fix(count-in): follow the song's meter and its pickup measure

The count-in always clicked exactly four beats, so a 3/4 song was counted
in 4/4, and a song opening with a pickup (anacrusis) had the pickup enter
where the downbeat belonged — putting the player a beat ahead all song.

Bar length now comes from the song_timeline beats already on the highway
(measure >= 0 marks downbeats), so no new plumbing: the time_signatures
map is streamed to plugins rather than stored in the frontend. A first bar
shorter than that meter shortens the count by its length — a 1-beat pickup
in 4/4 counts "1 2 3" and the music enters on 4.

Bar length is the mode of the downbeat gaps, not the first gap, so a
pickup's own short gap can't be read as the meter; the beats trailing the
last downbeat count as a candidate too, or a song of pickup + one bar
offers only the pickup's gap. Pickup shortening is scoped to the song's
first bar — a short bar elsewhere is a meter change, and is counted by its
own length instead. Songs without beats (pre-chart, minigames, synthetic
highways) still get four.

Applies to both count-in paths: loop wrap / section practice, and the
start-of-song 'Countdown before song' setting.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: gionnibgud <gionnibgud@gmail.com>

CHANGELOG.md

@@ -8,6 +8,28 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
+- **Drum-part picker (feedpak 1.17.0 "drums as arrangements").** When a song
+  carries several drum charts, a **Drum part** selector appears beside the
+  arrangement switcher (advanced settings) so a player can choose which drummer
+  to play. Selecting one re-streams that part's tab over the highway WS
+  (`?drum_part=<id>`, mirroring the arrangement switch); the choice persists
+  across an arrangement change, and the picker reflects the server's
+  authoritative part (unknown/absent selection falls back to the primary). The
+  row hides for single-drum and non-drum songs, so nothing changes there. Builds
+  on the loader below; no plugin change needed — the drum renderer just draws
+  whatever tab streams.
+- **Multiple drum parts (feedpak 1.17.0 "drums as arrangements").** The sloppak
+  loader now reads `type: drums` arrangement entries carrying per-arrangement
+  `drum_tab` file pointers — a song can ship several drum charts (a second
+  drummer, an aux-percussion layer). Parts surface as `LoadedSloppak.drum_parts`
+  (primary first; the entry aliasing the song-level `drum_tab:` key is the
+  primary and is never loaded twice), the highway WS `song_info` gains a
+  `drum_parts` name list, and `?drum_part=<id>` on the WS URL selects which
+  part's tab streams (`drum_tab` messages carry `part_id` when multiple parts
+  exist; unknown ids fall back to the primary). Pointer entries are **never**
+  loaded as fretted arrangements — the loader's file/notation gate keeps a drum
+  part out of the fretted pipeline (and out of note-detection grading), pinned
+  by test. Legacy single-drum packs read exactly as before, as a one-part list.
 - **`chart-transform` capability domain (#952)** — plugins can now remap the
   chart before rendering and scoring through a core-owned provider
   coordinator. Synchronous transforms run after difficulty filtering; host
@@ -253,6 +275,17 @@ 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
+- **Count-in follows the song's meter and its pickup measure.** The count-in
+  (loop wrap, section practice, and the "Countdown before song" setting) always
+  clicked exactly four beats, so a 3/4 song was counted in 4/4, and a song
+  opening with a pickup (anacrusis) had the pickup enter where the downbeat
+  belonged — putting the player a beat ahead for the whole song. The bar length
+  now comes from the `song_timeline` beats already on the highway
+  (`measure >= 0` marks downbeats; no new plumbing, since the `time_signatures`
+  map is streamed to plugins rather than stored in the frontend), and a first
+  bar shorter than that meter shortens the count by its length: a 1-beat pickup
+  in 4/4 counts "1 2 3" and the music enters on 4. Songs without beats — pre-chart,
+  minigames, synthetic highways — still get four.
 - **GP8 asset resolution honours the directory the registry named.**
   `<EmbeddedFilePath>` is matched on filename stem so a format variant of the
   same recording can win (an `.ogg` beside the declared `.mp3` is copied out

lib/routers/ws_highway.py

@@ -26,6 +26,7 @@ from fastapi import APIRouter, WebSocket, WebSocketDisconnect
 
 from song import (
     anchor_to_wire,
+    arrangement_is_bass,
     arrangement_string_count,
     base_open_string_midis,
     chord_template_to_wire,
@@ -143,9 +144,21 @@ def _sanitize_authors(manifest: dict | None) -> list[dict]:
     return out
 
 
+def _drum_part_id_for_wire(drum_parts: list[dict] | None, selected_id: str | None) -> str | None:
+    """Expose a part id only when the pack genuinely has multiple parts."""
+    return selected_id if selected_id is not None and len(drum_parts or []) > 1 else None
+
+
 @router.websocket("/ws/highway/{filename:path}")
-async def highway_ws(websocket: WebSocket, filename: str, arrangement: int = -1, naming_mode: str = "legacy"):
-    """Stream song data for the highway renderer over WebSocket."""
+async def highway_ws(websocket: WebSocket, filename: str, arrangement: int = -1,
+                     naming_mode: str = "legacy", drum_part: str = ""):
+    """Stream song data for the highway renderer over WebSocket.
+
+    `drum_part` selects WHICH drum part's tab streams when the pack carries
+    several (feedpak 1.17.0 "drums as arrangements") — a part id from
+    song_info's `drum_parts`. Empty / unknown ids fall back to the primary,
+    so a stale or mistyped selection degrades to today's behavior instead of
+    silencing drums."""
     await websocket.accept()
     structlog.contextvars.bind_contextvars(ws_conn_id=uuid.uuid4().hex[:8])
 
@@ -261,9 +274,8 @@ async def highway_ws(websocket: WebSocket, filename: str, arrangement: int = -1,
                 bass_idxs = [
                     i
                     for i, a in enumerate(song.arrangements)
-                    if getattr(a, "path_bass", False)
+                    if arrangement_is_bass(a)
                     or (smart_names[i] or "").lower().startswith("bass")
-                    or "bass" in (getattr(a, "name", "") or "").lower()
                 ]
                 if bass_idxs:
                     # Among the bass parts: (1) honor the saved default-arrangement
@@ -564,6 +576,15 @@ async def highway_ws(websocket: WebSocket, filename: str, arrangement: int = -1,
             "has_drum_tab": bool(
                 is_slop and loaded_slop is not None and loaded_slop.drum_tab is not None
             ),
+            # The song's DRUM PARTS (feedpak 1.17.0 "drums as arrangements"),
+            # primary first — names only; the selected part's payload streams
+            # as the `drum_tab`/`drum_hits` messages below. Always a list
+            # (empty when the pack has no drums, and a single entry for a
+            # legacy one-drum pack), so a part picker can bind unconditionally.
+            "drum_parts": [
+                {"id": p["id"], "name": p["name"]}
+                for p in (loaded_slop.drum_parts or [])
+            ] if is_slop and loaded_slop is not None else [],
             "has_notation": bool(
                 is_slop
                 and loaded_slop is not None
@@ -587,18 +608,36 @@ async def highway_ws(websocket: WebSocket, filename: str, arrangement: int = -1,
         # client-side drums plugin keeps a fallback decoder for them.
         if is_slop and loaded_slop is not None and loaded_slop.drum_tab is not None:
             dt = loaded_slop.drum_tab
+            # Multiple drum parts: `?drum_part=<id>` picks which part's tab
+            # streams; the default (and any unknown id) is the PRIMARY —
+            # exactly the pre-parts behavior, so legacy clients notice nothing.
+            _dt_part_id = None
+            if loaded_slop.drum_parts:
+                _dt_part_id = loaded_slop.drum_parts[0]["id"]
+                if drum_part:
+                    for _p in loaded_slop.drum_parts:
+                        if _p["id"] == drum_part:
+                            dt = _p["drum_tab"]
+                            _dt_part_id = _p["id"]
+                            break
             kit = drums_mod.normalise_kit(dt.get("kit"))
             hits_wire = drums_mod.hits_to_wire(dt.get("hits") or [])
             _dt_name = dt.get("name")
             _dt_name = _dt_name if isinstance(_dt_name, str) and _dt_name else "Drums"
+            _dt_msg = {
+                "type": "drum_tab",
+                "version": int(dt.get("version", drums_mod.SCHEMA_VERSION)),
+                "name": _dt_name,
+                "kit": kit,
+                "total": len(hits_wire),
+            }
+            # Only multi-part packs identify a part on the wire. Legacy packs
+            # synthesize a one-item list internally but keep their old frame.
+            _wire_part_id = _drum_part_id_for_wire(loaded_slop.drum_parts, _dt_part_id)
+            if _wire_part_id is not None:
+                _dt_msg["part_id"] = _wire_part_id
             try:
-                await websocket.send_json({
-                    "type": "drum_tab",
-                    "version": int(dt.get("version", drums_mod.SCHEMA_VERSION)),
-                    "name": _dt_name,
-                    "kit": kit,
-                    "total": len(hits_wire),
-                })
+                await websocket.send_json(_dt_msg)
                 for i in range(0, len(hits_wire), 500):
                     await websocket.send_json({
                         "type": "drum_hits",
@@ -975,7 +1014,7 @@ async def highway_ws(websocket: WebSocket, filename: str, arrangement: int = -1,
         # base[string] + offset + capo + fret (matches the tuner / open-string
         # labels). arrangement_string_count is O(notes), so compute once here.
         _base = base_open_string_midis(
-            arrangement_string_count(arr), "bass" in (arr.name or "").lower())
+            arrangement_string_count(arr), arrangement_is_bass(arr))
         _capo = int(getattr(arr, "capo", 0) or 0)
 
