Stop flagging snake_case is_error results as tool errors in OTel span

The tools/call result match arm treated {"is_error": True} as a tool error,
but serialize_server_result validates with by_name=False (alias-only), so the
snake_case key is dropped and the client receives a success. The span then
contradicted the wire response. Only CallToolResult and the camelCase
{"isError": True} dict survive serialization as errors; drop the snake_case
arm and assert the result stays a success.

Author: Kludex

src/mcp/server/_otel.py

@@ -59,8 +59,9 @@ async def __call__(self, ctx: ServerRequestContext[Any, Any], call_next: CallNex
                 span.set_status(StatusCode.ERROR, str(e))
                 raise
             if ctx.method == "tools/call":
+                # Only shapes that survive wire serialization (alias-only) are real tool errors.
                 match result:
-                    case CallToolResult(is_error=True) | {"isError": True} | {"is_error": True}:
+                    case CallToolResult(is_error=True) | {"isError": True}:
                         span.set_attribute("error.type", "tool_error")
                         span.set_status(StatusCode.ERROR)
                     case _:

tests/server/test_otel.py

@@ -92,19 +92,22 @@ async def err_tool(ctx: Ctx, params: CallToolRequestParams) -> CallToolResult:
 
 
 @pytest.mark.anyio
-async def test_tool_error_snake_case_dict_result_sets_error_type(server: SrvT, spans: SpanCapture):
+async def test_snake_case_dict_result_is_not_a_tool_error(server: SrvT, spans: SpanCapture):
+    # `is_error` is alias-only on the wire, so serialization drops it; the result reaches the
+    # client as a success and the span must not contradict that.
     async def err_tool(ctx: Ctx, params: CallToolRequestParams) -> dict[str, Any]:
         return {"content": [], "is_error": True}
 
     server.add_request_handler("tools/call", CallToolRequestParams, err_tool)
     server.middleware.append(OpenTelemetryMiddleware())
     async with connected_runner(server) as (client, _):
         spans.clear()
-        await client.send_raw_request("tools/call", {"name": "mytool", "arguments": {}})
+        result = await client.send_raw_request("tools/call", {"name": "mytool", "arguments": {}})
+    assert result == {"content": []}
     [span] = [s for s in spans.finished() if s.kind == SpanKind.SERVER]
     assert span.attributes is not None
-    assert span.attributes["error.type"] == "tool_error"
-    assert span.status.status_code == StatusCode.ERROR
+    assert "error.type" not in span.attributes
+    assert span.status.status_code == StatusCode.UNSET
 
 
 @pytest.mark.anyio