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feat(core): Continue MCP server traces from params._meta - #26308
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size-limit report 📦Compared: d90ea11 (PR test merge using develop at c9e61e3) Against: c9e61e3 (develop) · base run #65906 ✅ Baseline matches: this diff contains only this PR's changes.
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mydea
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Nov 12, 2026
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Fixes #17152 Co-Authored-By: Francesco Gringl-Novy <francesco.novy@sentry.io>
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Why: MCP clients propagate trace context per message in
params._meta(SEP-414, OTel MCP semconv), but our server instrumentation ignored it, so MCP server spans never connected to the calling agent's trace. Over stdio there was no other way to connect them at all. Incoming requests and notifications now continue the trace from_meta. The span that was active when the message arrived (e.g. thehttp.serverspan on Streamable HTTP) is added as a span link, which is what the semconv asks for, since MCP and transport contexts are independent.sentry-trace+baggagein_metaalways continue the trace. W3Ctraceparentis only read whenpropagateTraceparentis enabled, matching how the SDK treats W3C elsewhere: opt-in, and with no default continuation from incomingtraceparent. Malformed values fall back to today's behavior instead of starting a fresh trace. The W3C →sentry-traceconverter moved from the Flue integration into core so both can share it.How to review: Start with
packages/core/src/integrations/mcp-server/tracePropagation.ts. The transport and span changes just pass its result through. The node integration test uses the real@modelcontextprotocol/clientto make sure_metasurvives end to end.Unsure about:
_meta, the MCP span moves out of the HTTP request's trace into the caller's trace, and the HTTP span is only reachable via the link. That's spec behavior but a visible change, so it probably deserves a changelog line.traceparentwon't connect until users setpropagateTraceparent: true.sentry.link.type, because none of the defined values fit a transport relationship.Closes #17152
via Francesco Gringl-Novy.
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