/// warning

These are technical details that you might skip if it's not important for you now.

///

`RequestValidationError` is a sub-class of Pydantic's <a href="https://docs.pydantic.dev/latest/concepts/models/#error-handling" class="external-link" target="_blank">`ValidationError`</a>.

**FastAPI** uses it so that, if you use a Pydantic model in `response_model`, and your data has an error, you will see the error in your log.

Sebastián Ramírez <******@****.***> 1731896744 +0100

Sebastián Ramírez <******@****.***> 1731896744 +0100

timothy <******@****.***> 1748612313 +0900

Sebastián Ramírez <******@****.***> 1731896744 +0100

Sebastián Ramírez <******@****.***> 1731896744 +0100

0F73          ; mapped                 ; 0F71 0F72     # 2.0  TIBETAN VOWEL SIGN II
0F74          ; valid                                  # 2.0  TIBETAN VOWEL SIGN U
0F75          ; mapped                 ; 0F71 0F74     # 2.0  TIBETAN VOWEL SIGN UU
0F76          ; mapped                 ; 0FB2 0F80     # 2.0  TIBETAN VOWEL SIGN VOCALIC R

Sebastián Ramírez <******@****.***> 1735583217 +0000

* data validation
* data serialization
* data documentation, etc.

This works the same way as with Pydantic models. And it is actually achieved in the same way underneath, using Pydantic.

/// info

Keep in mind that dataclasses can't do everything Pydantic models can do.

So, you might still need to use Pydantic models.

You can see those schemas because they were declared with the models in the app.

That information is available in the app's **OpenAPI schema**, and then shown in the API docs.

That same information from the models that is included in OpenAPI is what can be used to **generate the client code**.

### Hey API { #hey-api }