diff --git a/lib/gp2notation.py b/lib/gp2notation.py
index 0787ec4..65b9b33 100644
--- a/lib/gp2notation.py
+++ b/lib/gp2notation.py
@@ -522,6 +522,10 @@ def attach_notation_to_sloppak(sloppak_dir: str | Path, arr_id: str, payload: di
         json.dumps(payload, separators=(",", ":")), encoding="utf-8"
     )
     entry["notation"] = filename
+    # Stamp the format version while we're rewriting the manifest (spec §4),
+    # without downgrading an existing (possibly higher) declared version.
+    from sloppak import FEEDPAK_VERSION
+    manifest.setdefault("feedpak_version", FEEDPAK_VERSION)
     manifest_path.write_text(
         yaml.safe_dump(manifest, sort_keys=False, allow_unicode=True),
         encoding="utf-8",
diff --git a/lib/sloppak.py b/lib/sloppak.py
index 90c41c4..2b9bf83 100644
--- a/lib/sloppak.py
+++ b/lib/sloppak.py
@@ -24,6 +24,11 @@ from pathlib import Path
 
 log = logging.getLogger("slopsmith.lib.sloppak")
 
+# The feedpak format version this build targets / writes (manifest
+# `feedpak_version`, a semver string per spec §4). Readers tolerate any version
+# (additive/MINOR compatibility); writers stamp this.
+FEEDPAK_VERSION = "1.2.0"
+
 import yaml
 
 from safepath import safe_join
@@ -316,6 +321,9 @@ class LoadedSloppak:
     stems: list[dict]           # [{"id": str, "file": str, "default": bool}]
     source_dir: Path
     manifest: dict
+    # The pack's declared format version (manifest `feedpak_version`, a semver
+    # string per spec §4). None when absent (legacy / pre-versioning packs).
+    feedpak_version: str | None = None
     # Parsed `drum_tab.json` payload when the manifest carries a `drum_tab:`
     # key pointing at a readable, schema-valid file. None otherwise (older
     # sloppaks, sloppaks without drums, sloppaks whose drum tab failed to
@@ -789,11 +797,13 @@ def load_song(
                         "events": clean_events,
                     }
 
+    _fpv = manifest.get("feedpak_version")
     return LoadedSloppak(
         song=song,
         stems=stems,
         source_dir=source_dir,
         manifest=manifest,
+        feedpak_version=_fpv if isinstance(_fpv, str) and _fpv else None,
         drum_tab=drum_tab_data,
         song_timeline=song_timeline_data,
         tempos=tempos_data,
diff --git a/lib/songmeta.py b/lib/songmeta.py
index 55352ae..362e042 100644
--- a/lib/songmeta.py
+++ b/lib/songmeta.py
@@ -49,6 +49,15 @@ def _apply_to_sloppak_manifest(manifest: dict, fields: dict) -> bool:
     if "year" in fields:
         manifest["year"] = _coerce_year(fields["year"])
         dirty = True
+    # Opportunistically declare the format version (spec §4) when we're already
+    # rewriting because a metadata field was supplied. Gated on `dirty` (i.e. a
+    # field was given) so this never forces a *standalone* rewrite with no fields
+    # passed, and `not in` so an existing (possibly higher) version is preserved,
+    # never downgraded. NB `dirty` here means "a field was supplied" — a
+    # supplied-but-identical value already triggers a rewrite (pre-existing).
+    if dirty and "feedpak_version" not in manifest:
+        from sloppak import FEEDPAK_VERSION
+        manifest["feedpak_version"] = FEEDPAK_VERSION
     return dirty
 
 
diff --git a/tests/test_gp2notation.py b/tests/test_gp2notation.py
index 3669102..d00bde1 100644
--- a/tests/test_gp2notation.py
+++ b/tests/test_gp2notation.py
@@ -502,8 +502,12 @@ def test_attach_notation_to_sloppak(tmp_path):
     entries = {e["id"]: e for e in rewritten["arrangements"]}
     assert entries["keys"]["notation"] == "notation_keys.json"
     assert "notation" not in entries["lead"]
-    # Key order preserved (sort_keys=False round-trip).
-    assert list(rewritten.keys()) == ["title", "artist", "arrangements", "stems"]
+    # Original key order preserved (sort_keys=False round-trip); the manifest
+    # rewrite also stamps feedpak_version (spec §4), appended at the end.
+    assert list(rewritten.keys()) == [
+        "title", "artist", "arrangements", "stems", "feedpak_version"]
+    from sloppak import FEEDPAK_VERSION
+    assert rewritten["feedpak_version"] == FEEDPAK_VERSION
 
