fix(server): allow tools with outputSchema to surface errors via unstructured content#2447
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…ructured content Per modelcontextprotocol#2429, a low-level tool with an `outputSchema` cannot return a tool-execution failure (e.g. "Resource not found") via unstructured `TextContent` because the call_tool decorator unconditionally validates the result against `outputSchema` and replaces the original error message with a hard "outputSchema defined but no structured output returned" error. Mirror the TypeScript SDK fix (modelcontextprotocol/typescript-sdk#655): when an outputSchema-bound tool returns no structured content, treat the unstructured payload as a tool-error result (`isError=True`) instead of replacing it. Tools that *do* return structured output keep their existing validation contract, and the explicit `CallToolResult`-returning path is unchanged. Github-Issue:modelcontextprotocol#2429
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Per #2429, a low-level tool with an
outputSchemacannot return a tool-execution failure (e.g. "Resource not found") via unstructuredTextContentbecause thecall_tooldecorator unconditionally validates the result againstoutputSchemaand replaces the original error message with a hard "outputSchema defined but no structured output returned" error.This PR mirrors the TypeScript SDK fix in modelcontextprotocol/typescript-sdk#655: when an outputSchema-bound tool returns no structured content, treat the unstructured payload as a tool-error result (
isError=True) instead of replacing it. Tools that do return structured output keep their existing validation contract, and the explicitCallToolResult-returning path is unchanged.Closes #2429