modelcontextprotocol · Kludex · Feb 10, 2026 · Feb 10, 2026 · Feb 10, 2026 · Feb 10, 2026
diff --git a/CLAUDE.md b/CLAUDE.md
@@ -28,6 +28,7 @@ This document contains critical information about working with this codebase. Fo
    - Bug fixes require regression tests
    - IMPORTANT: The `tests/client/test_client.py` is the most well designed test file. Follow its patterns.
    - IMPORTANT: Be minimal, and focus on E2E tests: Use the `mcp.client.Client` whenever possible.
+   - IMPORTANT: Do NOT test private functions (prefixed with `_`). Test them indirectly through the public API.
 
 Test files mirror the source tree: `src/mcp/client/streamable_http.py` → `tests/client/test_streamable_http.py`
 Add tests to the existing file for that module.

diff --git a/pyproject.toml b/pyproject.toml
@@ -40,6 +40,7 @@ dependencies = [
     "pyjwt[crypto]>=2.10.1",
     "typing-extensions>=4.13.0",
     "typing-inspection>=0.4.1",
+    "opentelemetry-api>=1.28.0",
 ]
 
 [project.optional-dependencies]
@@ -71,6 +72,7 @@ dev = [
     "coverage[toml]>=7.10.7,<=7.13",
     "pillow>=12.0",
     "strict-no-cover",
+    "opentelemetry-sdk>=1.28.0",
 ]
 docs = [
     "mkdocs>=1.6.1",

diff --git a/src/mcp/shared/session.py b/src/mcp/shared/session.py
@@ -8,12 +8,14 @@
 
 import anyio
 from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
+from opentelemetry import trace
 from pydantic import BaseModel, TypeAdapter
 from typing_extensions import Self
 
 from mcp.shared.exceptions import MCPError
 from mcp.shared.message import MessageMetadata, ServerMessageMetadata, SessionMessage
 from mcp.shared.response_router import ResponseRouter
+from mcp.shared.tracing import end_span_error, end_span_ok, start_client_span, start_server_span
 from mcp.types import (
     CONNECTION_CLOSED,
     INVALID_PARAMS,
@@ -77,6 +79,7 @@ def __init__(
         session: BaseSession[SendRequestT, SendNotificationT, SendResultT, ReceiveRequestT, ReceiveNotificationT],
         on_complete: Callable[[RequestResponder[ReceiveRequestT, SendResultT]], Any],
         message_metadata: MessageMetadata = None,
+        span: trace.Span | None = None,
     ) -> None:
         self.request_id = request_id
         self.request_meta = request_meta
@@ -87,6 +90,7 @@ def __init__(
         self._cancel_scope = anyio.CancelScope()
         self._on_complete = on_complete
         self._entered = False  # Track if we're in a context manager
+        self._span = span
 
     def __enter__(self) -> RequestResponder[ReceiveRequestT, SendResultT]:
         """Enter the context manager, enabling request cancellation tracking."""
@@ -126,6 +130,12 @@ async def respond(self, response: SendResultT | ErrorData) -> None:
         if not self.cancelled:  # pragma: no branch
             self._completed = True
 
+            if self._span is not None:
+                if isinstance(response, ErrorData):
+                    end_span_error(self._span, MCPError(code=response.code, message=response.message))
+                else:
+                    end_span_ok(self._span)
+
             await self._session._send_response(  # type: ignore[reportPrivateUsage]
                 request_id=self.request_id, response=response
             )
@@ -139,6 +149,10 @@ async def cancel(self) -> None:
 
         self._cancel_scope.cancel()
         self._completed = True  # Mark as completed so it's removed from in_flight
+
+        if self._span is not None:
+            end_span_error(self._span, MCPError(code=0, message="Request cancelled"))
+
         # Send an error response to indicate cancellation
         await self._session._send_response(  # type: ignore[reportPrivateUsage]
             request_id=self.request_id,
@@ -260,6 +274,9 @@ async def send_request(
             # Store the callback for this request
             self._progress_callbacks[request_id] = progress_callback
 
