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fcp-midi

fcp-midi

by os-tack
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fcp-midi

MCP server for semantic MIDI composition.

What It Does

fcp-midi lets LLMs compose music by describing musical intent -- notes, chords, dynamics, tempo changes -- and renders it into standard MIDI files. Instead of manipulating raw bytes or MIDI events, the LLM works with operations like note Bass E2 at:1.1 dur:quarter vel:90 and crescendo @track:Lead vel:45-75 @range:3.1-6.4. Built on the FCP framework; a mido.MidiFile is the source of truth throughout, so there's no separate serialization step.

Related MCP server: Vibe Composer MIDI MCP

Quick Example

midi_session('new "Voltage Drop" tempo:140 key:E-minor')

midi([
  'note Drums kick at:1.1 dur:eighth vel:100',
  'note Bass E2 at:1.1 dur:quarter vel:90',
  'chord Pad Em at:1.1 dur:whole vel:70',
  'crescendo @track:Lead vel:45-75 @range:3.1-6.4',
  'tempo 174 at:11.1',
])

midi_session('save as:./voltage_drop.mid')

Available MCP Tools

Tool

Purpose

midi(ops)

Batch mutations -- notes, chords, tracks, tempo, dynamics, copy/transpose

midi_query(q)

Inspect the composition -- map, tracks, events, piano-roll, instruments, find

midi_session(action)

Lifecycle -- new, open, save, checkpoint, undo, redo

midi_help()

Full reference card

Hero Examples

Plumber's Journey -- Classic game theme faithfully recreated: 4 tracks, 288 notes, 16 seconds, 180 BPM.

Voltage Drop -- A drum-and-bass track with tempo acceleration (140 -> 155 -> 174 BPM), 5 tracks, breakbeats, sub-bass, arpeggios, and a signature DROP section. Also used in the FCP vs raw Python agent battle -- FCP produced 1,674 notes across 12 tracks in 87 seconds, compared to 694 notes from ~689 lines of hand-written Python.

See docs/examples/ for MIDI files and the full writeup.

Installation

Requires Python >= 3.11.

pip install fcp-midi

MCP Client Configuration

{
  "mcpServers": {
    "midi": {
      "command": "uv",
      "args": ["run", "python", "-m", "fcp_midi"]
    }
  }
}

Architecture

Mido-native — no parallel semantic model between the op handlers and the file format:

MCP Server (fcp-core create_fcp_server + MidiAdapter)
  Parses op strings, dispatches to verb handlers, session lifecycle
        |
MidiModel (mido.MidiFile is the source of truth)
  Op handlers read/write mido messages directly on the tracks;
  a NoteIndex gives fast selector lookups. Undo/redo and batch
  atomicity are byte snapshots of the file, not event replay.

Key features:

  • Instrument library -- GM instruments by name (acoustic-grand-piano, synth-bass-1, standard-kit)

  • Soundfont support -- Load custom instrument banks

  • Beat addressing -- at:1.1 (bar 1, beat 1), at:3.2.240 (bar 3, beat 2, tick 240)

  • Duration vocabulary -- whole, half, quarter, eighth, sixteenth, triplet

  • Dynamics -- crescendo, diminuendo across ranges

  • Copy/transpose -- Duplicate and shift musical phrases

Development

uv sync
uv run pytest       # 620 tests
uv run pytest -m "not slow"  # skip stress tests (604 tests, ~0.25s)
uv run ruff check   # linting
uv run pyright      # type checking

License

MIT

Available Tools

4 tools
midiA

Execute midi operations. Each op string follows: VERB TARGET [key:value ...] Call midi_help for the full reference card.

