**FastAPI** will take that model, generate its JSON Schema and include it in the correct place in OpenAPI.

For example, to declare another response with a status code `404` and a Pydantic model `Message`, you can write:

{* ../../docs_src/additional_responses/tutorial001.py hl[18,22] *}

/// note

Keep in mind that you have to return the `JSONResponse` directly.

///

/// info

* **`400`** and above are for "Client error" responses. These are the second type you would probably use the most.
    * An example is `404`, for a "Not Found" response.
    * For generic errors from the client, you can just use `400`.

<span style="color: green;">INFO</span>:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
```

</div>

Then if you go to the URLs at `/openapi.json`, `/docs`, or `/redoc` you will just get a `404 Not Found` error like:

```JSON
{
    "detail": "Not Found"
}

* **`400`** und darüber stehen für „Client error“-Responses („Client-Fehler“). Auch diese verwenden Sie am häufigsten.
    * Ein Beispiel ist `404`, für eine „Not Found“-Response („Nicht gefunden“).
    * Für allgemeine Fehler beim Client können Sie einfach `400` verwenden.

@app.get("/items/{item_id}")
def get_item(item_id: str, username: Annotated[str, Depends(get_username)]):
    if item_id not in data:
        raise HTTPException(status_code=404, detail="Item not found")
    item = data[item_id]
    if item["owner"] != username:
        raise OwnerError(username)
    return item
```

---

{\n    a {\n      color: $white;\n    }\n  }\n}\n","//\n// Pages: 400 and 500 error pages\n//\n\n.error-page {\n  margin: 20px auto 0;\n  width: 600px;\n\n  @include media-breakpoint-down(sm) {\n    width: 100%;\n  }\n\n  //For the error number e.g: 404\n  > .headline {\n    float: left;\n    font-size: 100px;\n    font-weight: 300;\n\n    @include media-breakpoint-down(sm) {\n      float: none;\n      text-align: center;\n    }\n  }\n\n  //For the message\n  > .error-content {\n    display: block;\n...