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Free sheet · Lesson 1.3 of MCP Production

Was it the transport, the protocol or the tool?

The triage table from lesson 1.3: each observation, the boundary it points to, and the evidence to collect next, without guessing at the server.

By Runs Clean Sources checked 9 September 2026 Spec revision 2026-07-28 Free, no signup
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A4, prints in colour or greyscale. Same sheet the course hands out.

Where this goes next

Lab 1 throws valid and malformed messages at a real server and checks each error code against the spec, so the table above is something you watch happen.

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ObservationBoundaryNext evidence
No usable responseTransport outcome uncertainConnection diagnostics and application commit evidence
HTTP 404 without an MCP bodyEndpoint or intermediaryDestination and routing logs
HTTP 404, -32601Unknown protocol methodServer-supported methods
HTTP 400, -32020Missing/malformed/mismatched mirrorExact header/body comparison
HTTP 400, -32022Unsupported versionerror.data.supported and client support
-32700JSON parse failureOriginal bytes with sensitive values removed
-32600Invalid request envelopeObject shape, method, and ID
-32602Invalid parametersMethod/tool contract
result.isError: trueTool execution failureTool result and application evidence

Synthetic examples, not captured incidents:

{"jsonrpc":"2.0","id":"diagnostic-a","error":{"code":-32020,"message":"Header mismatch: Mcp-Name"}}
{"jsonrpc":"2.0","id":"diagnostic-b","result":{"resultType":"complete","isError":true,"content":[{"type":"text","text":"The requested document is unavailable."}]}}

Do not infer a committed write from a status code or infer no write from a timeout.

Sources: Envelope and errors, tool errors, checked 2026-09-09.

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