bass_offsets([-2, 0, 0, 0, 0, 0])) == \
+        bass_tuning_key(normalize_bass_offsets([-2, 0, 0, 0]))
+    assert bass_tuning_key([-2, 0, 0, 0]) != bass_tuning_key([-3, 0, 0, 0])
+    # Absolute open pitches of a standard 4-string bass (E1 A1 D2 G2).
+    assert bass_tuning_key([0, 0, 0, 0]) == "bass:28:33:38:43"
+
+
+def test_custom_bass_facet_row_selects_exactly_what_it_counted(client, server_mod):
+    """Whatever the grouping rule, the invariant that must NEVER break: every
+    facet row's count equals the number of songs its own key returns. This is
+    what makes the seam safe to change — a normalization that merged rows but
+    not the filter would fail here."""
+    _put(server_mod, filename="x6.sloppak", title="X6",
+         bass_tuning_name="Custom Tuning", bass_tuning_sort_key=28,
+         bass_tuning_offsets="5 5 5 5 4 4")
+    _put(server_mod, filename="x4.sloppak", title="X4",
+         bass_tuning_name="Custom Tuning", bass_tuning_sort_key=20,
+         bass_tuning_offsets="5 5 5 5")
+    _put(server_mod, filename="plain.sloppak", title="Plain",
+         bass_tuning_name="E Standard", bass_tuning_offsets="0 0 0 0 0 0")
+    for row in client.get("/api/library/tuning-names",
+                          params={"instrument": "bass"}).json()["tunings"]:
+        got = _files(client, tunings=row["key"], instrument="bass")
+        assert len(got) == row["count"], (
+            f"facet row {row['key']!r} counted {row['count']} but selects {len(got)}")
+
+
+# ── 10. THE HEADLINE REGRESSION ──────────────────────────────────────────────
+
+def test_covet_shibuya_is_findable_by_a_bassist(tmp_path, server_mod, client):
+    """Covet - "Shibuya" (Effloresce): the guitar is in a custom tuning
+    [-2,0,0,-1,-2,0] while the bass is dead standard. This is the tester's bug
+    in one song — a bassist filtering "E Standard" never saw it, because the
+    library only knew the guitar's custom tuning.
+
+    Round-tripped through the REAL extractor and scanner derivation, not
+    hand-written columns, so it covers the whole chain."""
+    d = _pack(tmp_path, "shibuya.sloppak", [
+        {"name": "Lead", "tuning": [-2, 0, 0, -1, -2, 0]},
+        {"name": "Bass", "tuning": [0, 0, 0, 0, 0, 0]},
+    ])
+    meta = _extract_meta_for_file(d)
+    server_mod.meta_db.put("shibuya.sloppak", 1.0, 1, {
+        **meta, "title": "Shibuya", "artist": "Covet", "album": "Effloresce"})
+
+    # The guitar chart really is a custom tuning…
+    assert meta["tuning_name"] == "Custom Tuning"
+    # …and the bass chart really is standard.
+    assert meta["bass_tuning_name"] == "E Standard"
+
+    # Before the fix a bassist filtering E Standard got nothing.
+    assert _files(client, tunings="E Standard") == set()
+    assert _files(client, tunings="E Standard", instrument="bass") == {"shibuya.sloppak"}
+
+    # And it appears in the bass facet under E Standard, as a REAL bass chart
+    # (not an inferred fallback).
+    row = next(t for t in client.get(
+        "/api/library/tuning-names", params={"instrument": "bass"}).json()["tunings"]
+        if t["name"] == "E Standard")
+    assert row["count"] == 1 and row["inferred_count"] == 0
+
+
+# ── 11. Provenance: the fallback must be honest, never silent ────────────────
+
+def test_facet_reports_how_many_rows_are_inferred_from_the_guitar_chart(
+        client, facet_seeded):
+    """The fallback keeps no-bass-chart songs visible (a third of a real
+    library), but the UI must be able to say so. `nobass` has no bass chart and
+    rides under the guitar's Drop D; `match` has a real one."""
