fix(mcp): never let instrumentation errors break the traced MCP call - #4464
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fix(mcp): never let instrumentation errors break the traced MCP call#4464chengwudi1 wants to merge 2 commits into
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BaseSession.send_request was wrapped with @dont_throw, which swallows any exception raised by the instrumentation and returns None. send_request's return value is the RPC result, so a tracing error replaced the real CallToolResult with None and crashed callers with "AttributeError: 'NoneType' object has no attribute 'isError'". Two throwers were observed: - carrier["traceparent"] KeyError when inject() writes nothing, which happens whenever the call is made outside any span (e.g. the first call after instrumenting) or with an invalid span context; - result.content[0].text AttributeError for error results carrying non-text content blocks (image, audio, embedded resource), raised after the RPC had already succeeded. Fix: - patch_mcp_client: drop @dont_throw. Only the pre-call metadata extraction/propagation is wrapped in try/except; a failure there is logged at debug level and the request is still sent, uninstrumented. - _execute_and_handle_result: split the wrapped call from the result decoration. Exceptions from the wrapped call are still recorded on the span and re-raised; decoration failures are logged and the real result is returned. content[0].text is guarded for non-TextContent. Fixes traceloop#4463
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Included review availability: Your plan provides up to 8 included reviews per hour; 7 remain after this review. 📝 WalkthroughWalkthroughMCP request instrumentation isolates metadata, propagation, and span-processing failures from wrapped calls. Wrapped MCP results are preserved and returned. Error results with non-text content receive fallback status messages. Regression tests cover root calls and image content. ChangesMCP instrumentation safety
Estimated code review effort: 3 (Moderate) | ~20 minutes Severity of issue fixed: Medium Merge Risk: ⚪ Minimal · up to MCP tracing failures are isolated so request results and RPC exceptions retain their original behavior, including for unavailable tracing context and non-text error content. No merge-blocking risk remains. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
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✨ Finishing Touches🧪 Generate unit tests (beta)
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Inline comments:
In
`@packages/opentelemetry-instrumentation-mcp/opentelemetry/instrumentation/mcp/instrumentation.py`:
- Around line 211-215: Replace the debug-level logging for swallowed
instrumentation failures with the project logger at WARNING or another
production-visible level. Update both the MCP request metadata extraction/trace
propagation failure handler around the existing log call and the
output-decoration failure handler at
packages/opentelemetry-instrumentation-mcp/opentelemetry/instrumentation/mcp/instrumentation.py
lines 388-392, preserving the existing error details and message context.
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packages/opentelemetry-instrumentation-mcp/opentelemetry/instrumentation/mcp/instrumentation.pypackages/opentelemetry-instrumentation-mcp/tests/test_send_request_safety.py
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CodeRabbit review on traceloop#4464: the tracing failures these wrappers deliberately suppress were logged at debug level, which is filtered out in production — leaving operators with no record of the very failures that silenced a span. Log both sites (metadata extraction / trace-context propagation, and result decoration) at warning level.
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What changed?
BaseSession.send_requestwas wrapped with@dont_throw, which swallows any exception raised by the instrumentation and returnsNone. Butsend_request's return value is the RPC result — so a failure inside tracing replaced the realCallToolResultwithNoneand broke the traced call:Two throwers observed:
carrier["traceparent"]KeyError —inject()writes no traceparent when the request is made outside any span (e.g. the first call after instrumenting) or the span context is invalid.result.content[0].textAttributeError — raised after the RPC already succeeded, for error results carrying non-text content blocks (image, audio, embedded resource). OnlyTextContentexposes.text.How?
patch_mcp_client: drop@dont_throw. Only the pre-call metadata extraction + trace-context propagation is wrapped in try/except; a failure there is logged at debug level and the request is still sent, uninstrumented (fail-open)._execute_and_handle_result: split the wrapped call from the result decoration. Exceptions from the wrapped call are still recorded on the span and re-raised; decoration failures are logged and the real result is returned.content[0].textis guarded for non-TextContent blocks.Tests
Two regression tests in
tests/test_send_request_safety.py:test_tool_call_outside_active_span_returns_real_result— real FastMCP client, call with ametawhile no span is current (KeyError path).test_error_result_with_non_text_content_block_is_returned— error result with anImageContentblock is still returned to the caller (AttributeError path).Both fail on main (None / AttributeError) and pass with this change.
tests/passes (7 tests) andruff checkis clean.Fixes #4463
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