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Surface: GET /agents/runs/{id}, error.type = "ToolValidation"

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When this error occurs

The model produced arguments that didn’t satisfy the tool’s input schema:
  • Required field missing.
  • Field type didn’t match (string expected, integer given).
  • additionalProperties: false rejected an extra key.
The MCP server rejected the call with JSON-RPC -32602. (The platform doesn’t carry a copy of the input schema for MCP tools — it discovers tools/list per run and forwards the schema to the model verbatim, then forwards model output to the server. Validation happens server-side.)

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How to fix

  • Improve the input_schema in the run’s config to be tighter — required fields, strict enums, additionalProperties: false. Tighter schemas → fewer model mistakes.
  • Strengthen the tool description with a concrete example so the model copies the shape.
  • This error is not retried by the workflow; the activity tag is in RetryOptions.setDoNotRetry. The run fails fast and the caller can re-prompt.