         def _fill_scale_degree(wire: dict, n, t: float) -> None:

lib/sloppak.py

@@ -730,6 +730,125 @@ class LoadedSloppak:
     # separated stems the moment one drops below 100% — demucs recombination is
     # lossy, so the mixdown is strictly the better audio when nothing is muted.
     full_mix: str | None = None
+    # The song's DRUM PARTS (feedpak 1.17.0 "drums as arrangements"): one dict
+    # {"id", "name", "drum_tab"} per part, primary FIRST. A part comes from a
+    # `type: drums` arrangement entry carrying a per-arrangement `drum_tab`
+    # file pointer and NO note `file` — entries this loader deliberately never
+    # turns into fretted Arrangements (see the file/notation gate in
+    # load_song; that skip IS the grading invariant). The primary part's
+    # payload is the SAME object as `drum_tab` above (the song-level key is
+    # its back-compat alias). None when the pack has no drums at all; a
+    # single-part list for a legacy pack with only the song-level key.
+    drum_parts: list[dict] | None = None
+
+
+def _load_drum_tab_file(source_dir: Path, rel: str, label: str) -> dict | None:
+    """Load + schema-validate one drum-tab JSON named by a manifest-relative
+    path. Shared by the song-level `drum_tab:` key and the per-arrangement
+    drum-part pointers (feedpak 1.17.0), so every tab gets the same posture:
+    permissive — a missing file disables that part silently; a traversal,
+    parse, or validation failure disables it with a warning, never aborting
+    the load."""
+    # Constrain to source_dir to prevent a crafted manifest from reading
+    # files outside the sloppak directory via path traversal (e.g. ../../etc).
+    # Wrap both resolve() calls in a broad handler: symlink loops and
+    # permission errors on .resolve() should disable drums, not abort load.
+    try:
+        dt_path = (source_dir / rel).resolve()
+        dt_path.relative_to(source_dir.resolve())
+    except ValueError:
+        log.warning("sloppak: %s path %r escapes source_dir — skipped", label, rel)
+        return None
+    except OSError as e:
+        log.warning("sloppak: %s path resolution failed (%s) — skipped", label, e)
+        return None
+    if not dt_path.exists():
+        return None
+    try:
+        raw = load_json(dt_path)
+    except Exception as e:
+        log.warning("sloppak: failed to parse %s %r: %s", label, rel, e)
+        return None
+    ok, reason = drums_mod.validate_drum_tab(raw)
+    if not ok:
+        log.warning("sloppak: %s %r failed validation: %s", label, rel, reason)
+        return None
+    return raw
+
+
+def _resolve_drum_parts(
+    source_dir: Path,
+    drum_tab_rel: object,
+    drum_tab_data: dict | None,
+    drum_pointer_entries: list[dict],
+) -> tuple[dict | None, list[dict] | None]:
+    """Resolve drum pointers into a primary-first list with unique ids."""
+    if drum_tab_data is None and not drum_pointer_entries:
+        return drum_tab_data, None
+
+    primary_id = "drums"
+    primary_name = None
+    extra_parts: list[dict] = []
+    seen_rels: set[str] = set()
+    # Use the same canonical, traversal-safe identity as zip member lookup so
+    # equivalent spellings ("x.json", "./x.json", or backslashes) identify
+    # one file. Otherwise an alias pointer can reload and duplicate the primary.
+    primary_rel_key = (
+        _zip_member_key(drum_tab_rel.strip())
+        if isinstance(drum_tab_rel, str) and drum_tab_rel.strip() else None
+    )
+    for entry in drum_pointer_entries:
+        rel = str(entry.get("drum_tab") or "").strip()
+        rel_key = _zip_member_key(rel) if rel else None
+        rel_identity = rel_key or rel
+        if not rel or rel_identity in seen_rels:
+            continue
+        seen_rels.add(rel_identity)
+        entry_id = str(entry.get("id") or "").strip()
+        entry_name = str(entry.get("name") or "").strip()
+        if primary_rel_key is not None and rel_key == primary_rel_key:
+            if entry_id:
+                primary_id = entry_id
+            if entry_name:
+                primary_name = entry_name
+            continue
+        tab = _load_drum_tab_file(source_dir, rel, f"drum part {entry_id or rel}")
+        if tab is None:
+            continue
+        tab_name = tab.get("name")
+        extra_parts.append({
+            "id": entry_id,
+            "name": entry_name
+                or (tab_name if isinstance(tab_name, str) and tab_name else "Drums"),
+            "drum_tab": tab,
+        })
+
+    parts: list[dict] = []
+    used_ids: set[str] = set()
+    if drum_tab_data is not None:
+        if primary_name is None:
+            tab_name = drum_tab_data.get("name")
+            primary_name = tab_name if isinstance(tab_name, str) and tab_name else "Drums"
+        parts.append({"id": primary_id, "name": primary_name, "drum_tab": drum_tab_data})
+        used_ids.add(primary_id)
+
+    next_generated_id = 2
+    for part in extra_parts:
+        part_id = part["id"]
+        if not part_id or part_id in used_ids:
+            while f"drums-{next_generated_id}" in used_ids:
+                next_generated_id += 1
+            part_id = f"drums-{next_generated_id}"
+            next_generated_id += 1
+        part["id"] = part_id
+        used_ids.add(part_id)
+        parts.append(part)
+
+    if not parts:
+        return drum_tab_data, None
+    if drum_tab_data is None:
+        drum_tab_data = parts[0]["drum_tab"]
+    return drum_tab_data, parts
 
 
 def load_song(
@@ -754,6 +873,7 @@ def load_song(
     notation_acc: dict[str, dict] = {}
     any_notation = False
     arrangement_ids_acc: list[str | None] = []  # parallel to song.arrangements
+    drum_pointer_entries: list[dict] = []  # feedpak 1.17.0 drum-part pointers
     for entry in manifest.get("arrangements", []) or []:
         if not isinstance(entry, dict):
             log.warning("sloppak: non-dict arrangement entry skipped (%r)", type(entry).__name__)
@@ -762,7 +882,30 @@ def load_song(
         rel = rel_raw.strip() if isinstance(rel_raw, str) else ""
         notation_raw = entry.get("notation")
         has_notation_key = isinstance(notation_raw, str) and bool(notation_raw.strip())
-        if not rel and not has_notation_key:
+        _etype = str(entry.get("type") or "").strip().lower()
+        is_drums = _etype in ("drums", "drum")
+        # A drums-typed entry MUST NEVER become a fretted Arrangement (grading
+        # invariant, spec §5.2/§7.5): route on `type` FIRST, not on file
+        # absence — a malformed drums entry that also carries a note file/
+        # notation would otherwise fall through and grade as garbage.
+        if is_drums or (not rel and not has_notation_key):
+            # A DRUM-PART POINTER entry (feedpak 1.17.0 "drums as
+            # arrangements"): `type: drums` with a per-arrangement `drum_tab`
+            # file. Collect it for the drum-parts load after this loop.
+            if is_drums and isinstance(entry.get("drum_tab"), str):
+                drum_pointer_entries.append(entry)
+            elif is_drums:
+                # Drums-typed but no drum_tab pointer — drop it (any note
+                # file/notation it carries is ignored), never fret it.
+                log.warning(
+                    "sloppak: drums-typed arrangement entry %r has no drum_tab pointer — dropped",
+                    entry.get("id"),
+                )
+            elif isinstance(entry.get("drum_tab"), str):
+                log.warning(
+                    "sloppak: arrangement entry has drum_tab %r but type=%r — ignored",
+                    entry.get("drum_tab"), entry.get("type"),
+                )
             continue
         data = None
         if rel:
@@ -792,6 +935,11 @@ def load_song(
         # the arrangement JSON (name, tuning, capo, centOffset).
         if entry.get("name"):
             arr.name = str(entry["name"])
+        # Editor-authored instrument type (feedpak-spec §5.2 / editor PR #335).
+        # Drives arrangement_string_count's bass fallback so a bass authored on
+        # an arrangement whose NAME doesn't say "bass" still reports 4 strings.
+        if entry.get("type"):
+            arr.type = str(entry["type"]).strip().lower()
         if "tuning" in entry:
             arr.tuning = list(entry["tuning"])
         if "capo" in entry:
@@ -868,32 +1016,13 @@ def load_song(
     drum_tab_data: dict | None = None
     drum_tab_rel = manifest.get("drum_tab")
     if isinstance(drum_tab_rel, str) and drum_tab_rel:
-        # Constrain to source_dir to prevent a crafted manifest from reading
-        # files outside the sloppak directory via path traversal (e.g. ../../etc).
-        # Wrap both resolve() calls in a broad handler: symlink loops and
-        # permission errors on .resolve() should disable drums, not abort load.
-        try:
-            dt_path = (source_dir / drum_tab_rel).resolve()
-            dt_path.relative_to(source_dir.resolve())
-        except ValueError:
-            log.warning("sloppak: drum_tab path %r escapes source_dir — skipped", drum_tab_rel)
-            dt_path = None
-        except OSError as e:
-            log.warning("sloppak: drum_tab path resolution failed (%s) — skipped", e)
-            dt_path = None
-        if dt_path is not None and dt_path.exists():
-            try:
-                raw = load_json(dt_path)
-            except Exception as e:
-                log.warning("sloppak: failed to parse drum_tab %r: %s", drum_tab_rel, e)
-                raw = None
-            if raw is not None:
-                ok, reason = drums_mod.validate_drum_tab(raw)
-                if ok:
-                    drum_tab_data = raw
-                else:
-                    log.warning("sloppak: drum_tab %r failed validation: %s",
-                                drum_tab_rel, reason)
+        drum_tab_data = _load_drum_tab_file(source_dir, drum_tab_rel, "drum_tab")
+
+    # Keep the dense compatibility logic independently testable and guarantee
+    # ids are unique before the highway exposes them as selectors.
+    drum_tab_data, drum_parts = _resolve_drum_parts(
+        source_dir, drum_tab_rel, drum_tab_data, drum_pointer_entries,
+    )
 