+    rows = {t["name"]: t for t in client.get(
+        "/api/library/tuning-names", params={"instrument": "bass"}).json()["tunings"]}
+    assert rows["Drop D"]["count"] == 2
+    assert rows["Drop D"]["inferred_count"] == 1      # nobass only
+    assert rows["E Standard"]["inferred_count"] == 0  # differ has a real bass chart
+
+
+def test_guitar_facet_reports_no_inferred_rows(client, facet_seeded):
+    """Guitar is never a fallback perspective, so nothing is ever inferred."""
+    rows = client.get("/api/library/tuning-names").json()["tunings"]
+    assert all(t["inferred_count"] == 0 for t in rows)
+
+
+def test_song_rows_mark_an_inferred_tuning(client, facet_seeded):
+    """A bass player's row must be distinguishable: native bass chart vs
+    borrowed from the guitar. Without this the card silently presents a guitar
+    tuning as the bass tuning — the original bug in a new place."""
+    rows = {s["filename"]: s for s in client.get(
+        "/api/library", params={"instrument": "bass"}).json()["songs"]}
+    assert rows["differ.sloppak"]["tuning_inferred"] is False
+    assert rows["nobass.sloppak"]["tuning_inferred"] is True
+    assert rows["differ.sloppak"]["tuning_perspective"] == "bass"
+
+
+def test_guitar_rows_carry_no_bass_perspective_fields(client, facet_seeded):
+    """The guitar payload is untouched — no perspective/inferred keys at all."""
+    row = client.get("/api/library").json()["songs"][0]
+    assert "tuning_inferred" not in row and "tuning_perspective" not in row
+
+
+def test_arrangements_has_bass_is_the_real_bass_chart_lever(server_mod, client):
+    """'Only songs with a real bass chart' is the EXISTING `arrangements_has`
+    filter — no new filter, no "confirmed tunings" checkbox. It composes with
+    the tuning filter, so a bassist who wants to exclude inferred rows already
+    can, and it is already expressible in a saved collection rule."""
+    def put_with_arrs(fn, arrs, **kw):
+        server_mod.meta_db.put(fn, 1.0, 1, {
+            "title": fn, "artist": "A", "album": "A - LP", "year": "2010",
+            "duration": 200.0, "tuning": "Drop D", "arrangements": arrs,
+            "has_lyrics": False, "format": "sloppak", "stem_ids": [],
+            "tuning_name": "Drop D", "tuning_sort_key": -2,
+            "tuning_offsets": "-2 0 0 0 0 0", **kw})
+
+    put_with_arrs("withbass.sloppak",
+                  [{"index": 0, "name": "Lead"}, {"index": 1, "name": "Bass"}],
+                  bass_tuning_name="Drop D", bass_tuning_sort_key=-2,
+                  bass_tuning_offsets="-2 0 0 0",
+                  bass_tuning_key=bass_tuning_key([-2, 0, 0, 0]))
+    put_with_arrs("nobass.sloppak", [{"index": 0, "name": "Lead"}])
+
+    # Both are reachable under the bass Drop D pill (the fallback keeps the
+    # no-bass-chart song visible)…
+    assert _files(client, tunings="Drop D", instrument="bass") == {
+        "withbass.sloppak", "nobass.sloppak"}
+    # …and the existing arrangements_has lever narrows to real bass charts.
+    assert _files(client, tunings="Drop D", instrument="bass",
+                  arrangements_has="Bass") == {"withbass.sloppak"}
+
+
+def test_song_rows_carry_the_bass_tuning_for_the_client(client, facet_seeded):
+    """The card renders the bass tuning client-side, so the row must ship it —
+    and ship '' (not the guitar value) when there is no bass chart, so the
+    client's fallback stays the client's decision."""
+    rows = {s["filename"]: s for s in client.get("/api/library").json()["songs"]}
+    assert rows["differ.sloppak"]["tuning_name"] == "D Standard"
+    assert rows["differ.sloppak"]["bass_tuning_name"] == "E Standard"
+    assert rows["differ.sloppak"]["bass_tuning_offsets"] == "0 0 0 0 0 0"
+    assert rows["nobass.sloppak"]["bass_tuning_name"] == ""

tests/test_library_tuning_perspectives.py

@@ -0,0 +1,294 @@
+"""The three-valued tuning PERSPECTIVE, and "playable without retuning".
