MCP Lab

mcp-lab: be the model

Use mcp-lab when you need to see EXACTLY what an LLM sees through dagger mcp — raw tool results, isError markers, builtin ReadLogs/ListServices output — rather than a TUI rendering or your own session's differently composed view.

The loop

1. (Engine-change QA) engine-lab start — note the printed tcp endpoint.
2. mcp-lab start with engine: <that endpoint>. Omit engine to attach to the current session's own engine instead (stock behavior, no rebuild).
3. tools — the verbatim tools/list a model would receive.
4. call(name, argsJson) — the verbatim tools/call result. isError results are prefixed with a marker, not raised: failure UX is the point.
5. After engine edits: engine-lab restart, then mcp-lab start again (the old session died with the old engine).

The bridge's own service logs (ListServices in YOUR session, then ReadLogs) carry the spawned CLI's stderr — progress and engine chatter — useful when a call hangs or the session fails to initialize.

Crash scenarios (fixtures/crasher)

The default fixture serves these MCP tools alongside the builtins:

• startCrasher ({"delaySeconds": N}, default 30): healthy service that prints a FATAL line and exits 7 after N seconds — crash mid-session.
• startDoomed: dies before its healthcheck, so the start call itself fails — crash on boot. The interesting part is the error text the model receives.
• poke ({"url": "http://<hostname>:8080"}): probe whether a service still answers, and observe the failure UX when it doesn't.

Questions worth asking in a crash QA pass: does the failure hand the model a usable span handle? Does ListServices admit the service ever existed? Does ReadLogs on the advertised handle actually reach the crash logs?

Pitfalls

• The first tools/call after start can take minutes (SDK builds in the attached engine). The bridge holds requests until init completes and reports init failures in the response body instead of dying.
• One MCP session = one dagger mcp process = one dagger session: services started by earlier calls stay alive for later ones. start again for a fresh session.
• call argsJson must be a JSON OBJECT ({} for no args).