Slim ServerMiddleware to (ctx, call_next) and add OpenTelemetryMiddleware#2941
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Kludex
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Jun 21, 2026
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I didn't realize I created those tests. I don't think they are necessary. We already have tests for otel.
Kludex
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Jun 21, 2026
Kludex
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Jun 21, 2026
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…dleware` Move `method` and `params` onto `ServerRequestContext` so context-tier middleware reads `ctx.method`/`ctx.params` instead of separate positional args. `CallNext` now takes the context, so middleware can rewrite the inbound message with `call_next(replace(ctx, params=...))`. Add a context-tier `OpenTelemetryMiddleware` alongside the existing dispatch-tier `otel_middleware`, which is left intact.
- OpenTelemetryMiddleware: nest under the ambient span when no traceparent is present. Passing an explicit empty Context to start_as_current_span orphans the span; extract_trace_context now returns None for an absent or malformed carrier so callers fall through to ambient parenting. Adds a regression test with both span tiers installed. - Rename 'mw' -> 'middleware' across runner.py; drop redundant CallNext annotation on the compose-loop accumulator. - Document that 'initialize' is observed by ServerMiddleware but not rewritable: the post-chain handshake commit reads the wire params, so a rewritten ctx.params on initialize does not reach connection state. TODO at the commit site names the desync triggers; resolves when initialize becomes a built-in handler.
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A non-empty _meta without a traceparent key (e.g. only a progressToken) made extract() return an empty Context, which orphans the span when passed explicitly to start_as_current_span. Check the extracted context carries a valid span and return None otherwise so callers fall through to ambient parenting.
felixweinberger
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Jun 22, 2026
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What
Reshapes the context-tier
ServerMiddlewareinterface and adds a context-tierOpenTelemetryMiddleware.ServerMiddleware.__call__goes from(ctx, method, params, call_next)to(ctx, call_next).methodandparamsnow live onServerRequestContextasctx.methodand rawctx.params.CallNexttakes the context:Callable[[ServerRequestContext], Awaitable[HandlerResult]]. Middleware passes it through withcall_next(ctx), and can rewrite the inbound message viacall_next(replace(ctx, params=...)).OpenTelemetryMiddleware(context-tier) inmcp/server/_otel.py, mirroring the span shape of the existing dispatch-tierotel_middleware, which is left intact. It spans both requests and notifications, settingjsonrpc.request.idonly when present.Why
The 4-arg signature duplicated
method/paramsthat already belong on the context, andcall_next()could only observe, not rewrite. The new shape matches the pure-ASGI middleware model (single context in, pass it through, mutate before next) and lets middleware redirect params before the handler runs.Notes
ctx.paramsis deliberate: middleware runs before method lookup and validation (it wraps the failure path too), so no single validated type exists at that tier. Authors validate on demand withModel.model_validate(ctx.params or {}).docs/migration.md(no compat shim - the 4-arg form never shipped in a release).request_id is Nonepath was confirmed correct.AI Disclaimer
This PR was developed with the assistance of either Claude or Codex. I've reviewed and verified the changes.