> For the complete documentation index, see [llms.txt](https://api.circle.so/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://api.circle.so/apis/headless/auth-sdk.md).

# Auth SDK

<figure><img src="/files/aLKFvLp7HQKVO4nsRAy0" alt=""><figcaption></figcaption></figure>

{% hint style="info" %}
**The Headless Auth SDK is intended to be used only with server-side apps to authenticate your website/app with your secure App Token. This token should be kept secret, and should not be made available to anyone on the client side for security reasons.**
{% endhint %}