> ## Documentation Index
> Fetch the complete documentation index at: https://developers.criteo.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Content-type mandatory in authentication header

If you have implemented your calls to Criteo authentication endpoint **/oauth2/token** without the header **Content-Type: application/x-www-form-urlencoded**, this header will be mandatory beginning on May 11th.

Please ensure your implementation is compliant with [our guide](https://developers.criteo.com/marketing-solutions/reference/oauth) before this date, otherwise, you will see the following error when calling the authentication endpoint:

```text theme={null}
{
    "error": "invalid_request",
    "error_description": "The request content-type header must be \"application/x-www-form-urlencoded\".",
    "error_uri": "https://tools.ietf.org/html/rfc6749#section-5.2"
}
```

These controls ensure we offer you the best developer experience and adhere to the latest API best practices.