What

Reshapes the context-tier ServerMiddleware interface and adds a context-tier OpenTelemetryMiddleware.

• ServerMiddleware.__call__ goes from (ctx, method, params, call_next) to (ctx, call_next). method and params now live on ServerRequestContext as ctx.method and raw ctx.params.
• CallNext takes the context: Callable[[ServerRequestContext], Awaitable[HandlerResult]]. Middleware passes it through with call_next(ctx), and can rewrite the inbound message via call_next(replace(ctx, params=...)).
• New OpenTelemetryMiddleware (context-tier) in mcp/server/_otel.py, mirroring the span shape of the existing dispatch-tier otel_middleware, which is left intact. It spans both requests and notifications, setting jsonrpc.request.id only when present.

Why

The 4-arg signature duplicated method/params that already belong on the context, and call_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

• Raw ctx.params is 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 with Model.model_validate(ctx.params or {}).
• Documented in docs/migration.md (no compat shim - the 4-arg form never shipped in a release).
• Reviewed with Codex; the notification request_id is None path was confirmed correct.

AI Disclaimer

This PR was developed with the assistance of either Claude or Codex. I've reviewed and verified the changes.