     # Drum-only sloppak: every GP track was percussion, so it ships a
     # drum_tab but no pitched arrangements. The highway WS rejects an empty
@@ -1221,6 +1350,7 @@ def load_song(
         manifest=manifest,
         feedpak_version=_fpv if isinstance(_fpv, str) and _fpv else None,
         drum_tab=drum_tab_data,
+        drum_parts=drum_parts,
         song_timeline=song_timeline_data,
         tempos=tempos_data,
         time_signatures=time_sigs_data,

lib/song.py

@@ -182,6 +182,12 @@ class Arrangement:
     # `base`/`changes` drive the highway tone-change markers; `definitions`
     # feed the Tones plugin gear panel.
     tones: dict | None = None
+    # Editor-authored instrument type (feedpak-spec §5.2 / editor PR #335):
+    # "bass" | "guitar" | "piano" | "keys" | "drums" | "". First-class DATA that
+    # lets a user author an instrument on an arrangement whose NAME doesn't say
+    # so. Lifted from the sloppak manifest entry by sloppak.load_song(); "" for
+    # archive/loose sources, which instead carry the path_* flags below.
+    type: str = ""
     # arrangement XML <arrangementProperties> flags for smart naming (feedBack feat/arrangement).
     # Populated from the XML; default False/0 for sloppak / GP-imported sources.
     path_lead: bool = False
@@ -503,8 +509,8 @@ def note_pitch_midi(arr: "Arrangement", note: "Note") -> int | None:
     O(notes) via ``arrangement_string_count`` — for a whole arrangement, hoist
     the base with :func:`base_open_string_midis` and call :func:`pitch_from_base`
     per note instead."""
-    is_bass = "bass" in (arr.name or "").lower()
-    base = base_open_string_midis(arrangement_string_count(arr), is_bass)
+    base = base_open_string_midis(arrangement_string_count(arr),
+                                  arrangement_is_bass(arr))
     return pitch_from_base(base, int(getattr(arr, "capo", 0) or 0),
                            arr.tuning or [], note.string, note.fret)
 
@@ -633,6 +639,23 @@ def phrase_from_wire(d: dict) -> Phrase:
     )
 
 
+def arrangement_is_bass(arr: Arrangement) -> bool:
+    """Whether ``arr`` is a bass, most-authoritative signal first: an
+    editor-authored ``type == "bass"`` (feedpak-spec §5.2 / editor PR #335,
+    lifted onto sloppaks by :func:`sloppak.load_song`), then the archive
+    ``path_bass`` <arrangementProperties> flag, then the legacy "bass"
+    case-insensitive substring in the name. Single source of the bass decision
+    so string-count derivation and the open-string pitch base (via
+    :func:`base_open_string_midis`) agree — a bass authored on an arrangement
+    whose NAME doesn't say "bass" must get both 4 lanes AND the bass MIDI base,
+    not 4 lanes on a guitar octave."""
+    return (
+        (arr.type or "").strip().lower() == "bass"
+        or bool(arr.path_bass)
+        or "bass" in (arr.name or "").lower()
+    )
+
+
 def arrangement_string_count(arr: Arrangement) -> int:
     """Derive the active arrangement's string count.
 
@@ -650,10 +673,17 @@ def arrangement_string_count(arr: Arrangement) -> int:
        But this is a LOWER BOUND only — a 6-string lead chart that
        never plays string 5 reports 5, undercounting by 1.
 
-    2. **Name-based fallback.** Arrangements named "Bass" (case-
-       insensitive substring match) default to 4; everything else
-       defaults to 6. This catches the partial-string-usage case
-       where notes don't span all the instrument's strings.
+    2. **Instrument-type fallback.** An arrangement whose authoritative
+       instrument signal says bass defaults to 4; everything else
+       defaults to 6. This catches the partial-string-usage case where
+       notes don't span all the instrument's strings. The bass signal is
+       any of: an editor-authored ``type == "bass"`` (feedpak-spec §5.2 /
+       editor PR #335 — lifted onto sloppaks by ``sloppak.load_song``),
+       the ``path_bass`` <arrangementProperties> flag (archive/DLC
+       sources), or the legacy "bass" case-insensitive substring in the
+       name. Trusting ``type``/``path_bass`` closes the gap where a user
+       authors a bass instrument on an arrangement whose NAME doesn't say
+       "bass" (the editor lays out 4 lanes; core must agree).
 
     A third signal — ``len(arr.tuning)`` when it isn't the arrangement XML
     padded value of 6 — folds in for sloppak / GP-imported sources
@@ -684,6 +714,10 @@ def arrangement_string_count(arr: Arrangement) -> int:
       max(0, 4, 0) = 4
     * Empty arrangement named "Lead" (tuning len 6) →
       max(0, 6, 0) = 6
+    * Editor-authored bass named "Low End" (type "bass", tuning len 6,
+      notes 0..3) → name_based=4 → max(4, 4, 0) = 4
+    * DLC bass named "Low End" (pathBass flag set, tuning len 6, notes
+      0..3) → name_based=4 → max(4, 4, 0) = 4
 
     Topkoa's issue argues plugins shouldn't do arrangement-name
     matching; server-side fallback IS the right place for it
@@ -699,7 +733,9 @@ def arrangement_string_count(arr: Arrangement) -> int:
             if cn.string > max_s:
                 max_s = cn.string
     notes_count = max_s + 1 if max_s >= 0 else 0
-    name_based = 4 if "bass" in arr.name.lower() else 6
+    # Bass signal (type / pathBass / name substring) — see arrangement_is_bass.
+    # Any one being bass pulls the fallback to 4.
+    name_based = 4 if arrangement_is_bass(arr) else 6
     # Tuning-length signal — only trustworthy when NOT the arrangement XML
     # padded value of 6. Length 4/5 indicates explicit bass / 5-string
     # bass; length 7/8 indicates an extended-range guitar from GP.

static/app.js

@@ -1150,7 +1150,7 @@ window.feedBack.on('song:ready', () => {
 let _arrBusyGen = 0;
 let _arrBusyTimeout = null;
 
-async function changeArrangement(index) {
+async function changeArrangement(index, drumPart) {
     if (currentFilename) {
         // Tear down any pending fresh-load credits before switching: the
         // no-count-in hold timer would otherwise fire togglePlay() against the
@@ -1276,11 +1276,38 @@ async function changeArrangement(index) {
         _resetSectionPracticeLog();
         invalidateParentCount();
 
