Oh My Cassette

The cassette MCP server is a local bridge to the Cassette video editor. Its editing tools come from the Cassette MCP service and change as Cassette evolves, so this skill deliberately names none of them. The server's instructions and each tool's description are the source of truth: read them, and when they disagree with anything you remember, they win.

Rules that hold for every version

1. Follow the server. Which tool to call, in what order, and how to route on a result are in the server instructions and the tool descriptions. Route on typed fields such as status, never on prose.
2. Relay the user's words. When a tool takes the user's editing request, pass it exactly as the user wrote it: no rewriting, summarizing, translating, or expanding. The Cassette agent reads the project media itself.
3. Ground every claim in a tool result. What is on the timeline, which version it is, and where the editor link points come from the latest result, not from memory.
4. Render only when asked. Export or render only when the user asks to export, render, download, or finish.
5. Say where the media goes. When the user has not asked for Cassette by name in this conversation, tell them before the first upload that the files they named go to the Cassette service for editing, and go on once they agree.

Local files

• Pass file paths as the user gave them, absolute or relative to the workspace. The bridge prepares and uploads them and hands Cassette references in their place.
• Only files the user named or that sit in the workspace. Never search the filesystem for media.
• Files outside the workspace are refused unless the user adds their directory to CASSETTE_ALLOWED_ROOTS. Only video, audio, and image files upload.
• Video is transcoded on this machine with ffmpeg before it uploads, and audio or images Cassette does not take as they are get converted. Long footage can take minutes.
• Exported files are downloaded into cassette-exports/ in the workspace. Use the local path the result gives you; never invent one.

Long waits

Long work never fails just for taking long. A tool waits a bounded time and then returns a normal, non-error result that says the work is still going:

• status: "preparing" comes from the bridge: the local files are still being prepared or uploaded, and that work continues in the background. Tell the user the progress it reports, then call the same tool again with exactly the same arguments to keep waiting; it picks up the same work instead of starting over. Never keep calling for longer than the user is willing to wait: ask before you continue a long wait.
• running, processing and similar statuses come from Cassette. How to continue (usually the same tool with the id the result returns) is in the server instructions and the tool description.
• Cancelling a call only stops waiting; the work goes on. Stop the work itself only when the user asks, with the tool the server provides for that.

When the bridge itself reports a problem

Errors whose code starts with bridge. come from the local bridge, not from Cassette. Any other error code is Cassette's own: follow its message and the tool description.

Code	What to do
bridge.backend_unreachable	A tool that names its request in an argument has already been sent again by the bridge while the call's wait lasted. If the message says the connection broke during the call, the work may still be running: call the same tool again with the same arguments to pick it up. If it says Cassette did not take the call, retry after retry_after_seconds when the error gives it. Otherwise call cassette_bridge_status once, tell the user the service is not reachable and at which URL, and retry only after it reports connected.
bridge.rate_limited	Cassette is taking too many requests and did not run the call. Wait retry_after_seconds (from the error), then call the same tool again with the same arguments; do not retry sooner.
bridge.forbidden	This account or grant cannot access Cassette. Ask the user to check that they signed in with the account of their projects. Do not repeatedly trigger login.
bridge.auth_unavailable	Online authentication or secure credential storage is unavailable. Tell the user to restore the connection or unlock the system keyring, then retry.
bridge.unauthorized	The service refused the token. Tell the user to run oh-my-cassette login. The browser handles email verification and consent. Restart the MCP host only if it cannot refresh tools. Do not repeatedly trigger login.
bridge.upgrade_required	Tell the user to update the oh-my-cassette version their MCP host runs (the message links the steps: bump the oh-my-cassette== pin, or update the plugin). Do not retry.
bridge.protocol_unsupported	The service at that URL is too old for this plugin (it does not speak MCP 2026-07-28). Tell the user the Cassette service must be updated. Do not retry.
bridge.file_not_found	Ask the user for the correct path.
bridge.file_outside_workspace	Ask the user to copy the file into the workspace, or to add its directory to CASSETTE_ALLOWED_ROOTS. When the message says the host started the bridge in a folder too wide to read from (the whole disk or the home folder), ask them to set CASSETTE_WORKSPACE to the project folder.
bridge.file_type_rejected	Tell the user which types this tool accepts (the message lists them).
bridge.file_too_large	Tell the user about the size limit (CASSETTE_MAX_UPLOAD_MB).
bridge.ffmpeg_unavailable	Video needs ffmpeg on this machine. Relay the install hint from the message, or ask the user to set CASSETTE_FFMPEG and CASSETTE_FFPROBE. Do not retry until it is installed.
bridge.prepare_failed	ffmpeg could not read or convert the file. Tell the user which file failed and what the message says; do not retry the same file unchanged.
bridge.upload_failed	Retry once, then report it.
bridge.invalid_argument	Fix the arguments: file parameters take a path or a list of paths.
bridge.contract_violation	A Cassette service bug. Report it to the user as is; do not retry.

A failed export download (bridge.download_failed) does not fail the call: the result says so and keeps the original URL, which you can give the user.

When only cassette_bridge_status is listed

The service is offline, refused the token, or needs a newer plugin. Call cassette_bridge_status once and tell the user what it reports (URL, error_code and reason). Do not poll it. When the service comes back, its tools appear by themselves; if this host does not refresh tool lists, ask the user to restart the MCP server.