+
+Two behaviours that extend the bass tuning fix (see
+test_library_tuning_instrument.py):
+
+1. `active_instrument_profile` has three values (guitar-lead / guitar-rhythm /
+   bass), so the tuning perspective must too. Lead and rhythm charts can be
+   tuned differently, which is the identical bug a bassist hit, inside guitar.
+
+2. Exact tuning match answers "which tuning is this labelled". A player
+   actually wants "will this cost me a retune". Both are offered; exact stays
+   the default.
+
+Everything round-trips through the real extractor, the real scanner
+derivation, the real schema and the real HTTP surface.
+"""
+
+import importlib
+import sys
+
+import pytest
+import yaml
+from fastapi.testclient import TestClient
+
+from scan_worker import _extract_meta_for_file
+from tunings import (
+    PERSPECTIVES, bass_tuning_key, chart_is_playable_in, perspective_tuning_key,
+)
+
+
+@pytest.fixture()
+def server_mod(tmp_path, monkeypatch):
+    monkeypatch.setenv("CONFIG_DIR", str(tmp_path))
+    sys.modules.pop("server", None)
+    mod = importlib.import_module("server")
+    yield mod
+    conn = getattr(getattr(mod, "meta_db", None), "conn", None)
+    if conn is not None:
+        getattr(sys.modules.get("server"), "_join_background_db_threads", lambda: None)()
+        conn.close()
+
+
+@pytest.fixture()
+def client(server_mod):
+    c = TestClient(server_mod.app)
+    try:
+        yield c
+    finally:
+        c.close()
+
+
+def _pack(root, name, arrangements):
+    d = root / name
+    d.mkdir(parents=True)
+    (d / "manifest.yaml").write_text(yaml.safe_dump({
+        "title": name, "artist": "A", "duration": 100,
+        "arrangements": arrangements, "stems": [],
+    }), encoding="utf-8")
+    return d
+
+
+def _files(client, **kw):
+    return {s["filename"] for s in client.get("/api/library", params=kw).json()["songs"]}
+
+
+# ── 1. The same bug WITHIN guitar: lead vs rhythm ────────────────────────────
+
+def test_rhythm_chart_tuning_is_indexed_separately(tmp_path):
+    """A song whose LEAD is in E standard but whose RHYTHM is in Drop D must
+    index both — through the real extractor + scanner derivation."""
+    d = _pack(tmp_path, "split.sloppak", [
+        {"name": "Lead", "tuning": [0, 0, 0, 0, 0, 0]},
+        {"name": "Rhythm", "tuning": [-2, 0, 0, 0, 0, 0]},
+    ])
+    meta = _extract_meta_for_file(d)
+    assert meta["tuning_name"] == "E Standard"        # song-level = guitar-first
+    assert meta["rhythm_tuning_name"] == "Drop D"     # the rhythm chart's own
+    assert meta["rhythm_tuning_offsets"] == "-2 0 0 0 0 0"
+    assert meta["rhythm_tuning_low_pitch"] == 38      # low D
+
+
+def test_rhythm_offsets_are_not_truncated(tmp_path):
+    """Only BASS truncates (its arrays are padded). A 7-string guitar array is
+    real data — cutting it to 6 would invent a tuning the chart doesn't have."""
+    d = _pack(tmp_path, "seven.sloppak", [
+        {"name": "Rhythm", "tuning": [-2, -2, -2, -2, -2, -2, -2]},
+    ])
+    meta = _extract_meta_for_file(d)
+    assert meta["rhythm_tuning_offsets"] == "-2 -2 -2 -2 -2 -2 -2"
+
+
+def test_no_rhythm_arrangement_leaves_the_columns_empty(tmp_path):
+    d = _pack(tmp_path, "leadonly.sloppak", [{"name": "Lead", "tuning": [0] * 6}])
+    meta = _extract_meta_for_file(d)
+    assert meta["rhythm_tuning_name"] == ""
+    assert meta["rhythm_tuning_key"] == ""
+
+
+def _put(server_mod, fn, **kw):
+    base = dict(title=fn, artist="A", album="LP", year="2010", duration=200.0,
+                tuning="E Standard", arrangements=[], has_lyrics=False,
+                format="sloppak", stem_ids=[], tuning_name="E Standard",
+                tuning_sort_key=0, tuning_offsets="0 0 0 0 0 0",
+                tuning_low_pitch=40)
+    base.update(kw)
+    server_mod.meta_db.put(fn, 1.0, 1, base)
+
+
+@pytest.fixture()
+def rhythm_seeded(server_mod):
+    """Both songs are E Standard by LEAD. One has a Drop D rhythm chart; the
+    other has no rhythm chart at all (so it falls back + is marked inferred)."""
