effectcraft/docs/mcp.md

MCP conventions

Run effectcraft-cli mcp for a headless session, or effectcraft-cli mcp --bridge 9877 to use the desktop app's control channel. Existing ports are unchanged.

The server provides the same core conventions as FilmCraft #28:

Tool	Arguments	Result
command_list	filter?, enabled_only?, schemas?	Command catalog
command_run	id, params?	Engine command result
command_batch	steps: [{id, params?}], stop_on_error?	completed, failed, per-step results
doc_inspect	none	Project overview and active composition
render_preview	comp?, time?, max_side?, transparent?	PNG and frame information

command_batch stops at the first error by default; false continues. Each edit has its own undo step. The existing batch tool retains its atomic undo grouping and result references. Existing tools, including list_commands, execute_command, and batch, remain listed because existing workflows use them; there are no hidden aliases. See agents.md for the full catalog. Each tool has a title and read-only, destructive, idempotent and open-world hints. Tools with an optional output path are conservatively annotated as file writers.

Unknown top-level tool argument keys return JSON-RPC -32602 naming the key and accepted arguments. EffectCraft already rejects unknown engine command parameters; that behaviour is preserved. Tool failures (including escaped panics) return isError: true, and so does any JSON result with a failed count above zero (a command_batch / batch with a failed step, a render whose queue items did not all finish), even though the other steps or items succeeded: read results / items for the details. Malformed JSON returns -32700 with a null id and the session keeps serving. The MCP backend owns its session without a mutex; the engine's render-job mutexes already recover poisoned locks.

Resources effectcraft://document and effectcraft://commands return JSON matching doc_inspect and command_list. Clients declaring MCP 2026-07-28 in per-request _meta receive resultType: complete and list/read cache hints; document reads are never cached.

{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"command_run","arguments":{"id":"comp.new","params":{"width":640,"height":360}}}}
{"jsonrpc":"2.0","id":2,"method":"resources/read","params":{"uri":"effectcraft://document"}}

Progress and cancellation

Headless command_run / execute_command calls to renderQueue.render (unless wait: false) run the existing background job while the MCP server keeps reading stdin. A params._meta.progressToken string or number opts into notifications/progress, at most ten per second, strictly increasing with a queue-item total. No token means no notifications. ping, inspection and other requests are served while rendering. Closing stdin lets the pending render finish and reply.

Requests during a render

A render job owns a snapshot of the project taken when it started. While it runs:

• Every way of starting a second render is refused with isError: true: a blocking renderQueue.render, one with wait: false, or one inside a batch.
• Other commands, including edits, run normally and change the project. They do not affect the running job, which keeps rendering its snapshot; to render an edit, render again after the job ends. Queue items are matched by id when the job reports their status, so an item removed mid-render is left out of the reply and counted in failed.

Send notifications/cancelled with params.requestId to stop the matching request at the next frame batch. No response is sent for that cancelled request. Unknown and completed ids are ignored. The exporter records the exact files it opens, including storage overflow paths, and removes those files on cancellation; it never scans for similarly named files. Finished earlier queue items and skipped existing frames remain. Custom exporters implement the same cleanup contract. A failed queue item gives isError: true.

Bridge calls, wait: false, batch tools and single-frame tools retain synchronous/polling behaviour and do not report MCP progress or cancellation. renderQueue.render {"wait": false} returns the ids being rendered at once; poll renderQueue.list for status and stop it with renderQueue.stop. Use a direct blocking command call for an MCP-cancellable render.