Protocol Conformance Test

Test the wire protocol directly, so a client bug and a server bug are distinguishable

Use when: a client reports a failure and the server's own behaviour is unproven

Avoid when: the server is a thin library wrapper with no wire protocol of its own

Procedure

1. Write the exact JSON-RPC lines the client should send
2. Pipe them into the server process with a bounded timeout
3. Assert the response shape, not only the presence of a reply
4. Check the jsonrpc field, id echo and error codes
5. Repeat for a malformed line and for an unknown method
6. Keep the transcript as a fixture so the test is reproducible

Signals

• every response echoes the request id
• malformed input produces a parse error, not a crash
• an unknown method produces method-not-found
• the server keeps serving after an error line

Patterns

1. Piped transcript

printf '%s\n' '{"jsonrpc":"2.0",...}' | python -m skillmcp serve

2. Shape assertion

jsonrpc == '2.0' and id == sent id

3. Survival check

send a bad line, then a good one, expect two replies

Pitfalls

• Testing through the client library, which hides the raw response
• Asserting only that some output appeared
• Forgetting that a server which crashes on a bad line fails every later call

Done when

• happy path, malformed line and unknown method each have a fixture
• the server is shown to survive an error line

References

• JSON-RPC 2.0
• MCP - lifecycle