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Stop flagging snake_case is_error results as tool errors in OTel span #2971

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Merged
Kludex merged 2 commits into from
Jun 25, 2026
+12 −5
Merged

Stop flagging snake_case is_error results as tool errors in OTel span #2971

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c0ab4e9
Stop flagging snake_case is_error results as tool errors in OTel span
Kludex Jun 25, 2026
3eda836
Document literal-bool limitation of tools/call error detection
Kludex Jun 25, 2026
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6 changes: 5 additions & 1 deletion src/mcp/server/_otel.py
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Original file line number Diff line number Diff line change
Expand Up @@ -59,8 +59,12 @@ async def __call__(self, ctx: ServerRequestContext[Any, Any], call_next: CallNex
span.set_status(StatusCode.ERROR, str(e))
raise
if ctx.method == "tools/call":
# Tool errors are detected pre-serialization, so only shapes that reach the wire as an error
# count: the model, or the camelCase alias (`is_error` is dropped by the alias-only wire
# validation). A raw-dict `isError` is matched as a literal bool only - non-bool coercible
# values (1, "true") would serialize to an error but are rare enough to leave undetected.
match result:
case CallToolResult(is_error=True) | {"isError": True} | {"is_error": True}:
case CallToolResult(is_error=True) | {"isError": True}:

@cubic-dev-ai cubic-dev-ai Bot Jun 25, 2026 •
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P2: Tool-error span detection now misses coercible isError inputs that serialize to true, so telemetry can still diverge from the actual wire error result.

Prompt for AI agents 
Check if this issue is valid — if so, understand the root cause and fix it. At src/mcp/server/_otel.py, line 64:

<comment>Tool-error span detection now misses coercible `isError` inputs that serialize to `true`, so telemetry can still diverge from the actual wire error result.</comment>

<file context>
@@ -59,8 +59,9 @@ async def __call__(self, ctx: ServerRequestContext[Any, Any], call_next: CallNex
+                # 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)
</file context>
Fix with cubic

span.set_attribute("error.type", "tool_error")
span.set_status(StatusCode.ERROR)
case _:
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11 changes: 7 additions & 4 deletions tests/server/test_otel.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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
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