 
 def test_attach_notation_unknown_arrangement_raises(tmp_path):
diff --git a/tests/test_sloppak_feedpak_version.py b/tests/test_sloppak_feedpak_version.py
new file mode 100644
index 0000000..03026d9
--- /dev/null
+++ b/tests/test_sloppak_feedpak_version.py
@@ -0,0 +1,86 @@
+"""feedpak_version (spec §4): read on load + opportunistic stamp on a metadata
+write. Core has no create-from-scratch path (RS-free repo); the editor plugin's
+create-mode save stamping the version is a separate follow-up."""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+
+import yaml
+
+import sloppak as sloppak_mod
+from sloppak import FEEDPAK_VERSION
+from songmeta import write_sloppak_metadata
+
+
+def _write_dir_sloppak(root: Path, manifest_extras: dict) -> Path:
+    pak = root / f"{root.name}.sloppak"
+    pak.mkdir()
+    arr_dir = pak / "arrangements"
+    arr_dir.mkdir()
+    (arr_dir / "lead.json").write_text(json.dumps({
+        "name": "Lead", "tuning": [0, 0, 0, 0, 0, 0], "capo": 0,
+        "notes": [], "chords": [], "anchors": [], "handshapes": [],
+        "templates": [], "beats": [], "sections": [],
+    }))
+    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))
+    return pak
+
+
+def _load(pak: Path, tmp_path: Path):
+    cache = tmp_path / "cache"
+    cache.mkdir()
+    return sloppak_mod.load_song(pak.name, pak.parent, cache)
+
+
+def _manifest(pak: Path) -> dict:
+    return yaml.safe_load((pak / "manifest.yaml").read_text(encoding="utf-8"))
+
+
+# ── read ─────────────────────────────────────────────────────────────────────
+
+def test_feedpak_version_read_from_manifest(tmp_path: Path):
+    pak = _write_dir_sloppak(tmp_path, {"feedpak_version": "1.2.0"})
+    assert _load(pak, tmp_path).feedpak_version == "1.2.0"
+
+
+def test_feedpak_version_none_when_absent(tmp_path: Path):
+    pak = _write_dir_sloppak(tmp_path, {})
+    assert _load(pak, tmp_path).feedpak_version is None
+
+
+def test_feedpak_version_none_when_not_a_string(tmp_path: Path):
+    pak = _write_dir_sloppak(tmp_path, {"feedpak_version": 12})
+    assert _load(pak, tmp_path).feedpak_version is None
+
+
+# ── opportunistic stamp on a metadata write ──────────────────────────────────
+
+def test_metadata_write_stamps_version_when_absent(tmp_path: Path):
+    pak = _write_dir_sloppak(tmp_path, {})
+    assert "feedpak_version" not in _manifest(pak)
+    assert write_sloppak_metadata(pak, {"title": "New"}) is True
+    m = _manifest(pak)
+    assert m["title"] == "New"
+    assert m["feedpak_version"] == FEEDPAK_VERSION
+
+
+def test_metadata_write_preserves_existing_version(tmp_path: Path):
+    pak = _write_dir_sloppak(tmp_path, {"feedpak_version": "9.9.9"})
+    write_sloppak_metadata(pak, {"artist": "X"})
+    assert _manifest(pak)["feedpak_version"] == "9.9.9"  # not downgraded
+
+
+def test_metadata_no_change_does_not_add_version(tmp_path: Path):
+    # A no-op metadata write must NOT stamp a version (no rewrite happens).
+    pak = _write_dir_sloppak(tmp_path, {})
+    assert write_sloppak_metadata(pak, {}) is False
+    assert "feedpak_version" not in _manifest(pak)