-        window.highway.reconnect(currentFilename, index);
+        // Carry the selected drum part across the re-stream. An explicit
+        // `drumPart` (a drum-part switch, from changeDrumPart) wins; otherwise
+        // preserve the current picker selection so an ARRANGEMENT switch keeps
+        // the chosen part (drum parts are song-level, not per-arrangement).
+        const part = drumPart !== undefined
+            ? drumPart
+            : (document.getElementById('drum-part-select')?.value || '');
+        window.highway.reconnect(currentFilename, index, part);
         window.feedBack.emit('arrangement:changed', { index, filename: currentFilename });
     }
 }
 
+// Switch which drum part plays (feedpak 1.17.0 "drums as arrangements"). A part
+// switch re-streams the same song with a different drum tab — the same
+// transition as an arrangement switch — so it delegates to changeArrangement
+// with the CURRENT arrangement held and the new part applied. Wired to
+// #drum-part-select's onchange; the select is populated + shown by
+// highway.js's song_info handler only when the song has 2+ drum parts.
+async function changeDrumPart(partId) {
+    if (!currentFilename) return;
+    let index = 0;
+    const si = window.highway && typeof window.highway.getSongInfo === 'function'
+        ? window.highway.getSongInfo() : null;
+    if (si && typeof si.arrangement_index === 'number' && si.arrangement_index >= 0) {
+        index = si.arrangement_index;
+    } else {
+        const arrSel = document.getElementById('arr-select');
+        if (arrSel && arrSel.value !== '') index = Number(arrSel.value) || 0;
+    }
+    return changeArrangement(index, partId);
+}
+
 // Restart the current song from the beginning (or from loop A when an A–B
 // loop is armed). Uses the canonical _audioSeek funnel only — never touches
 // audio.currentTime directly and never reloads via playSong().
@@ -2325,7 +2352,7 @@ configureHost({
 Object.assign(window, {
     _confirmDialog, _getArrangementNamingMode, _libraryLocalFilename, _librarySongArtUrl,
     _librarySongId, _onHeaderClick, _onNamingModeChange, _trapFocusInModal,
-    changeArrangement, checkPluginUpdates, clearLibFilters, clearLoop,
+    changeArrangement, changeDrumPart, checkPluginUpdates, clearLibFilters, clearLoop,
     deleteSelectedLoop, esc, exportDiagnostics, exportSettings, filterFavorites,
     filterLibrary, fullRescanLibrary, goFavPage, handleSliderInput,
     hideScanBanner, importSettings, loadPlugins, loadSavedLoop,

static/highway.js

@@ -2298,6 +2298,31 @@ function createHighway() {
                                         sel.appendChild(opt);
                                     }
                                 }
+                                // Drum-part picker (feedpak 1.17.0 "drums as
+                                // arrangements"): a song can carry several drum
+                                // charts. Populate the picker beside the
+                                // arrangement switcher; show it only when there
+                                // are 2+ parts to choose between. `drum_parts`
+                                // is always present (empty for non-drum songs),
+                                // so a single-drum / no-drum song hides it. The
+                                // currently-streaming part is marked selected by
+                                // the `drum_tab` handler below (authoritative
+                                // `part_id`), so we don't guess here.
+                                {
+                                    const dpSel = document.getElementById('drum-part-select');
+                                    if (dpSel) {
+                                        const parts = Array.isArray(msg.drum_parts) ? msg.drum_parts : [];
+                                        dpSel.textContent = '';
+                                        for (const p of parts) {
+                                            const opt = document.createElement('option');
+                                            opt.value = p.id;
+                                            opt.textContent = p.name || p.id;
+                                            dpSel.appendChild(opt);
+                                        }
+                                        const dpRow = document.getElementById('v3-drum-part-row');
+                                        if (dpRow) dpRow.classList.toggle('hidden', parts.length <= 1);
+                                    }
+                                }
                             }
                             // Plugin context API — broadcast current song state
                             if (window.feedBack) {
@@ -2380,7 +2405,22 @@ function createHighway() {
                                 name: (typeof msg.name === 'string' && msg.name) ? msg.name : 'Drums',
                                 kit: Array.isArray(msg.kit) ? msg.kit : [],
                                 hits: [],
+                                // Which drum part this stream carries (feedpak
+                                // 1.17.0). Present only for multi-part packs;
+                                // null otherwise. Plugins can read it via
+                                // bundle.drumTab.part_id.
+                                part_id: (typeof msg.part_id === 'string' && msg.part_id) ? msg.part_id : null,
                             };
+                            // Reflect the authoritative streaming part in the
+                            // picker (the server resolves an unknown/absent
+                            // selection to the primary, so this keeps the
+                            // dropdown honest even after a fallback).
+                            if (hwState.drumTab.part_id) {
+                                const dpSel = document.getElementById('drum-part-select');
+                                if (dpSel && dpSel.value !== hwState.drumTab.part_id) {
+                                    dpSel.value = hwState.drumTab.part_id;
+                                }
+                            }
                             break;
                         case 'drum_hits':
                             if (hwState.drumTab && Array.isArray(msg.data)) {
@@ -2773,7 +2813,7 @@ function createHighway() {
             localStorage.setItem('showFingerHints', String(hwState._showFingerHints));
         },
 
-        reconnect(filename, arrangement) {
+        reconnect(filename, arrangement, drumPart) {
             // Close old WS but keep audio + animation running
             if (hwState.ws) { hwState.ws.close(); hwState.ws = null; }
             hwState.ready = false;
@@ -2799,6 +2839,11 @@ function createHighway() {
             _resetChordRenderState();
             const wsParams = new URLSearchParams();
             if (arrangement !== undefined) wsParams.set('arrangement', arrangement);
+            // Multiple drum parts (feedpak 1.17.0 "drums as arrangements"):
+            // carry the selected part id so the WS streams ITS drum tab. Empty
+            // / undefined → the primary part (server default), i.e. today's
+            // one-drum behavior for any pack the picker never touched.
+            if (drumPart) wsParams.set('drum_part', drumPart);
             let namingMode = 'smart';
             if (typeof window._getArrangementNamingMode === 'function') {
                 const v = window._getArrangementNamingMode();

static/js/count-in.js

@@ -1,4 +1,4 @@
-// Count-in — the 1-2-3-4 click before playback, plus the song-credits overlay that
+// Count-in — the one-bar click before playback, plus the song-credits overlay that
 // shares its lifecycle and timers.
 //
 // The third slice out of app.js's strongly-connected core, and the first that had to
@@ -40,6 +40,75 @@ export function playClick(high = false) {
     osc.stop(_audioCtx.currentTime + 0.08);
 }
 
+// ── How many clicks lead into `startT` ──────────────────────────────────
+// One bar, derived from the song_timeline beats: `window.highway.getBeats()`
+// is the only meter data the frontend holds (the `time_signatures` map is
+// streamed to plugins, not stored here). Beats carry `measure >= 0` on
+// downbeats, so the gap between consecutive downbeats IS the bar length —
+// which is why a 3/4 song no longer gets four clicks.
+//
+// A first bar shorter than that is a pickup (anacrusis), and the count is
+// shortened by its length so the music enters on its real beat: a 1-beat
+// pickup in 4/4 counts "1 2 3" and the pickup lands on 4. Counting a full
+// four there puts the pickup where the downbeat belongs, and the player comes
+// in a beat late for the whole song.
+export function countInBeats(startT) {
+    const DEFAULT = 4;   // pre-chart, synthetic highway (minigames), or no beats
+    let beats = null;
+    try {
+        if (window.highway && typeof window.highway.getBeats === 'function') {
+            beats = window.highway.getBeats();
+        }
+    } catch (_) { /* fall through to the default */ }
+    if (!Array.isArray(beats) || beats.length < 2) return DEFAULT;
+
+    const downbeats = [];
+    for (let i = 0; i < beats.length; i++) {
+        if (beats[i] && beats[i].measure >= 0) downbeats.push(i);
+    }
+    if (downbeats.length < 2) return DEFAULT;
+
+    // Bar length = the most common gap between downbeats. The mode rather than
+    // the first gap: it ignores a short pickup bar and a short final bar, and
+    // survives an isolated meter change mid-song. The beats trailing the last
+    // downbeat count as a candidate too — otherwise a song of pickup + one bar
+    // offers only the pickup's own gap and the count collapses to it.
+    const gapCounts = new Map();
+    const addGap = (gap) => gapCounts.set(gap, (gapCounts.get(gap) || 0) + 1);
+    for (let k = 1; k < downbeats.length; k++) {
+        addGap(downbeats[k] - downbeats[k - 1]);
+    }
+    addGap(beats.length - downbeats[downbeats.length - 1]);
+    let barLen = DEFAULT;
+    let bestCount = 0;
+    for (const [gap, n] of gapCounts) {
+        // Tie → the longer bar: a pickup's short gap must not outvote the
+        // real meter when the song is too short to repeat it.
+        if (n > bestCount || (n === bestCount && gap > barLen)) {
+            barLen = gap;
+            bestCount = n;
+        }
+    }
+
+    // The beat playback resumes on. The 50 ms tolerance matches the seek
+    // precision the loop-wrap path already assumes.
+    const startIdx = beats.findIndex(b => b && b.time >= startT - 0.05);
+    if (startIdx === -1) return barLen;                  // past the last beat
+    if (!(beats[startIdx].measure >= 0)) return barLen;  // resuming mid-bar
+
+    const nextDownbeat = downbeats.find(d => d > startIdx);
+    if (nextDownbeat === undefined) return barLen;       // the last downbeat
+    const thisBar = nextDownbeat - startIdx;
+    if (thisBar <= 0) return barLen;
+
+    // Only the song's FIRST bar can be a pickup. A short bar anywhere else is
+    // a meter change (or a truncated final bar), and counting it as a pickup
+    // would leave almost no count-in at all — so elsewhere we simply count
+    // that bar's own length, which is also what a mid-song meter change wants.
+    if (startIdx === downbeats[0] && thisBar < barLen) return barLen - thisBar;
+    return thisBar;
+}
+
 let _countingIn = false;
 let _countOverlay = null;
 // Generation token so teardown can cancel an in-progress count-in. Each
@@ -273,12 +342,15 @@ export async function startCountIn(opts = {}) {
     function beginCount() {
         const bpm = window.highway.getBPM(loopA);
         const beatInterval = 60 / bpm;
+        // One bar of the meter at loop A (a short bar there is counted short,
+        // same as the song-start pickup).
+        const clicks = countInBeats(loopA);
         let count = 0;
 
