MCP 2026-07-28 (1/11): protocol foundations — versions, typed errors, resultType - #224
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MCP 2026-07-28 (1/11): protocol foundations — versions, typed errors, resultType#224simonx1 wants to merge 4 commits into
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…ltType Lay the groundwork for the stateless 2026-07-28 revision: - LATEST/MODERN/LEGACY protocol version constants; SUPPORTED is their union and initialize only accepts a legacy negotiated version. - ServerError carries the JSON-RPC code and data; ServerError.from_jsonrpc builds HeaderMismatchError (-32020), MissingRequiredClientCapabilityError (-32021) and UnsupportedProtocolVersionError (-32022). Errors::Codes holds the allocation-policy constants. - process_jsonrpc_response validates resultType: absent means complete, unrecognized values raise InvalidResultError (a non-retried ServerError). Stdio now routes every response through it so codes are preserved. - resources/read errors with -32602 (or legacy -32002) raise ResourceNotFound on every transport. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01MoErzDypnq7hhuFBtueML7
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Codex review: only callers of process_jsonrpc_response checked resultType. ServerSSE#check_for_result returned the stored result directly and the stdio initialize handshake read res['result'] itself, so an unrecognized resultType passed through on those paths. Both now run the same validation; a stdio initialize error still surfaces as ConnectionError. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01MoErzDypnq7hhuFBtueML7
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…ot-found Review findings (grok): - HTTP, Streamable HTTP and SSE discarded the JSON-RPC body of a 4xx, so the 2026-07-28 protocol errors (-32020/-32021/-32022, which the spec requires servers to send with HTTP 400) never became typed errors and a dual-era client could not recognize a modern server. The body is now parsed (gzip-aware) and the typed error raised with the HTTP status in the message; 5xx stays TransientServerError. - resources/read -32602 only means not-found on a modern session; on a legacy (2025-11-25) session it remains generic Invalid params. Added protocol_version/modern? readers to JsonRpcCommon for the gate. - A stdio initialize rejected with UnsupportedProtocolVersionError now names the versions the server supports; initialize answering with a modern version is covered by a test. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01MoErzDypnq7hhuFBtueML7
…and raise_error Second review round (codex + grok): - Only a JSON-RPC 2.0 error body carrying the data its schema mandates (data.supported for -32022, data.requiredCapabilities for -32021) counts as a recognized modern error, so a bare -3202x from a legacy endpoint or intermediary cannot suppress the fallback. ServerError#http_status records the HTTP status the error arrived with. - Faraday raise_error middleware no longer bypasses body inspection: a 4xx raised as Faraday::ClientError is parsed like the response path and never becomes a retryable TransportError. - HTTP error bodies are inspected only up to 64 KiB (gzip expansion included); larger bodies yield a plain ServerError without parsing. - Protocol-level errors (the typed -3202x errors and InvalidResultError, ServerError#protocol_error?) propagate through call_tool, get_prompt, read_resource and every stdio public method instead of being wrapped into ToolCallError and friends, so requiredCapabilities and supported versions reach the host. - A modern result must be an object; an explicit null resultType is invalid in either era. Legacy bare-array results stay tolerated. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01MoErzDypnq7hhuFBtueML7
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Summary
First of a stacked series implementing the MCP 2026-07-28 revision. This PR lays the base-protocol groundwork the later feature PRs build on; nothing changes on the wire yet.
LATEST_PROTOCOL_VERSION(2026-07-28),MODERN_PROTOCOL_VERSIONS,LEGACY_PROTOCOL_VERSIONS;SUPPORTED_PROTOCOL_VERSIONSis their union.PROTOCOL_VERSIONstays2025-11-25(the legacyinitializerequest version), andinitializenow only accepts a legacy negotiated version.Errors::Codesconstants;ServerErrorgainscode/data(backward-compatible ctor);ServerError.from_jsonrpcbuilds the typedHeaderMismatchError(-32020),MissingRequiredClientCapabilityError(-32021,#required_capabilities) andUnsupportedProtocolVersionError(-32022,#supported/#requested). All four transports raise them; stdio now routes every response throughprocess_jsonrpc_responseso codes are never dropped.resultType— absent ⇒"complete"(earlier-protocol servers),"input_required"passes through (handled by the MRTR PR), anything unrecognized raisesInvalidResultError(a non-retriedServerError). Transports can widen the accepted set viaaccepted_result_types(used by the tasks extension PR).resources/readerrors with-32602(or legacy-32002) raiseResourceNotFoundon every transport.Spec references
Test plan
bundle exec rspec— 1701 examples, 0 failures (44 new)bundle exec rubocop— clean🤖 Generated with Claude Code
https://claude.ai/code/session_01MoErzDypnq7hhuFBtueML7