plainsong-mcp

A Model Context Protocol server for Plainsong, so any agent can read, write and compile music notation — and so several agents can work on one score at the same time without overwriting each other.

plainsong-mcp                 # JSON-RPC over stdio, what most clients expect
plainsong-mcp --http          # loopback HTTP, for remote and multi-agent setups
plainsong-mcp --list-tools    # what it exposes

Install

pip install git+https://github.com/SuperInstance/plainsong-mcp

That brings in the compiler as well. Neither has any other dependency: the protocol, both transports, the session store and the compiler itself are written against the standard library. Python 3.10 or newer.

The compiler comes from PyPI as plainsong>=1.4.0. 1.1.0 is the first release carrying plainsong.features, the per-bar analysis this package re-exports rather than duplicating; 1.4.0 is the first carrying plainsong.runtime.localhost, the loopback check behind the same re-export, so 1.4.0 is now the binding floor. Either way the requirement is real: below it the install succeeds and the import fails.

Point a client at it

For a client that launches servers over stdio, the usual shape is:

{
  "mcpServers": {
    "plainsong": {
      "command": "plainsong-mcp"
    }
  }
}

Then ask it for music. The server exposes 27 tools, which is everything the plainsong CLI can do — write and validate notation, compile to MIDI and audio, transpose, search a library of several thousand pieces, analyse what each listener on a stage actually hears, and probe what the host machine can render.

Many agents, one score

The reason this exists. A session gives every agent a voice of its own:

ensemble_open    name a session, set key, tempo and metre
ensemble_join    claim a voice -- one owner at a time
ensemble_read    the whole current state, in one call
ensemble_write_part   write your voice, against the version you read
ensemble_render  merge the parts and compile
ensemble_log     what everyone has done

Because the parts are disjoint, the common case never conflicts — two agents writing two voices both succeed. When two do collide, the later write is refused and handed the current state to rebase onto, rather than silently overwriting somebody's work. Locks are held only around a read-modify-write of the manifest, never across a call to a model, because a model call takes seconds and a lock held that long is a lock nobody else can get.

Notation is parsed before a part is accepted, so invalid notation never lands. Writes are atomic, the merge is deterministic, and every change appends to a log a joining agent can read to find out what has happened so far.

See docs/ensemble.md for the whole protocol and docs/mcp.md for the tool and resource surface.

Reading a score as features

analyze_features computes sixteen per-bar features — note density, harmonic tension, syncopation, contour direction, register spread and so on — matching the vocabulary fleet-jepa-midi perceives. That lets a bandleader read a written score, and makes a corpus of notation usable as labelled training data.

Where this came from

This began inside the compiler's repository and was extracted once it was clear it had become its own thing rather than a feature of the compiler. Two names had also collided along the way: the analysis of who hears what on a physical stage, and a session shared by several agents, were both called "ensemble". The first is now plainsong stage and lives with the compiler; the second is what this repository means by the word.

The compiler still ships a copy of this server, which is what its plainsong mcp command runs. That copy is being retired — install from here.

Status

The protocol is exercised by a test suite that drives the server with real JSON-RPC: the initialize handshake, tools/list, tools/call, malformed input, unknown methods, notifications, and a tool that raises. Concurrency is tested with threads racing for the same voice, and merge determinism is checked by writing the same parts in three different orders and comparing bytes rather than assuming.

CI runs the suite on Python 3.10 through 3.13 across Linux, macOS and Windows, and then speaks the protocol to the built server over a pipe, so a handshake that regresses fails the build rather than a mock of one.

No third-party MCP client has connected to it yet. The protocol behaviour is verified against the specification rather than against a particular client — strong evidence, and not the same thing. Doing that is the most useful thing anyone could contribute.

90 tests.

Licence

MIT. See LICENSE.