         function tick() {
             if (gen !== _countInGen) return; // teardown mid-count
             count++;
-            if (count > 4) {
+            if (count > clicks) {
                 hideCountOverlay();
                 _countingIn = false;
                 if (window._juceMode) {
@@ -320,7 +392,7 @@ export async function startCountIn(opts = {}) {
     }
 }
 
-// Start-of-song count-in: a 4-beat click before playback begins, gated by the
+// Start-of-song count-in: a one-bar click before playback begins, gated by the
 // "Countdown before song" setting (Gameplay tab). Mirrors the loop count-in's
 // overlay + click + gen-token cancellation, but counts from the song's current
 // position (0 at song start) with no loop A/B rewind. startCountIn() is loop-
@@ -340,14 +412,15 @@ export async function startSongCountIn() {
     if (gen !== _countInGen) return; // teardown during pause
     const startT = S.lastAudioTime || 0;
     let bpm = window.highway.getBPM(startT);
-    // Pre-chart / malformed-tempo fallback: 4 beats at 120 BPM (500 ms each).
+    // Pre-chart / malformed-tempo fallback: 120 BPM (500 ms per beat).
     if (!Number.isFinite(bpm) || bpm <= 0) bpm = 120;
     const beatInterval = 60 / bpm;
+    const clicks = countInBeats(startT);
     let count = 0;
     function tick() {
         if (gen !== _countInGen) return; // teardown mid-count
         count++;
-        if (count > 4) {
+        if (count > clicks) {
             hideCountOverlay();
             _countingIn = false;
             // Hand off to the normal play path — togglePlay() flips isPlaying,

static/v3/index.html

@@ -1194,6 +1194,10 @@
                         <button id="arr-default-pin" type="button" onclick="pinCurrentArrangementDefault()" aria-pressed="false" aria-label="Select an arrangement to make it the default" class="v3-pop-pin" title="Select an arrangement to make it the default">☆</button>
                     </span>
                 </div>
+                <div class="v3-pop-row hidden" id="v3-drum-part-row">
+                    <span class="v3-pop-label">Drum part</span>
+                    <select id="drum-part-select" onchange="changeDrumPart(this.value)" class="v3-pop-select max-w-[130px]" title="Which drum chart to play — a song can carry several"></select>
+                </div>
                 <div class="v3-pop-row">
                     <span class="v3-pop-label" id="mastery-slider-label">Difficulty</span>
                     <span class="flex items-center gap-2">

tests/js/count_in_beats.test.js

@@ -0,0 +1,189 @@
+// Verify `countInBeats(startT)` in static/js/count-in.js sizes the count-in
+// to the song's own bar rather than a hardcoded four clicks.
+//
+// Two behaviours are under test:
+//   1. Meter — a 3/4 song gets three clicks, not four.
+//   2. Pickup (anacrusis) — a first bar shorter than the meter shortens the
+//      count so the pickup enters on its real beat (1-beat pickup in 4/4 →
+//      "1 2 3", music on 4). A full four there puts the pickup where the
+//      downbeat belongs and the player comes in a beat late all song.
+//
+// The meter is read from the song_timeline beats (`window.highway.getBeats()`,
+// `measure >= 0` on downbeats) because that is the only meter data the
+// frontend holds — the `time_signatures` map is streamed to plugins, not
+// stored here.
+//
+// Same extraction approach as loop_restart.test.js: pull the function source
+// out of the module and evaluate it in a vm sandbox with a stubbed highway,
+// rather than loading the ESM module and its DOM-coupled imports.
+
+const { test } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+const vm = require('node:vm');
+
+const COUNT_IN_JS = path.join(__dirname, '..', '..', 'static', 'js', 'count-in.js');
+
+// Brace-match the function body out of the source. Brittle by design:
+// a rename fails loudly here rather than silently skipping coverage.
+function extractFunction(src, signature) {
+    const start = src.indexOf(signature);
+    if (start === -1) throw new Error(`extractFunction: '${signature}' not found`);
+    const openBrace = src.indexOf('{', start + signature.length);
+    let depth = 1;
+    let i = openBrace + 1;
+    while (i < src.length && depth > 0) {
+        const ch = src[i];
+        if (ch === '{') depth++;
+        else if (ch === '}') depth--;
+        i++;
+    }
+    if (depth !== 0) throw new Error(`extractFunction: unbalanced braces after '${signature}'`);
+    return src.slice(start, i);
+}
+
+const src = fs.readFileSync(COUNT_IN_JS, 'utf8');
+// Drop the `export` keyword so the body evaluates as a plain declaration.
+const fnSrc = extractFunction(src, 'export function countInBeats')
+    .replace(/^export\s+/, '');
+
+// `beats` is the song_timeline shape: {time, measure}, measure >= 0 only on
+// downbeats. `getBeats` may also be absent entirely (pre-chart / minigame).
+function load(beats) {
+    const sandbox = {
+        window: beats === undefined
+            ? { highway: {} }
+            : { highway: { getBeats: () => beats } },
+    };
+    vm.createContext(sandbox);
+    vm.runInContext(`${fnSrc}; globalThis.__fn = countInBeats;`, sandbox);
+    return sandbox.__fn;
+}
+
+// Build a beats array: `bars` full bars of `beatsPerBar`, optionally preceded
+// by a pickup of `pickup` beats. One beat per 0.5 s throughout.
+function makeBeats({ beatsPerBar = 4, bars = 4, pickup = 0 } = {}) {
+    const out = [];
+    let t = 0;
+    let measure = 0;
+    if (pickup > 0) {
+        for (let i = 0; i < pickup; i++) {
+            out.push({ time: t, measure: i === 0 ? measure : -1 });
+            t += 0.5;
+        }
+        measure++;
+    }
+    for (let b = 0; b < bars; b++) {
+        for (let i = 0; i < beatsPerBar; i++) {
+            out.push({ time: t, measure: i === 0 ? measure : -1 });
+            t += 0.5;
+        }
+        measure++;
+    }
+    return out;
+}
+
+// ── Meter ────────────────────────────────────────────────────────────────
+
+test('countInBeats counts a full bar in 4/4', () => {
+    const countInBeats = load(makeBeats({ beatsPerBar: 4 }));
+    assert.equal(countInBeats(0), 4);
+});
+
+test('countInBeats counts three in 3/4 (was hardcoded four)', () => {
+    const countInBeats = load(makeBeats({ beatsPerBar: 3 }));
+    assert.equal(countInBeats(0), 3);
+});
+
+test('countInBeats counts six in 6/8', () => {
+    const countInBeats = load(makeBeats({ beatsPerBar: 6 }));
+    assert.equal(countInBeats(0), 6);
+});
+
+// ── Pickup (anacrusis) ───────────────────────────────────────────────────
+
+test('countInBeats shortens the count by a 1-beat pickup in 4/4', () => {
+    const countInBeats = load(makeBeats({ beatsPerBar: 4, pickup: 1 }));
+    assert.equal(countInBeats(0), 3, 'counts 1-2-3 so the pickup lands on 4');
+});
+
+test('countInBeats shortens the count by a 2-beat pickup in 4/4', () => {
+    const countInBeats = load(makeBeats({ beatsPerBar: 4, pickup: 2 }));
+    assert.equal(countInBeats(0), 2);
+});
+
+test('countInBeats handles a pickup in 3/4', () => {
+    const countInBeats = load(makeBeats({ beatsPerBar: 3, pickup: 1 }));
+    assert.equal(countInBeats(0), 2);
+});
+
+test('countInBeats finds the meter when the song is only a pickup plus one bar', () => {
+    // Gap counts tie (one 1-beat gap, one 4-beat gap) — the longer bar is the
+    // meter, so this must be 3 rather than 0.
+    const countInBeats = load(makeBeats({ beatsPerBar: 4, bars: 1, pickup: 1 }));
+    assert.equal(countInBeats(0), 3);
+});
+
+// ── Resuming somewhere other than the song top ───────────────────────────
+
+test('countInBeats counts a full bar at a mid-song downbeat, pickup notwithstanding', () => {
+    const beats = makeBeats({ beatsPerBar: 4, pickup: 1 });
+    const countInBeats = load(beats);
+    // Index 5 is the downbeat of the second full bar (1 pickup + 4 beats).
+    assert.equal(beats[5].measure >= 0, true, 'fixture sanity: index 5 is a downbeat');
+    assert.equal(countInBeats(beats[5].time), 4);
+});
+
+test('countInBeats counts a mid-song meter change by that bar, not as a pickup', () => {
+    // 4/4 throughout, except one 3-beat bar at index 8. Treating a short bar
+    // anywhere but the song's first as a pickup would count a single click.
+    const beats = [];
+    let t = 0;
+    const push = (n, measure) => {
+        for (let i = 0; i < n; i++) { beats.push({ time: t, measure: i === 0 ? measure : -1 }); t += 0.5; }
+    };
+    push(4, 0); push(4, 1); push(3, 2); push(4, 3); push(4, 4);
+    const countInBeats = load(beats);
+    assert.equal(beats[8].measure, 2, 'fixture sanity: index 8 opens the 3-beat bar');
+    assert.equal(countInBeats(beats[8].time), 3, 'counts the short bar itself');
+    assert.equal(countInBeats(0), 4, 'the 4/4 opening is unaffected');
+});
+
+test('countInBeats counts a full bar when resuming mid-bar', () => {
+    const beats = makeBeats({ beatsPerBar: 4 });
+    const countInBeats = load(beats);
+    assert.equal(countInBeats(beats[2].time), 4);   // third beat of bar 1
+});
+
+test('countInBeats tolerates a start time slightly past the beat (seek slop)', () => {
+    const countInBeats = load(makeBeats({ beatsPerBar: 4, pickup: 1 }));
+    assert.equal(countInBeats(0.02), 3);
+});
+
+// ── Fallbacks ────────────────────────────────────────────────────────────
+
+test('countInBeats falls back to four without a beats array', () => {
+    assert.equal(load(undefined)(0), 4, 'no getBeats (pre-chart / minigame)');
+    assert.equal(load([])(0), 4, 'empty beats');
+    assert.equal(load(null)(0), 4, 'null beats');
+});
+
+test('countInBeats falls back to four when beats carry no downbeat labels', () => {
+    const beats = [0, 0.5, 1.0, 1.5, 2.0].map(time => ({ time, measure: -1 }));
+    assert.equal(load(beats)(0), 4);
+});
+
+test('countInBeats falls back to four with only one downbeat', () => {
+    const beats = [
+        { time: 0, measure: 0 },
+        { time: 0.5, measure: -1 },
+        { time: 1.0, measure: -1 },
+    ];
+    assert.equal(load(beats)(0), 4);
+});
+
+test('countInBeats counts a full bar past the last beat', () => {
+    const beats = makeBeats({ beatsPerBar: 3 });
+    assert.equal(load(beats)(9999), 3);
+});

