So, you will have to make sure your data is of the correct type. E.g. it is compatible with JSON, if you are returning a `JSONResponse`.

    And also that you are not sending any data that should have been filtered by a `response_model`.

### More info

!!! note "Technical Details"
    You could also use `from starlette.responses import Response` or `from starlette.responses import JSONResponse`.

This includes, for example:

* Email notifications sent after performing an action:
    * As connecting to an email server and sending an email tends to be "slow" (several seconds), you can return the response right away and send the email notification in the background.
* Processing data:

    !!! tip
        Prefer to use the `Annotated` version if possible.

    ```Python hl_lines="9"
    {!> ../../../docs_src/header_params/tutorial003.py!}
    ```

If you communicate with that *path operation* sending two HTTP headers like:

```
X-Token: foo
X-Token: bar
```

The response would be like:

```JSON
{
    "X-Token values": [
        "bar",
        "foo"
    ]
}
```

You could use it while developing your app to log the body and debug it, return it to the user, etc.

```Python hl_lines="14"
{!../../../docs_src/handling_errors/tutorial005.py!}
```

Now try sending an invalid item like:

```JSON
{
  "title": "towel",
  "size": "XL"
}
```

You will receive a response telling you that the data is invalid containing the received body:

!!! info
    To send data, you should use one of: `POST` (the more common), `PUT`, `DELETE` or `PATCH`.

    Sending a body with a `GET` request has an undefined behavior in the specifications, nevertheless, it is supported by FastAPI, only for very complex/extreme use cases.

3. Install Poetry in this Docker stage.

4. Copy the `pyproject.toml` and `poetry.lock` files to the `/tmp` directory.

    Because it uses `./poetry.lock*` (ending with a `*`), it won't crash if that file is not available yet.

5. Generate the `requirements.txt` file.

6. This is the final stage, anything here will be preserved in the final container image.