+    _put(server_mod, "rdiffer.sloppak",
+         rhythm_tuning_name="Drop D", rhythm_tuning_sort_key=-2,
+         rhythm_tuning_offsets="-2 0 0 0 0 0",
+         rhythm_tuning_key=perspective_tuning_key(
+             [-2, 0, 0, 0, 0, 0], PERSPECTIVES["guitar-rhythm"]),
+         rhythm_tuning_low_pitch=38)
+    _put(server_mod, "rnone.sloppak")
+
+
+def test_rhythm_filter_excludes_a_lead_only_tuning_match(client, rhythm_seeded):
+    """THE WITHIN-GUITAR BUG. Filtering rhythm "E Standard" must not return
+    rdiffer — that is its LEAD tuning; its rhythm chart is in Drop D."""
+    assert _files(client, tunings="E Standard") == {"rdiffer.sloppak", "rnone.sloppak"}
+    # rnone has no rhythm chart, so it falls back to its lead tuning and stays.
+    assert _files(client, tunings="E Standard", instrument="guitar-rhythm") == {
+        "rnone.sloppak"}
+    assert _files(client, tunings="Drop D", instrument="guitar-rhythm") == {
+        "rdiffer.sloppak"}
+    # …and Drop D finds nothing from the lead perspective.
+    assert _files(client, tunings="Drop D") == set()
+
+
+def test_rhythm_perspective_marks_inferred_rows(client, rhythm_seeded):
+    rows = {s["filename"]: s for s in client.get(
+        "/api/library", params={"instrument": "guitar-rhythm"}).json()["songs"]}
+    assert rows["rdiffer.sloppak"]["tuning_inferred"] is False
+    assert rows["rnone.sloppak"]["tuning_inferred"] is True
+    assert rows["rdiffer.sloppak"]["tuning_perspective"] == "guitar-rhythm"
+
+
+def test_rhythm_facet_reports_inferred_portion(client, rhythm_seeded):
+    rows = {t["name"]: t for t in client.get(
+        "/api/library/tuning-names",
+        params={"instrument": "guitar-rhythm"}).json()["tunings"]}
+    assert rows["Drop D"]["count"] == 1 and rows["Drop D"]["inferred_count"] == 0
+    assert rows["E Standard"]["count"] == 1 and rows["E Standard"]["inferred_count"] == 1
+
+
+def test_facet_row_selects_exactly_what_it_counted_for_rhythm(client, rhythm_seeded):
+    """The invariant that must hold for EVERY perspective."""
+    for row in client.get("/api/library/tuning-names",
+                          params={"instrument": "guitar-rhythm"}).json()["tunings"]:
+        got = _files(client, tunings=row["key"], instrument="guitar-rhythm")
+        assert len(got) == row["count"], row["key"]
+
+
+def test_guitar_lead_is_byte_identical_to_the_legacy_default(client, rhythm_seeded):
+    """The majority path must not regress: the default payload gains no keys,
+    and the legacy two-valued vocabulary still resolves to it."""
+    default = client.get("/api/library").json()
+    explicit = client.get("/api/library", params={"instrument": "guitar-lead"}).json()
+    legacy = client.get("/api/library", params={"instrument": "guitar"}).json()
+    assert default == explicit == legacy
+    row = default["songs"][0]
+    assert "tuning_inferred" not in row and "tuning_perspective" not in row
+
+
+def test_unknown_perspective_falls_back_to_lead(client, rhythm_seeded):
+    """An unrecognised value must never silently change filter semantics."""
