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mcp: omit structured output from error results - #1397
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guglielmo-san merged 1 commit intoOct 13, 2026
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A typed tool handler that reports a tool execution error by returning a result with IsError set (and a nil error) still has its Out value marshaled into structuredContent and validated against the output schema. Skip that step for error results, as is already done when the handler asks for input. Fixes modelcontextprotocol#1387
guglielmo-san
approved these changes
Oct 13, 2026
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A typed tool handler that reports a tool execution error by returning a result with
IsError: trueand a nil error still has itsOutvalue marshaled intostructuredContentand validated against the output schema. Clients that readstructuredContentget a zero value such as{"items":null}next to the error, and an output schema that rejects the zero value turns the tool error into a protocol error (validating tool output: ...).This change skips the output step for an error result, as
toolForErralready does when the handler asks for input. A handler that wants structured data on an error result can still setres.StructuredContentitself; it is passed through unchanged.Fixes #1387
Tested with
go test ./...on Go 1.27.1, darwin/arm64; the new test fails without the change.