Return -32022 for strict-mode protocol version rejections - #15
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The draft MCP spec requires version-mismatch rejections to use the UnsupportedProtocolVersionError code -32022 (with data.supported / data.requested), not JSON-RPC -32602, which is reserved for malformed params. The strict initialize path in fast-time-server used the wrong code, causing spec-conformant clients to misclassify the server during version negotiation. Use the existing ERR_UNSUPPORTED_PROTOCOL_VERSION constant in mcp_initialize_response, update the strict-mode tests, and correct the README. Fixes #13 Signed-off-by: Jonathan Springer <jps@s390x.com>
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Summary
Fixes #13.
When the strict server (
--protocol 2026-07-28 --strict) rejects aninitializefor protocol-version reasons, it returned JSON-RPC-32602("Invalid params"). The draft spec requires anUnsupportedProtocolVersionErrorwith code-32022anddata: {supported, requested}— the payload was already correct, only the code was wrong. Spec-conformant clients (e.g. MCP SDKnegotiate_auto()) key on the code, so they misclassified the server's era during negotiation.Changes
src/main.rs:mcp_initialize_responsenow uses the existingERR_UNSUPPORTED_PROTOCOL_VERSION(-32022) constant for strict-mode version rejections.-32602is kept for genuinely malformed params (mcp_invalid_params_response), and-32020for header/body mismatches, per the issue's suggested fix.test_strict_initialize_rejects_unsupported_versionandtest_strict_modern_only_rejects_all_initializenow assert-32022.--strictsection updated to name-32022.Verification
cargo fmt --check,cargo clippy --all-targetsclean;cargo test: 36/36 pass.--protocol 2026-07-28 --strict:{"jsonrpc":"2.0","id":2,"error":{"code":-32022,"message":"Unsupported protocol version","data":{"requested":"2025-11-25","supported":["2026-07-28"]}}}