# Missing application ID

Resolve missing application ID errors.

The `X-App-Id` header is absent or empty.

Client-side requests must include a valid [application ID](/explanation/application) through the `X-App-Id` header.

## Response

All error responses follow the [Problem Details (RFC 9457)](/reference/api/overview#errors) format. The `type`, `title`, `status`, and `detail` fields are always present, while `instance` may be omitted depending on the API.

Here is an example of the response returned for this error:

```json
{
  "type": "https://croct.help/api/authentication/missing-public-id",
  "title": "The application ID is missing.",
  "status": 401,
  "detail": "The request must include an application public ID, but none was provided.",
  "instance": "/client/web/content"
}
```