fix(mcp): propagate trace context when metadata is absent

packages/opentelemetry-instrumentation-mcp/opentelemetry/instrumentation/mcp/instrumentation.py

@@ -187,16 +187,20 @@ async def traced_method(wrapped, instance, args, kwargs):
                 method = args[0].root.method
             if len(args) > 0 and hasattr(args[0].root, "params"):
                 params = args[0].root.params
-            if params:
-                if hasattr(args[0].root.params, "meta"):
-                    meta = args[0].root.params.meta
+            if params is not None and hasattr(params, "meta"):
+                meta = params.meta
+                if meta is None:
+                    from mcp.types import RequestParams
 
-            # Handle trace context propagation
-            if meta and len(args) > 0:
+                    meta = RequestParams.Meta()
+
+            # Handle trace context propagation, including requests without _meta.
+            if meta is not None:
                 carrier = {}
                 TraceContextTextMapPropagator().inject(carrier)
-                meta.traceparent = carrier["traceparent"]
-                args[0].root.params.meta = meta
+                for key, value in carrier.items():
+                    setattr(meta, key, value)
+                params.meta = meta
 
             # Create different span types based on method
             if method == "tools/call":

packages/opentelemetry-instrumentation-mcp/tests/test_client_trace_context.py

@@ -0,0 +1,42 @@
+from types import SimpleNamespace
+from unittest.mock import AsyncMock
+
+import pytest
+from mcp.types import CallToolRequestParams, RequestParams
+from opentelemetry.instrumentation.mcp import instrumentation
+from opentelemetry.instrumentation.mcp.instrumentation import McpInstrumentor
+
+
+@pytest.mark.parametrize(
+    ("meta", "expected_custom_value"),
+    [
+        (None, None),
+        (RequestParams.Meta(custom_value="preserved"), "preserved"),
+    ],
+)
+async def test_client_trace_context_is_injected_without_replacing_metadata(monkeypatch, meta, expected_custom_value):
+    """Verify ordinary tool calls propagate trace context and preserve metadata."""
+    propagator = AsyncMock()
+    propagator.inject = lambda carrier: carrier.update(
+        {
+            "traceparent": "00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01",
+            "tracestate": "vendor=value",
+        }
+    )
+    monkeypatch.setattr(instrumentation, "TraceContextTextMapPropagator", lambda: propagator)
+
+    # BaseInstrumentor is a singleton. Bypass it so this unit test cannot mutate
+    # the session-scoped instrumentor used by the integration tests.
+    instrumentor = object.__new__(McpInstrumentor)
+    instrumentor._handle_tool_call = AsyncMock(return_value="sent")
+    params = CallToolRequestParams(name="test", arguments={}, _meta=meta)
+    message = SimpleNamespace(root=SimpleNamespace(method="tools/call", params=params))
+
+    wrapped = AsyncMock()
+    result = await instrumentor.patch_mcp_client(AsyncMock())(wrapped, None, (message,), {})
+
+    assert result == "sent"
+    assert params.meta is not None
+    assert params.meta.traceparent == "00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01"
+    assert params.meta.tracestate == "vendor=value"
+    assert getattr(params.meta, "custom_value", None) == expected_custom_value