+    assert client.get("/api/library", params={"instrument": "kazoo"}).json() == \
+        client.get("/api/library").json()
+
+
+def test_tuning_sort_respects_the_rhythm_perspective(client, rhythm_seeded):
+    """Sort is musical distance from standard. rdiffer is 0 away by lead but
+    -2 by rhythm, so the perspective changes its position."""
+    def order(**kw):
+        return [s["filename"] for s in client.get(
+            "/api/library", params={"sort": "tuning", **kw}).json()["songs"]]
+    assert order()[0] == "rdiffer.sloppak"                            # tie → filename
+    assert order(instrument="guitar-rhythm")[0] == "rnone.sloppak"    # 0 beats -2
+
+
+# ── 2. "Playable without retuning" ───────────────────────────────────────────
+
+@pytest.mark.parametrize("your_low,chart_low,expected", [
+    (23, 28, True),     # 5-string bass (low B) plays a 4-string standard chart
+    (23, 26, True),     # …and a drop-D chart: the low D is fretted on the B string
+    (28, 26, False),    # 4-string standard CANNOT reach a drop-D chart's low D
+    (28, 28, True),     # identical tuning
+    (40, 38, False),    # guitar standard vs a drop-D chart
+    (38, 40, True),     # a drop-D guitar covers a standard chart
+    (None, 28, False),  # unknown chart pitch is never claimed playable
+    (28, None, False),
+])
+def test_playability_rule(your_low, chart_low, expected):
+    """The core comparison as a property: your lowest open string vs the
+    chart's lowest required pitch. Unknown => not playable (conservative)."""
+    assert chart_is_playable_in(chart_low, your_low) is expected
+
+
+@pytest.fixture()
+def pitched(server_mod):
+    _put(server_mod, "std.sloppak", tuning_low_pitch=40)
+    _put(server_mod, "dropd.sloppak", tuning="Drop D", tuning_name="Drop D",
+         tuning_offsets="-2 0 0 0 0 0", tuning_sort_key=-2, tuning_low_pitch=38)
+    _put(server_mod, "dropc.sloppak", tuning="Drop C", tuning_name="Drop C",
+         tuning_offsets="-4 -2 -2 -2 -2 -2", tuning_sort_key=-14, tuning_low_pitch=36)
+
+
+def _playable(client, offsets, instrument="guitar", sc=6, **kw):
+    return {s["filename"] for s in client.get("/api/library", params={
+        "tuning_match": "playable", "playable_offsets": offsets,
+        "playable_instrument": instrument, "playable_string_count": str(sc), **kw,
+    }).json()["songs"]}
+
+
+def test_playable_from_standard_excludes_lower_tuned_charts(client, pitched):
+    """In E standard you can play the standard chart, but the drop-D and
+    drop-C charts need a retune — exactly what the tester wants surfaced."""
+    assert _playable(client, "0,0,0,0,0,0") == {"std.sloppak"}
+
+
+def test_playable_from_drop_c_covers_everything_above_it(client, pitched):
+    """Tuned DOWN to drop C, every higher-tuned chart is reachable by fretting
+    — the dominant real case this feature exists for."""
+    assert _playable(client, "-4,-2,-2,-2,-2,-2") == {
+        "std.sloppak", "dropd.sloppak", "dropc.sloppak"}
+
+
+def test_playable_is_a_mode_not_a_replacement_for_exact(client, pitched):
+    """Exact match still works untouched, and returns something DIFFERENT from
+    playable — they answer different questions."""
+    exact = {s["filename"] for s in client.get(
+        "/api/library", params={"tunings": "Drop D"}).json()["songs"]}
+    assert exact == {"dropd.sloppak"}
+    assert _playable(client, "-2,0,0,0,0,0") == {"std.sloppak", "dropd.sloppak"}
+
+
+def test_playable_excludes_rows_with_no_indexed_pitch(client, server_mod, pitched):
+    """Conservative by construction: a chart whose low pitch we could not
+    compute is EXCLUDED, never assumed playable. Wrongly claiming playability
+    costs a mid-practice retune — the failure this feature prevents."""
