Errors

Every error response is a JSON object with a single error string field:

{ "error": "Project not found" }

Status codes

Status	Meaning
`400`	Malformed JSON, or a request body/param that failed validation (e.g. missing `content`).
`401`	Missing or invalid API key.
`403`	No credits balance (and not on a usage-based subscription). See mathify.dev/pricing.
`404`	The resource doesn’t exist, or doesn’t belong to the authenticated key owner. Not-found and not-yours are intentionally indistinguishable.
`409`	A conflicting job is already in progress (e.g. applying music while a music job is already running for that fragment).
`422`	The request is well-formed but the target isn’t in a state that supports the operation (e.g. applying music to a fragment with no rendered animation yet).
`502`	An upstream service (the render backend) failed or was unreachable. Safe to retry.
`503`	Generation is temporarily in maintenance mode.

Generation and music application are asynchronous. Poll the relevant GET endpoint rather than expecting a synchronous result:

After POST /projects/:id/messages → poll GET /fragments/:fragmentId until animationUrl is non-null.
After POST /fragments/:fragmentId/music → poll GET /fragments/:fragmentId/music until status is completed or failed.