WaveeMusic: Skill for Claude Code

Wavee playlist mutations (wire + sync)

The write half of the library: every playlist and rootlist edit Wavee sends, every playlist frame the dealer sends back, and the invariants that keep the two from corrupting each other.

Read this before touching: Backend/Playlists/**, Backend/Mutation.cs, Backend/Sync/LibrarySync.cs (playlist/rootlist/permission arms), Backend/Realtime/DealerRouter.cs, IPlaylistMutationSource (Wavee.Core/Sources/SeamPorts.cs:74), or the detail page's edit/reorder/notice surfaces.

Every wire shape below is desktop-verified against two Fiddler captures + one dealer archive (2026-08-15) and pinned by byte-exact goldens. Design record: docs/plans/wavee/playlist-saz-fable5-handoff.md (read its Errata section first — the original body is wrong on several load-bearing points).

1. Outbound wire contracts

Envelope (PlaylistWireMapper.Build, :214) — POST /playlist/v2/{path}/changes, body = raw ListChanges under a lying Content-Type: application/x-www-form-urlencoded (SpotifyHeaders.PlaylistV2Mutation):

Ops (ToWireOp :252):

• ADD — attrs{timestamp, item_id} per item; add_first/add_last emitted only when true, else from_index. item_ids are client-minted (SpotifyIds.NewItemId, 8 random bytes) and the server keeps them (verified: A046's 50 ids came back byte-identical on the next GET). added_by is never sent.
• Keyed REM — Rem{items_as_key=1, items:[Item{uri, attrs{item_id}}]}, one op per row, all in one Delta. No index at all. Index REM (from,length) is the fallback for rows whose id we never learned — all-or-nothing per batch (PlaylistMutationSource.BuildRemoveOps :176), because mixing shapes in one Delta shifts indices.
• Keyed MOV — Mov{items[], add_after_item | add_first | add_last}, fields 1–3 absent. Anchor derivation lives in PlaylistMutationSource.BuildKeyedMove :218: toIndex<=0→add_first; >=Count→add_last; else the row at toIndex-1 walking back over rows that are themselves moving (that is what makes a gapped multi-select land as one contiguous run); nothing left → add_first. One op for the whole selection.
• UPDATE_LIST — new_attributes only. collaborative=false and deleted_by_owner=false travel as values, never also as no_value (the no_value form is parse-side only — it is what the dealer's old_attributes uses).
• Rootlist ops are positional — Mov{from,length,to}, index Rem, index Add. Playlist row ADD carries attrs{ts, public=1}; folder markers never carry public (IsGroupMarker :359).