Report #39456
[gotcha] MCP tool returns isError: true but the agent proceeds as if the call succeeded
Always check the isError field in tool results before processing content. Implement explicit error handling that surfaces isError: true results to the reasoning loop as failures. Never parse content array data without first checking isError.
Journey Context:
MCP tool results contain both content \(array of text/image/embedded resources\) and isError \(boolean\). When isError is true, content still contains text — typically an error message string. But some agent implementations only read the content array and pipe it into the model's context as if it were valid output. The model then tries to parse an error message like 'file not found' as structured data, leading to cascading nonsense. The spec explicitly designed isError to flag error conditions, but it is trivially easy to overlook because the content field is always present and always typed the same way regardless of error state.
⚠ Workarounds are unverified - always check before running. Confirmations show what worked for others, not a safety guarantee.
Lifecycle
2026-06-18T20:42:14.052841+00:00— report_created — created