+    _put(server_mod, "unknown.sloppak", tuning_low_pitch=None)
+    assert "unknown.sloppak" not in _playable(client, "-4,-2,-2,-2,-2,-2")
+    # …but it is still reachable normally, so it isn't lost from the library.
+    assert any(s["filename"] == "unknown.sloppak"
+               for s in client.get("/api/library").json()["songs"])
+
+
+def test_malformed_playable_tuning_applies_no_filter(client, pitched):
+    """A tuning we cannot resolve must not silently claim everything is
+    playable OR that nothing is — it applies no filter at all."""
+    everything = {s["filename"] for s in client.get("/api/library").json()["songs"]}
+    assert _playable(client, "not,a,tuning") == everything
+    assert _playable(client, "") == everything
+    # A string count that disagrees with the offsets is equally unusable.
+    assert _playable(client, "0,0,0,0", instrument="guitar", sc=6) == everything
+
+
+def test_playable_respects_the_bass_perspective(client, server_mod):
+    """A 5-string bass (low B) can play a 4-string standard bass chart. The
+    comparison must run on the BASS tuning — this song's GUITAR chart is tuned
+    far lower, so reading the wrong column would flip the answer."""
+    _put(server_mod, "bassy.sloppak",
+         tuning="Custom Tuning", tuning_name="Custom Tuning",
+         tuning_offsets="-4 -2 -2 -1 -2 0", tuning_sort_key=-11,
+         tuning_low_pitch=36,
+         bass_tuning_name="E Standard", bass_tuning_sort_key=0,
+         bass_tuning_offsets="0 0 0 0",
+         bass_tuning_key=bass_tuning_key([0, 0, 0, 0]),
+         bass_tuning_low_pitch=28)
+    # 5-string bass low B (23) <= the chart low E (28) → playable.
+    got = {s["filename"] for s in client.get("/api/library", params={
+        "tuning_match": "playable", "playable_offsets": "0,0,0,0,0",
+        "playable_instrument": "bass", "playable_string_count": "5",
+        "instrument": "bass"}).json()["songs"]}
+    assert got == {"bassy.sloppak"}
+    # A 4-string bass tuned UP a semitone (low F, 29) cannot reach the low E.
+    got_up = {s["filename"] for s in client.get("/api/library", params={
+        "tuning_match": "playable", "playable_offsets": "1,1,1,1",
+        "playable_instrument": "bass", "playable_string_count": "4",
+        "instrument": "bass"}).json()["songs"]}
+    assert got_up == set()
+
+
+def test_playable_and_stats_agree(client, pitched):
+    """The count surface must apply the same predicate as the grid."""
+    body = client.get("/api/library/stats", params={
+        "tuning_match": "playable", "playable_offsets": "0,0,0,0,0,0",
+        "playable_instrument": "guitar", "playable_string_count": "6"}).json()
+    assert body["total_songs"] == 1

tests/test_loosefolder.py

@@ -302,3 +302,44 @@ def test_extract_meta_uses_lead_tuning_when_bass_sorts_first(tmp_path):
 
     meta = loosefolder.extract_meta(tmp_path)
     assert meta["tuning_offsets"] == [0, 0, 0, 0, 0, 0]
+    # …and the bass chart's OWN tuning is indexed alongside it, so a bass
+    # player's library filter isn't answered with the guitar tuning.
+    assert meta["bass_tuning_offsets"] == [-4, -4, -4, -4, 0, 0]
+
+
+def test_extract_meta_bass_tuning_absent_without_bass_arrangement(tmp_path):
+    """A folder with no bass chart leaves the bass tuning EMPTY (None) rather
+    than echoing the guitar tuning — the library then falls back explicitly,
+    and 'no bass part' stays distinguishable from 'bass part in E Standard'."""
+    (tmp_path / "audio.wem").write_bytes(b"\0")
+    (tmp_path / "lead.xml").write_text(_LEAD_STD_XML, encoding="utf-8")
+
+    meta = loosefolder.extract_meta(tmp_path)
+    assert meta["tuning_offsets"] == [0, 0, 0, 0, 0, 0]
+    assert meta["bass_tuning_offsets"] is None
+
+
+def test_extract_meta_bass_tuning_matches_guitar_is_still_indexed(tmp_path):
+    """The COMMON case: bass and guitar in the same tuning. The bass column
+    must still be populated — an empty one would be read as 'no bass chart'."""
