[Bug]: Output validation error: outputSchema defined but no structured output returned when not setting any output schema.

[ikauwsp]

🐞 Bug Summary

For restAPI tool, tool is working fine. However, it always returns Output validation error from outputSchema, even though I left it empty. It always says that outputSchema is defined and hence unable to get the correct response from the tool.

🤔 Expected Behavior

When leaving the optional output schema empty the tool should return the expected response without getting the above error.

[rakdutta]

Tested the REST API tool — when output_schema is not defined, it works correctly and returns the expected response. Could you share the steps to reproduce?

[SirSerch]

I’m experiencing the same problem. After investigating, I discovered that when a tool is edited from the admin dashboard and the outputSchema field is deleted, it is automatically saved as {}.

As a result, during output validation (in server.py, line 524), the following logic always evaluates tool.outputSchema as not None, since {} is considered a valid (non-null) value:

# output validation
if tool and tool.outputSchema is not None:
    if maybe_structured_content is None:
        return self._make_error_result(
            "Output validation error: outputSchema defined but no structured output returned"
        )

Because of this behavior, the code always raises an “Output validation error” exception, instead of returning the actual tool result.

---

Root Cause

The outputSchema is stored as {} instead of None when removed from the dashboard. Consequently, the validation logic interprets it as a defined schema.

---

Proposed Solutions

1. Save outputSchema as None when no schema is defined.
   • This would correctly skip the validation when a schema is not intended.
2. Add a checkbox in the dashboard to explicitly indicate whether output validation should be applied for a given tool.
   • This would make the behavior more transparent for users.

---

Expected Behavior

When no outputSchema is defined (i.e., set to None), the tool output should bypass schema validation and return the actual result without raising an exception.

[rakdutta]

The issue has been successfully reproduced. I’m now working on implementing a fix.