tests/js/highway_chart_transform.test.js

@@ -316,7 +316,7 @@ test('anchor zoom helpers read the staged anchors first', () => {
 test('init and reconnect clear the stage but keep the provider', () => {
     const src = fs.readFileSync(highwayJs, 'utf8');
     const initBody = extractBlock(src, 'init(canvasEl, container)');
-    const reconnectBody = extractBlock(src, 'reconnect(filename, arrangement)');
+    const reconnectBody = extractBlock(src, 'reconnect(filename, arrangement, drumPart)');
     assert.match(initBody, /_clearChartTransformStage\(\);/, 'init clears the stage');
     assert.match(reconnectBody, /_clearChartTransformStage\(\);/, 'reconnect clears the stage');
     assert.ok(!/init\([\s\S]{0,2000}_xfProvider = null/.test(src.slice(src.indexOf('const api = {'))),

tests/js/loop_restart.test.js

@@ -88,6 +88,10 @@ function buildSandbox() {
         playClick: () => {},
         showCountOverlay: () => {},
         hideCountOverlay: () => {},
+        // beginCount sizes the count to the bar at loop A; the wrap-path
+        // assertions below don't depend on how many clicks it decides on.
+        // Covered directly in count_in_beats.test.js.
+        countInBeats: () => 4,
 
         // Stubbed DOM access. Anything querying for a button just gets a
         // permissive object that ignores writes.

tests/test_highway_ws_drum_parts.py

@@ -0,0 +1,17 @@
+"""Wire-compatibility coverage for selectable drum parts."""
+
+from routers.ws_highway import _drum_part_id_for_wire
+
+
+def test_single_synthesized_part_keeps_legacy_frame_without_part_id():
+    parts = [{"id": "drums", "name": "Drums", "drum_tab": {}}]
+    assert _drum_part_id_for_wire(parts, "drums") is None
+
+
+def test_multiple_parts_expose_selected_part_id():
+    parts = [
+        {"id": "drums", "name": "Drums", "drum_tab": {}},
+        {"id": "drums-2", "name": "Aux", "drum_tab": {}},
+    ]
+    assert _drum_part_id_for_wire(parts, "drums-2") == "drums-2"
+    assert _drum_part_id_for_wire(parts, None) is None