+    (tmp_path / "audio.wem").write_bytes(b"\0")
+    _write_min_xml(tmp_path / "lead.xml", arrangement="Lead")
+    _write_min_xml(tmp_path / "bass.xml", arrangement="Bass")
+
+    meta = loosefolder.extract_meta(tmp_path)
+    assert meta["bass_tuning_offsets"] == [0, 0, 0, 0, 0, 0]
+
+
+def test_extract_meta_manifest_tuning_does_not_become_the_bass_tuning(tmp_path):
+    """A manifest `tuning_offsets` overrides the SONG tuning but says nothing
+    about which chart it describes, so it must never be mistaken for the bass
+    part's tuning — with no bass chart the bass column stays empty."""
+    (tmp_path / "audio.wem").write_bytes(b"\0")
+    (tmp_path / "lead.xml").write_text(_LEAD_STD_XML, encoding="utf-8")
+    (tmp_path / "manifest.json").write_text(json.dumps({
+        "tuning_offsets": [-2, -2, -2, -2, -2, -2],
+    }), encoding="utf-8")
+
+    meta = loosefolder.extract_meta(tmp_path)
+    assert meta["tuning_offsets"] == [-2, -2, -2, -2, -2, -2]
+    assert meta["bass_tuning_offsets"] is None

tests/test_tunings.py

@@ -4,10 +4,15 @@ import pytest
 
 from tunings import (
     DEFAULT_TUNINGS,
+    PERSPECTIVES,
     TUNING_PRESET_MIDIS,
     _valid_tuning_for_key,
     apply_flat_instrument_patch_to_profiles,
+    normalize_offsets,
     open_midis_to_freqs,
+    perspective_low_pitch,
+    perspective_tuning_key,
+    perspective_tuning_name,
     settings_with_instrument_profiles,
     tuning_midis_from_offsets,
     tuning_name,
@@ -214,7 +219,11 @@ def test_settings_profiles_migrate_legacy_flat_bass_selection():
     })
     assert settings["active_instrument_profile"] == "bass"
     assert settings["instrument_profiles"]["bass"]["string_count"] == 6
-    assert settings["instrument_profiles"]["bass"]["tuning"] == "C Standard"
+    # The legacy 6-string-bass name migrates to the corrected one: the
+    # pitches [19,24,29,34,39,44] sound lowest G, and extended-range bass is
+    # named off its actual lowest string. Same tuning, right label — and the
+    # alias is what keeps this profile VALID rather than rejected.
+    assert settings["instrument_profiles"]["bass"]["tuning"] == "G Standard"
     assert settings["reference_pitch"] == 432
     assert settings["pathway"] == "practice"
     assert settings["instrument_profiles"]["bass"]["pathway"] == "practice"
@@ -279,3 +288,97 @@ def test_freqs_to_midis_rejects_garbage():
     assert freqs_to_midis([float("inf")]) is None      # non-finite
     assert freqs_to_midis([float("-inf")]) is None     # non-finite
     assert freqs_to_midis([]) == []                    # vacuously fine
+
+
+# ── Extended-range BASS naming (feedBack: 6-string bass read as guitar) ──────
+# A 6-string bass has SIX offsets exactly like a 6-string guitar, but its
+# lowest string is B, not E. `tuning_name` gated its guitar ladder on
+# `len(offsets) == 6` alone, so a bass got guitar names: an all-zeros bass
+# read "E Standard" (it is Standard/B) and a whole-step-down bass read
+# "D Standard" (it is A Standard). Reported from a real Sleep Token chart
+# tuned A0 D1 G1 C2 F2 A#2; the player called it A standard and was right.
+# Convention (bass- and guitar-pedagogy seats, 2026-07-18): name extended
+# range by the ACTUAL lowest string, matching the 7-string guitar presets.
+
+def test_bass_perspective_keeps_proven_six_string_tuning_but_truncates_padding():
+    bass = PERSPECTIVES["bass"]
+
+    # Legacy four-string Rocksmith data pads its unused tail with zeroes.
