Fcp Midi

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.

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/](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

Source & license

This open-source MCP server is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: os-tack
• Source: os-tack/fcp-midi
• License: MIT

Install and usage instructions live in the source repository linked above.