MUSIC: note TRACK PITCH at:POS dur:DUR [vel:V] [ch:N] chord TRACK SYMBOL at:POS dur:DUR [vel:V] [ch:N] track add|remove NAME [instrument:INST] [program:N] [ch:N] [bank:MSB[.LSB]] cc TRACK CC_NAME VALUE at:POS [ch:N] bend TRACK VALUE at:POS [ch:N] tracker TRACK import at:POS [res:RES] ... tracker end

STATE: mute TRACK solo TRACK program TRACK INSTRUMENT|program:N [at:POS] [bank:MSB[.LSB]]

META: tempo BPM [at:POS] time-sig N/D [at:POS] key-sig KEY-MODE [at:POS] marker TEXT at:POS title TEXT

EDITING: remove SELECTORS move SELECTORS to:POS copy SELECTORS to:POS transpose SELECTORS SEMITONES velocity SELECTORS DELTA quantize SELECTORS grid:DUR modify SELECTORS [pitch:P] [vel:V] [dur:D] [at:POS] [ch:N] repeat SELECTORS [to:POS] count:N crescendo SELECTORS from:VEL to:VEL decrescendo SELECTORS from:VEL to:VEL

SELECTORS: @track:NAME @channel:N @range:M.B-M.B @pitch:PITCH @velocity:N-M @all @recent @recent:N @not:TYPE:VALUE Combine to intersect: @track:Piano @range:1.1-4.4

POSITION: M.B (1.1 = start) M.B.T (tick offset) tick:N +DUR -DUR end

DURATION: whole, half, quarter, eighth, sixteenth, 32nd 1n, 2n, 4n, 8n, 16n, 32n dotted-quarter, triplet-eighth, ticks:N

PITCH: C4, D#5, Bb3 (note+accidental+octave) midi:60 (raw MIDI number)

CHORDS: Cmaj, Am, Dm7, G7, Bdim, Faug, Csus4, Asus2 Cmaj7, Am7, Dm7b5, G9, Cm6, Cadd9, Dm/F (slash)

VELOCITY: 0-127 (numeric) ppp, pp, p, mp, mf, f, ff, fff (dynamic names)

CC NAMES: volume, pan, modulation, expression, sustain, reverb, chorus, brightness, portamento, breath

GM INSTRUMENTS (EXAMPLES): acoustic-grand-piano, electric-piano-1, vibraphone acoustic-guitar-nylon, electric-bass-finger, violin trumpet, alto-sax, flute, string-ensemble-1 program:N (raw 0-127) bank:MSB[.LSB]

RESPONSE PREFIXES:

  • note/chord added ~ event modified

  • track modified - event removed ! meta event @ bulk operation

CONVENTIONS:

  • Positions are 1-based: measure 1, beat 1 = 1.1

    • Channels are 1-indexed user-facing (ch:1 through ch:16)

    • Channel 10 is drums (GM standard)

    • Track names are unique identifiers

    • Batch multiple ops in one call for efficiency

    • Call midi_help after context truncation for full reference

Inputs: ops (required).

midi_helpA

Returns the midi reference card with all syntax.

Inputs: no named parameters.

midi_queryD

Query midi state.

Inputs: q (required).

midi_sessionC

Session: 'new "Title"', 'open ./file', 'save', 'checkpoint v1', 'undo', 'redo'

Inputs: action (required).

Tool Schema Changelog

Recent tool additions, removals, and schema changes observed during successful MCP inspections.

Loading tool schema changelog…

    Deploy Server
    One endpoint for every MCP toolRayrun runs every MCP server behind one URL and holds the credentials. Each call is decided per client, tool, and argument, then recorded.ray.runAd

    TDQS

    B3.3/5.0

    Scored across 4 tools

    Disambiguation5/5

    Each tool has a clearly distinct role: midi for editing/creating music, midi_query for reading state, midi_session for session/file management, and midi_help for documentation. There is no overlap in their primary purposes.

    Naming Consistency5/5

    All tools follow a consistent 'midi[_suffix]' pattern using snake_case. The base 'midi' denotes the core operation, while suffixes like '_query', '_session', and '_help' clearly indicate auxiliary functions. This is a predictable and uniform convention.

    Tool Count5/5

    With exactly 4 tools, the set is well-scoped and right-sized. The main tool centralizes a rich DSL for MIDI operations, while the supporting tools handle query, session, and help functions. No redundancy or excessive splitting.

    Completeness5/5

    The tool set provides full lifecycle coverage for MIDI work: creating/editing via midi, reading state via midi_query, session control (new/open/save/undo/redo) via midi_session, and reference via midi_help. There are no obvious missing operations within the domain.

    Maintenance

    ActivityInactive
    ResponsivenessNo recent community issues

    Resources

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