Interoperating with OpenTelemetry instrumentation in the MCP SDK #3451

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1. aabmass

   added

   enhancementImprovement to existing functionality. For issues and smaller PR improvements.Improvement to existing functionality. For issues and smaller PR improvements.

   on Mar 10, 2026
2. marvin-context-protocol

   added

   serverRelated to FastMCP server implementation or server-side functionality.Related to FastMCP server implementation or server-side functionality.

   on Mar 10, 2026
3. jlowin commented on Mar 13, 2026

   

   jlowin

   on Mar 13, 2026

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   Thanks @aabmass! Copying @chrisguidry for visibility, as he's the OTEL guru on the FastMCP side
4. chrisguidry commented on Mar 19, 2026

   

   chrisguidry

   on Mar 19, 2026

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   Thanks for the heads up, @aabmass! I think we can definitely unify things once the SDK's spans are in place. We tried to stick as close to the OTel semantic conventions for MCP, so I'm betting that once you have spans in place, we can just remove ours and add back any fastmcp-specific attributes to them where we need. Theoretically, our unit test suite should remain unchanged 🤞

   👍React with 👍2Reacted by Bill Easton and Aaron Abbott
5. strawgate commented on Mar 25, 2026

   

   strawgate

   on Mar 25, 2026

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   Happy to help look at this too
6. ringerc commented on May 8, 2026

   

   ringerc

   on May 8, 2026

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   I just hit this issue with httpx not seeing trace context. Quite confusing.

   Context propagation should still work at application layer if _meta is decoded by the request receiver, but two parallel traces may be generated (one for transport, one for application) which is very confusing.

   Workaround

   Patch StreamableHTTPTransport.post_writer to restore trace context from _meta before spawning request handler tasks:

   1. Extract traceparent/tracestate from params._meta when processing each message
   2. Wrap the task function to attach the extracted OTel context before execution
   3. The httpx instrumentation then sees the correct parent and injects matching HTTP headers

   # Extract trace context from _meta
   traceparent = params.get("_meta", {}).get("traceparent")
   if traceparent:
       carrier = {"traceparent": traceparent}
       trace_ctx = extract(carrier)
   
   # Wrap handler to run with restored trace context
   async def handle_with_context():
       token = otel_context.attach(trace_ctx)
       try:
           await handle_request_async()
       finally:
           otel_context.detach(token)
   
   tg.start_soon(handle_with_context)