issue #4038 - fix(mcp): handle dict/object result shapes in stream writer error parsing

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

@@ -547,6 +547,29 @@ async def __aenter__(self) -> Any:
     async def __aexit__(self, exc_type: Any, exc_value: Any, traceback: Any) -> Any:
         return await self.__wrapped__.__aexit__(exc_type, exc_value, traceback)
 
+    @staticmethod
+    def _get_value(data: Any, key: str, default: Any = None) -> Any:
+        if isinstance(data, dict):
+            return data.get(key, default)
+        return getattr(data, key, default)
+
+    @classmethod
+    def _extract_result_error(cls, result: Any) -> Tuple[bool, Union[str, None]]:
+        is_error = cls._get_value(result, "isError", False) is True
+        if not is_error:
+            return False, None
+
+        content = cls._get_value(result, "content", [])
+        if not content:
+            return True, None
+
+        first_item = content[0]
+        message = cls._get_value(first_item, "text")
+        if message is None:
+            return True, None
+
+        return True, str(message)
+
     @dont_throw
     async def send(self, item: Any) -> Any:
         from mcp.types import JSONRPCMessage, JSONRPCRequest
@@ -567,14 +590,9 @@ async def send(self, item: Any) -> Any:
                 span.set_attribute(
                     SpanAttributes.MCP_RESPONSE_VALUE, f"{serialize(request.result)}"
                 )
-                if "isError" in request.result:
-                    if request.result["isError"] is True:
-                        span.set_status(
-                            Status(
-                                StatusCode.ERROR,
-                                f"{request.result['content'][0]['text']}",
-                            )
-                        )
+                is_error, error_message = self._extract_result_error(request.result)
+                if is_error:
+                    span.set_status(Status(StatusCode.ERROR, error_message))
             if hasattr(request, "id"):
                 span.set_attribute(SpanAttributes.MCP_REQUEST_ID, f"{request.id}")
 

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

@@ -1,3 +1,8 @@
+from types import SimpleNamespace
+
+from opentelemetry.instrumentation.mcp.instrumentation import InstrumentedStreamWriter
+
+
 async def test_fastmcp_instrumentor(span_exporter, tracer_provider) -> None:
     from fastmcp import FastMCP, Client
 
@@ -146,3 +151,33 @@ def get_greeting() -> str:
         assert workflow_name == 'test-server.mcp', (
             f"Expected workflow name 'test-server.mcp' on tool span, got '{workflow_name}'"
         )
+
+
+def test_extract_result_error_with_dict_shape():
+    result = {"isError": True, "content": [{"text": "dict error"}]}
+
+    is_error, message = InstrumentedStreamWriter._extract_result_error(result)
+
+    assert is_error is True
+    assert message == "dict error"
+
+
+def test_extract_result_error_with_object_shape():
+    result = SimpleNamespace(
+        isError=True,
+        content=[SimpleNamespace(text="object error")],
+    )
+
+    is_error, message = InstrumentedStreamWriter._extract_result_error(result)
+
+    assert is_error is True
+    assert message == "object error"
+
+
+def test_extract_result_error_with_missing_content():
+    result = {"isError": True, "content": []}
+
+    is_error, message = InstrumentedStreamWriter._extract_result_error(result)
+
+    assert is_error is True
+    assert message is None