tests/test_sloppak_drum_parts.py

@@ -0,0 +1,295 @@
+"""Loader coverage for MULTIPLE drum parts (feedpak 1.17.0 "drums as
+arrangements").
+
+A drum part rides the manifest as a `type: drums` arrangement entry carrying
+a per-arrangement `drum_tab` file pointer and NO note `file`. The loader:
+
+  - NEVER turns a pointer entry into a fretted Arrangement — that skip is
+    the grading invariant (an empty drum chart must not reach the fretted
+    pipeline, where note detection would grade it as garbage);
+  - resolves the parts into `LoadedSloppak.drum_parts`, primary FIRST: the
+    entry aliasing the song-level `drum_tab:` file contributes its id/name
+    but is never loaded twice (its payload IS `loaded.drum_tab`);
+  - loads each extra part's file with the same permissive posture as the
+    song-level tab (a bad part disables that part only, never the load);
+  - copes with a pointer-only pack (no song-level key): the first part
+    becomes the primary so every legacy consumer keeps working.
+"""
+
+from __future__ import annotations
+
+import json
+import logging
+from pathlib import Path
+
+import yaml
+
+import sloppak as sloppak_mod
+
+
+def _tab(name: str, hits: list[dict] | None = None) -> dict:
+    return {
+        "version": 1,
+        "name": name,
+        "kit": [{"id": "kick", "name": "Kick"}],
+        "hits": hits if hits is not None else [{"t": 1.0, "p": "kick", "v": 100}],
+    }
+
+
+def _write_pak(root: Path, manifest_extras: dict, files: dict[str, dict | str]) -> Path:
+    """A minimal directory-form sloppak with one Lead arrangement plus the
+    given extra files ({relpath: json-dict-or-raw-text})."""
+    pak = root / f"{root.name}.sloppak"
+    pak.mkdir()
+    arr_dir = pak / "arrangements"
+    arr_dir.mkdir()
+    arr = {
+        "name": "Lead", "tuning": [0, 0, 0, 0, 0, 0], "capo": 0,
+        "notes": [], "chords": [], "anchors": [], "handshapes": [],
+        "templates": [], "beats": [], "sections": [],
+    }
+    (arr_dir / "lead.json").write_text(json.dumps(arr))
+    manifest = {
+        "title": "Test", "artist": "Tester", "album": "", "year": 2026,
+        "duration": 10.0,
+        "arrangements": [
+            {"id": "lead", "name": "Lead", "file": "arrangements/lead.json"},
+        ],
+        "stems": [{"id": "full", "file": "stems/full.ogg", "default": True}],
+    }
+    manifest.update(manifest_extras)
+    (pak / "manifest.yaml").write_text(yaml.safe_dump(manifest, sort_keys=False))
+    for rel, payload in files.items():
+        text = payload if isinstance(payload, str) else json.dumps(payload)
+        (pak / rel).write_text(text)
+    return pak
+
+
+def _load(pak_path: Path, tmp_path: Path):
+    dlc_root = pak_path.parent
+    cache = tmp_path / "cache"
+    cache.mkdir()
+    return sloppak_mod.load_song(pak_path.name, dlc_root, cache)
+
+
+def _two_part_manifest() -> dict:
+    """The exact shape the editor writes: primary alias entry + one extra."""
+    return {
+        "drum_tab": "drum_tab.json",
+        "arrangements": [
+            {"id": "lead", "name": "Lead", "file": "arrangements/lead.json"},
+            {"id": "drums", "name": "Drums", "type": "drums",
+             "drum_tab": "drum_tab.json"},
+            {"id": "drums-2", "name": "Drums (Live)", "type": "drums",
+             "drum_tab": "drum_tab_drums-2.json"},
+        ],
+    }
+
+
+# ── The grading invariant ────────────────────────────────────────────────────
+
+def test_pointer_entries_never_become_fretted_arrangements(tmp_path: Path):
+    pak = _write_pak(tmp_path, _two_part_manifest(), {
+        "drum_tab.json": _tab("Drums"),
+        "drum_tab_drums-2.json": _tab("Drums (Live)"),
+    })
+    loaded = _load(pak, tmp_path)
+    # Only the Lead chart is an Arrangement — neither drum part enters the
+    # fretted pipeline (song.arrangements is what note detection grades).
+    assert [a.name for a in loaded.song.arrangements] == ["Lead"]
+    # And the ids list stays parallel to song.arrangements (skipped entries
+    # contribute nothing) — a misalignment here would remap every chart edit.
+    assert loaded.arrangement_ids == ["lead"]
+
+
+def test_drums_typed_entry_with_note_file_never_frets(tmp_path: Path):
+    # A malformed entry: type:drums but ALSO carrying a note `file`. Keying the
+    # skip on file absence would let it through as a fretted, selectable,
+    # gradeable Arrangement (spec §5.2/§7.5 MUST-NOT). Routing on `type` first
+    # drops it instead — it never reaches song.arrangements.
+    bogus = {
+        "name": "Bogus", "tuning": [0, 0, 0, 0, 0, 0], "capo": 0,
+        "notes": [], "chords": [], "anchors": [], "handshapes": [],
+        "templates": [], "beats": [], "sections": [],
+    }
+    pak = _write_pak(tmp_path, {
+        "arrangements": [
+            {"id": "lead", "name": "Lead", "file": "arrangements/lead.json"},
+            {"id": "bad", "name": "Bogus", "type": "drums",
+             "file": "arrangements/bogus.json"},
+        ],
+    }, {"arrangements/bogus.json": bogus})
+    loaded = _load(pak, tmp_path)
+    assert [a.name for a in loaded.song.arrangements] == ["Lead"]
+    assert loaded.arrangement_ids == ["lead"]
+
+
+# ── Parts resolution ─────────────────────────────────────────────────────────
+
+def test_two_parts_resolve_primary_first_with_alias_identity(tmp_path: Path):
+    pak = _write_pak(tmp_path, _two_part_manifest(), {
+        "drum_tab.json": _tab("Drums"),
+        "drum_tab_drums-2.json": _tab("Drums (Live)", [{"t": 2.0, "p": "kick", "v": 90}]),
+    })
+    loaded = _load(pak, tmp_path)
+    assert loaded.drum_parts is not None
+    assert [(p["id"], p["name"]) for p in loaded.drum_parts] == [
+        ("drums", "Drums"), ("drums-2", "Drums (Live)"),
+    ]
+    # The primary's payload IS the song-level tab — same object, loaded once.
+    assert loaded.drum_parts[0]["drum_tab"] is loaded.drum_tab
+    assert loaded.drum_parts[1]["drum_tab"]["hits"][0]["t"] == 2.0
+
+
+def test_primary_pointer_equivalent_path_is_not_duplicated(tmp_path: Path):
+    manifest = {
+        "drum_tab": "drum_tab.json",
+        "arrangements": [
+            {"id": "lead", "name": "Lead", "file": "arrangements/lead.json"},
+            {"id": "kit", "name": "Live Kit", "type": "drums",
+             "drum_tab": "./drum_tab.json"},
+        ],
+    }
+    pak = _write_pak(tmp_path, manifest, {"drum_tab.json": _tab("Drums")})
+    loaded = _load(pak, tmp_path)
+    assert loaded.drum_parts is not None
+    assert [(p["id"], p["name"]) for p in loaded.drum_parts] == [
+        ("kit", "Live Kit"),
+    ]
+    assert loaded.drum_parts[0]["drum_tab"] is loaded.drum_tab
+
+
+def test_legacy_single_drum_pack_gets_a_one_part_list(tmp_path: Path):
+    pak = _write_pak(tmp_path, {"drum_tab": "drum_tab.json"}, {
+        "drum_tab.json": _tab("Drums"),
+    })
+    loaded = _load(pak, tmp_path)
+    assert loaded.drum_parts is not None and len(loaded.drum_parts) == 1
+    assert loaded.drum_parts[0]["id"] == "drums"
+    assert loaded.drum_parts[0]["drum_tab"] is loaded.drum_tab
+
+
+def test_no_drums_means_no_parts(tmp_path: Path):
+    pak = _write_pak(tmp_path, {}, {})
+    loaded = _load(pak, tmp_path)
+    assert loaded.drum_parts is None
+    assert loaded.drum_tab is None
+
+
+def test_pointer_only_pack_promotes_the_first_part_to_primary(tmp_path: Path):
+    # A writer that omitted the song-level alias: readers must cope (the
+    # spec keeps the alias, but a reader never crashes on its absence).
+    pak = _write_pak(tmp_path, {
+        "arrangements": [
+            {"id": "lead", "name": "Lead", "file": "arrangements/lead.json"},
+            {"id": "kit", "name": "Kit", "type": "drums",
+             "drum_tab": "drum_tab_kit.json"},
+        ],
+    }, {"drum_tab_kit.json": _tab("Kit")})
+    loaded = _load(pak, tmp_path)
+    assert loaded.drum_parts is not None and len(loaded.drum_parts) == 1
+    # The part's tab becomes THE drum tab, so has_drum_tab / the default
+    # stream / the drum-only placeholder all keep working.
+    assert loaded.drum_tab is loaded.drum_parts[0]["drum_tab"]
+    assert loaded.drum_parts[0]["id"] == "kit"
+
+
+# ── Permissive per-part failure ──────────────────────────────────────────────
+
+def test_a_bad_extra_part_disables_that_part_only(tmp_path: Path):
+    manifest = _two_part_manifest()
+    manifest["arrangements"].append(
+        {"id": "drums-3", "name": "Broken", "type": "drums",
+         "drum_tab": "drum_tab_broken.json"})
+    pak = _write_pak(tmp_path, manifest, {
+        "drum_tab.json": _tab("Drums"),
+        "drum_tab_drums-2.json": _tab("Drums (Live)"),
+        "drum_tab_broken.json": "not json {{{",
+    })
+    loaded = _load(pak, tmp_path)
+    assert [p["id"] for p in loaded.drum_parts] == ["drums", "drums-2"]
+
+
+def test_a_traversal_part_path_is_skipped(tmp_path: Path):
+    manifest = _two_part_manifest()
+    manifest["arrangements"][2]["drum_tab"] = "../outside.json"
+    (tmp_path / "outside.json").write_text(json.dumps(_tab("Evil")))
+    pak = _write_pak(tmp_path, manifest, {"drum_tab.json": _tab("Drums")})
+    loaded = _load(pak, tmp_path)
+    assert [p["id"] for p in loaded.drum_parts] == ["drums"]
+
+
+def test_duplicate_pointer_rels_load_once(tmp_path: Path):
+    manifest = _two_part_manifest()
+    manifest["arrangements"].append(
+        {"id": "drums-dup", "name": "Dup", "type": "drums",
+         "drum_tab": "drum_tab_drums-2.json"})
+    pak = _write_pak(tmp_path, manifest, {
+        "drum_tab.json": _tab("Drums"),
+        "drum_tab_drums-2.json": _tab("Drums (Live)"),
+    })
+    loaded = _load(pak, tmp_path)
+    assert [p["id"] for p in loaded.drum_parts] == ["drums", "drums-2"]
+
+
+def test_duplicate_part_ids_are_made_unique(tmp_path: Path):
+    manifest = _two_part_manifest()
+    manifest["arrangements"][2]["id"] = "drums"
+    manifest["arrangements"].append(
+        {"id": "drums-2", "name": "Aux", "type": "drums",
+         "drum_tab": "drum_tab_aux.json"})
+    pak = _write_pak(tmp_path, manifest, {
+        "drum_tab.json": _tab("Drums"),
+        "drum_tab_drums-2.json": _tab("Drums (Live)"),
+        "drum_tab_aux.json": _tab("Aux"),
+    })
+    loaded = _load(pak, tmp_path)
+    assert [p["id"] for p in loaded.drum_parts] == ["drums", "drums-2", "drums-3"]
+
+
+def test_drum_pointer_with_wrong_type_logs_warning(tmp_path: Path, caplog):
+    pak = _write_pak(tmp_path, {
+        "arrangements": [
+            {"id": "lead", "name": "Lead", "file": "arrangements/lead.json"},
+            {"id": "typo", "type": "druns", "drum_tab": "drum_tab_typo.json"},
+        ],
+    }, {"drum_tab_typo.json": _tab("Typo")})
+    # feedBack sets propagate=False, so pytest's root capture sees nothing from
+    # it — attach caplog's handler to the feedBack logger and pin WARNING
+    # regardless of ambient level (a sibling test can leak ERROR onto this tree).
+    lg = logging.getLogger("feedBack")
+    orig_level = lg.level
+    lg.addHandler(caplog.handler)
+    lg.setLevel(logging.WARNING)
+    try:
+        loaded = _load(pak, tmp_path)
+    finally:
+        lg.removeHandler(caplog.handler)
+        lg.setLevel(orig_level)
+    assert loaded.drum_parts is None
+    assert "has drum_tab" in caplog.text and "type='druns'" in caplog.text
+
+
+# ── Drum-only pack with parts ────────────────────────────────────────────────
+
+def test_drum_only_pack_with_pointer_entries_still_synthesizes_placeholder(tmp_path: Path):
+    # No pitched arrangements at all, drums via pointer entries only: the
+    # placeholder "Drums" arrangement must still appear so the highway WS
+    # proceeds and the tab reaches the drum highway.
+    pak = _write_pak(tmp_path, {
+        "arrangements": [
+            {"id": "kit", "name": "Kit", "type": "drums",
+             "drum_tab": "drum_tab_kit.json"},
+        ],
+    }, {"drum_tab_kit.json": _tab("Kit", [{"t": 5.0, "p": "kick", "v": 100}])})
+    # Remove the Lead arrangement _write_pak added to the manifest.
+    manifest_path = pak / "manifest.yaml"
+    manifest = yaml.safe_load(manifest_path.read_text())
+    manifest["arrangements"] = [e for e in manifest["arrangements"] if e.get("id") != "lead"]
+    manifest.pop("duration", None)
+    manifest_path.write_text(yaml.safe_dump(manifest, sort_keys=False))
+    loaded = _load(pak, tmp_path)
+    assert [a.name for a in loaded.song.arrangements] == ["Drums"]
+    assert loaded.drum_parts is not None and loaded.drum_parts[0]["id"] == "kit"
+    # Song length derived from the last hit (the drum-only path's rule).
+    assert loaded.song.song_length > 5.0

