feat(core): reserved trace context _meta keys and propagation docs (SEP-414)#2270
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feat(core): reserved trace context _meta keys and propagation docs (SEP-414)#2270mattzcarey wants to merge 1 commit into
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…EP-414) Add TRACEPARENT_META_KEY, TRACESTATE_META_KEY, and BAGGAGE_META_KEY constants for the unprefixed _meta keys reserved by SEP-414, export them from core/public, document trace context propagation in the client and server guides (with type-checked examples), and add a passthrough regression test covering request, response, and notification _meta.
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Implements the SDK side of SEP-414: OpenTelemetry Trace Context Propagation Conventions.
The draft MCP specification (
basic/index.mdx, "Distributed Tracing Conventions") reserves the keystraceparent,tracestate, andbaggagein_metaas an explicit exception to the_metakey prefix rule. When present, the values MUST follow the W3C Trace Context (traceparent,tracestate) and W3C Baggage formats. Spec PR: modelcontextprotocol/modelcontextprotocol#414.Note: passthrough already worked — the SDK never touches user-supplied
_metabeyond the envelope keys it owns, so trace context already propagated end to end. This PR adds the named constants, documentation, and a regression test to lock the behavior in:packages/core/src/types/constants.ts:TRACEPARENT_META_KEY,TRACESTATE_META_KEY,BAGGAGE_META_KEYwith JSDoc citing SEP-414 and the W3C formats, placed alongside the other reserved_metakey constants and exported viacore/public(and therefore re-exported by@modelcontextprotocol/client/@modelcontextprotocol/server).docs/client.md/docs/server.md: new "Trace context propagation" sections with type-checked examples (per-request_meta, fetch-middleware injection on the client; readingctx.mcpReq._metain a handler on the server), synced viapnpm sync:snippets.packages/core/test/shared/traceContextMeta.test.ts: regression test asserting_meta.traceparent/tracestate/baggagesurvive the round trip to the server-side handler, back on the response_meta, and through notifications.@modelcontextprotocol/core.Validation
pnpm build:all— passpnpm check:all(typecheck + lint, all packages) — passpnpm --filter @modelcontextprotocol/core test— 467/467 pass (incl. 4 new)pnpm --filter @modelcontextprotocol/client test— 367/367 passpnpm --filter @modelcontextprotocol/server test— 72/72 passpnpm test:all— all tests pass; the only flagged item is the pre-existing e2e fake-timer/SSE unhandled-rejection flake inscenarios/protocol.test.ts(protocol:timeout:max-total [sse]), which reproduces identically on cleanmain.Downstream impact: none for cloudflare/agents; enables Workers trace propagation later.
Note for maintainers: if the constants are considered clutter, happy to reduce this to docs-only.
Closes #2196