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Log MCPServer handler exceptions by kind and keep crash details off the wire (#3314)

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Co-authored-by: Marcelo Trylesinski <marcelotryle@gmail.com>
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Max 2026-08-24 15:51:18 +01:00 • committed by GitHub
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docs/servers/structured-output.md
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@ -182,18 +182,19 @@ You don't notice while you build the value by hand: Pydantic already made sure y
The annotation promises `WeatherData`. The upstream response stopped sending `humidity`.
!!! check
Call `get_weather` and it does not quietly hand the client a half-empty object. The call fails,
and the first lines of the error name the field:
Call `get_weather` and it does not quietly hand the client a half-empty object. The call fails:
the client gets `is_error=True` with `Error executing tool get_weather`, so the model knows the
call failed instead of confidently reading weather that isn't there. The field name is for you,
in the server log at `ERROR`:
```text
Error executing tool get_weather: 1 validation error for WeatherData
Tool 'get_weather' raised an unexpected exception
...
pydantic_core._pydantic_core.ValidationError: 1 validation error for WeatherData
humidity
Field required [type=missing, input_value={'temperature': 16.2, 'conditions': 'Overcast'}, input_type=dict]
```
That text comes back as the tool result with `is_error=True`, so the model knows the call failed
instead of confidently reading weather that isn't there.
Returning a plain `dict` from a `-> WeatherData` tool is fine, by the way. That's exactly what `json.loads` produced. Validation is on the value, not on the Python type.
## Opting out