+        method = request_data["method"]
+        span = start_client_span(method, request_data.get("params"))
+
         try:
             jsonrpc_request = JSONRPCRequest(jsonrpc="2.0", id=request_id, **request_data)
             await self._write_stream.send(SessionMessage(message=jsonrpc_request, metadata=metadata))
@@ -278,7 +295,15 @@ async def send_request(
             if isinstance(response_or_error, JSONRPCError):
                 raise MCPError.from_jsonrpc_error(response_or_error)
             else:
-                return result_type.model_validate(response_or_error.result, by_name=False)
+                result = result_type.model_validate(response_or_error.result, by_name=False)
+                if span is not None:
+                    end_span_ok(span)
+                return result
+
+        except BaseException as exc:
+            if span is not None:
+                end_span_error(span, exc)
+            raise
 
         finally:
             self._response_streams.pop(request_id, None)
@@ -339,13 +364,16 @@ async def _receive_loop(self) -> None:
                                 message.message.model_dump(by_alias=True, mode="json", exclude_none=True),
                                 by_name=False,
                             )
+                            request_data = message.message.model_dump(by_alias=True, mode="json", exclude_none=True)
+                            server_span = start_server_span(request_data["method"], request_data.get("params"))
                             responder = RequestResponder(
                                 request_id=message.message.id,
                                 request_meta=validated_request.params.meta if validated_request.params else None,
                                 request=validated_request,
                                 session=self,
                                 on_complete=lambda r: self._in_flight.pop(r.request_id, None),
                                 message_metadata=message.metadata,
+                                span=server_span,
                             )
                             self._in_flight[responder.request_id] = responder
                             await self._received_request(responder)

diff --git a/src/mcp/shared/tracing.py b/src/mcp/shared/tracing.py
@@ -0,0 +1,81 @@
+from __future__ import annotations
+
+from typing import Any
+
+from opentelemetry import trace
+from opentelemetry.trace import StatusCode
+
+_tracer = trace.get_tracer("mcp")
+
+_EXCLUDED_METHODS: frozenset[str] = frozenset({"notifications/message"})
+
+# Semantic convention attribute keys
+ATTR_MCP_METHOD_NAME = "mcp.method.name"
+ATTR_ERROR_TYPE = "error.type"
+
+# Methods that have a meaningful target name in params
+_TARGET_PARAM_KEY: dict[str, str] = {
+    "tools/call": "name",
+    "prompts/get": "name",
+    "resources/read": "uri",
+}
+
+
+def _extract_target(method: str, params: dict[str, Any] | None) -> str | None:
+    """Extract the target (e.g. tool name, prompt name) from request params."""
+    key = _TARGET_PARAM_KEY.get(method)
+    if key is None or params is None:
+        return None
+    value = params.get(key)
+    if isinstance(value, str):
+        return value
+    return None
+
+
+def start_client_span(method: str, params: dict[str, Any] | None) -> trace.Span | None:
+    """Start a CLIENT span for an outgoing MCP request.
+
+    Returns None if the method is excluded from tracing.
+    """
+    if method in _EXCLUDED_METHODS:
+        return None
+
+    target = _extract_target(method, params)
+    span_name = f"{method} {target}" if target else method
+    span = _tracer.start_span(
+        span_name,
+        kind=trace.SpanKind.CLIENT,
+        attributes={ATTR_MCP_METHOD_NAME: method},
+    )
+    return span
+
+
+def start_server_span(method: str, params: dict[str, Any] | None) -> trace.Span | None:
+    """Start a SERVER span for an incoming MCP request.
+
+    Returns None if the method is excluded from tracing.
+    """
+    if method in _EXCLUDED_METHODS:
+        return None
+
+    target = _extract_target(method, params)
+    span_name = f"{method} {target}" if target else method
+    span = _tracer.start_span(
+        span_name,
+        kind=trace.SpanKind.SERVER,
+        attributes={ATTR_MCP_METHOD_NAME: method},
+    )
+    return span
+
+
+def end_span_ok(span: trace.Span) -> None:
+    """Mark a span as successful and end it."""
+    span.set_status(StatusCode.OK)
+    span.end()
+
+
+def end_span_error(span: trace.Span, error: BaseException) -> None:
+    """Mark a span as errored and end it."""
+    span.set_status(StatusCode.ERROR, str(error))
+    span.set_attribute(ATTR_ERROR_TYPE, type(error).__qualname__)
+    span.end()