tests/test_song.py

@@ -329,6 +329,31 @@ def test_note_pitch_midi_bass_uses_bass_base():
     assert note_pitch_midi(bass, Note(time=0, string=0, fret=0)) == 28
 
 
+def test_note_pitch_midi_authored_bass_type_uses_bass_base():
+    """Editor PR #335: a bass authored via `type` on an arrangement whose NAME
+    doesn't say "bass". arrangement_string_count now returns 4 for it, so the
+    open-string base MUST also be the bass base (low E1 = 28), not the guitar
+    octave (40). Pre-fix note_pitch_midi keyed is_bass off the name only, so
+    this returned 40 (4 lanes on a guitar octave — the exact inconsistency)."""
+    bass = Arrangement(
+        name="Low End", type="bass",
+        tuning=[0, 0, 0, 0],
+        notes=[Note(time=float(i), string=i, fret=0) for i in range(4)],
+    )
+    assert note_pitch_midi(bass, Note(time=0, string=0, fret=0)) == 28
+
+
+def test_note_pitch_midi_path_bass_flag_uses_bass_base():
+    """Same guarantee via the archive <arrangementProperties> pathBass flag on a
+    non-"bass"-named arrangement: bass base (28), not guitar (40)."""
+    bass = Arrangement(
+        name="Low End", path_bass=True,
+        tuning=[0, 0, 0, 0],
+        notes=[Note(time=float(i), string=i, fret=0) for i in range(4)],
+    )
+    assert note_pitch_midi(bass, Note(time=0, string=0, fret=0)) == 28
+
+
 def test_note_pitch_midi_out_of_range_string_is_none():
     arr = Arrangement(name="Lead", tuning=[0, 0, 0, 0, 0, 0])
     assert note_pitch_midi(arr, Note(time=0, string=9, fret=0)) is None
@@ -1153,6 +1178,60 @@ def test_string_count_ignores_rs_padded_tuning_for_bass():
     assert arrangement_string_count(arr) == 4
 
 
+def test_string_count_4_for_path_bass_flag_without_bass_in_name():
+    # Archive/DLC bass whose manifest ArrangementName isn't "Bass" but whose
+    # <arrangementProperties> pathBass flag is set. Notes on 0..3, tuning
+    # padded to the arrangement-XML length of 6. Pre-fix, name_based forced
+    # 6 (name has no "bass"), so this returned 6 despite the authoritative
+    # instrument flag saying bass.
+    arr = Arrangement(
+        name="Low End",
+        path_bass=True,
+        tuning=[0, 0, 0, 0, 0, 0],
+        notes=[Note(time=float(i), string=i, fret=0) for i in range(4)],
+    )
+    assert arrangement_string_count(arr) == 4
+
+
+def test_string_count_4_for_authored_bass_type_without_bass_in_name():
+    # Editor PR #335: an instrument `type` authored as bass on an arrangement
+    # whose NAME does not contain "bass" (the sloppak loader lifts the manifest
+    # `type` onto Arrangement.type). Notes on 0..3, tuning padded to 6.
+    # The editor lays out 4 lanes off the type; core must agree.
+    arr = Arrangement(
+        name="Low End",
+        type="bass",
+        tuning=[0, 0, 0, 0, 0, 0],
+        notes=[Note(time=float(i), string=i, fret=0) for i in range(4)],
+    )
+    assert arrangement_string_count(arr) == 4
+
+
+def test_string_count_6_for_authored_guitar_type_no_regression():
+    # A non-bass authored type on a generic name still resolves to the
+    # canonical 6 — the type signal only pulls DOWN to 4 for bass.
+    arr = Arrangement(
+        name="Track 1",
+        type="guitar",
+        notes=[Note(time=float(i), string=i, fret=0) for i in range(5)],
+    )
+    assert arrangement_string_count(arr) == 6
+
+
+def test_arrangement_is_bass_signal_safety():
+    # The manifest `type` is lifted onto arr.type verbatim; the helper must be
+    # safe against the messy shapes a hand-edited/loose source can produce.
+    from song import arrangement_is_bass
+    assert arrangement_is_bass(Arrangement(name="Low End", type="bass"))
+    assert arrangement_is_bass(Arrangement(name="Low End", type=" BASS "))  # ws/case
+    assert arrangement_is_bass(Arrangement(name="Low End", path_bass=True))
+    assert arrangement_is_bass(Arrangement(name="Slap Bass"))               # legacy name
+    # Non-bass / absent signals stay False (back-compat: no bass signal → 6).
+    assert not arrangement_is_bass(Arrangement(name="Lead", type=""))
+    assert not arrangement_is_bass(Arrangement(name="Rhythm", type="guitar"))
+    assert not arrangement_is_bass(Arrangement(name="", type=""))
+
+
 # ── compute_smart_names ───────────────────────────────────────────────────────
 
 def _sarr(path_lead=False, path_rhythm=False, path_bass=False,

tests/test_tuning_provider_isolation.py

@@ -60,12 +60,14 @@ def test_the_failure_is_actually_logged(registry, caplog):
     # capture_logger() context manager for this, but it is not importable from here:
     # pyproject pins pythonpath to [".", "lib"], so `tests` is not a package.)
     lg = logging.getLogger("feedBack")
+    orig_level = lg.level
     lg.addHandler(caplog.handler)
     lg.setLevel(logging.ERROR)
     try:
         registry.get_merged()
     finally:
         lg.removeHandler(caplog.handler)
+        lg.setLevel(orig_level)  # restore, or ERROR leaks onto the feedBack tree
 
     assert any("bad-plugin" in r.getMessage() for r in caplog.records), (
         "the raising provider was never named in the logs"