+    assert normalize_offsets([-2, -2, -2, -2, 0, 0], bass) == [-2] * 4
+
+    # A uniform non-zero six-string tuning cannot be that padding shape.
+    extended = normalize_offsets([-2] * 6, bass)
+    assert extended == [-2] * 6
+    assert perspective_tuning_name(extended, bass) == "A Standard"
+    assert perspective_tuning_key(extended, bass) == "bass:21:26:31:36:41:46"
+    assert perspective_low_pitch(extended, bass) == 21
+
+
+BASS_STANDARD_CASES = [
+    # 4-string bass is E-A-D-G — the guitar ladder's low four, names unchanged.
+    ([0, 0, 0, 0], "E Standard"),
+    ([-1, -1, -1, -1], "Eb Standard"),
+    ([-2, -2, -2, -2], "D Standard"),
+    # 5-string adds a low B → the B ladder.
+    ([0] * 5, "Standard"),
+    ([-1] * 5, "Bb Standard"),
+    ([-2] * 5, "A Standard"),
+    ([-3] * 5, "G# Standard"),
+    ([-4] * 5, "G Standard"),
+    # 6-string: same names, extra top string.
+    ([0] * 6, "Standard"),
+    ([-1] * 6, "Bb Standard"),
+    ([-2] * 6, "A Standard"),
+    ([-3] * 6, "G# Standard"),
+    ([-4] * 6, "G Standard"),
+]
+
+
+@pytest.mark.parametrize("offsets,expected", BASS_STANDARD_CASES)
+def test_bass_standard_tunings(offsets, expected):
+    assert tuning_name(offsets, is_bass=True) == expected
+
+
+def test_six_offsets_alone_do_not_imply_a_guitar():
+    """The regression the bug report came from."""
+    sleep_token = [-2] * 6           # A0 D1 G1 C2 F2 A#2
+    assert tuning_name(sleep_token, is_bass=True) == "A Standard"
+    # ...and the identical offsets on a guitar keep the guitar name.
+    assert tuning_name(sleep_token) == "D Standard"
+    # A STANDARD 6-string bass is not "E Standard" either.
+    assert tuning_name([0] * 6, is_bass=True) == "Standard"
+    assert tuning_name([0] * 6) == "E Standard"
+
+
+def test_bass_drop_tunings_name_the_resulting_low_string():
+    # 5-string B standard, low string dropped a whole step → A.
+    assert tuning_name([-2, 0, 0, 0, 0], is_bass=True) == "Drop A"
+    # 4-string E standard → D.
+    assert tuning_name([-2, 0, 0, 0], is_bass=True) == "Drop D"
+
+
+def test_bass_presets_are_named_off_their_lowest_string():
+    """Every bass preset's name must match the note its low string sounds."""
+    names = ["C", "C#", "D", "Eb", "E", "F", "F#", "G", "Ab", "A", "Bb", "B"]
+    alt = {"Ab": "G#", "G#": "Ab", "Bb": "A#", "A#": "Bb", "Eb": "D#", "D#": "Eb"}
+    for key in ("bass-4", "bass-5", "bass-6"):
+        for name, midis in TUNING_PRESET_MIDIS[key].items():
+            if not name.endswith("Standard") or name == "Standard":
+                continue
+            root = name.rsplit(" ", 1)[0]
+            low = names[midis[0] % 12]
+            assert root in (low, alt.get(low)), (
+                f"{key} {name!r} lowest string sounds {low}"
+            )
+
+
+def test_superseded_bass_names_migrate_instead_of_being_rejected():
+    """Renaming must not invalidate saved profiles (both pedagogy seats)."""
+    for key in ("bass-5", "bass-6"):
+        assert _valid_tuning_for_key(key, "D Standard") == "A Standard"
+        assert _valid_tuning_for_key(key, "C Standard") == "G Standard"
+        # Current names still pass straight through.
+        assert _valid_tuning_for_key(key, "A Standard") == "A Standard"
+    # The rename must not leak into other instruments.
+    assert _valid_tuning_for_key("guitar-6", "D Standard") == "D Standard"
+    assert _valid_tuning_for_key("bass-4", "D Standard") == "D Standard"