How it works

Add users to your site effortlessly with our simple, developer-friendly API

1

Create Your Site

Set up a new site in your dashboard. Configure your redirect URLs and get your unique authentication endpoints that you'll integrate into your website.

2

Add Signin Links

Place our signup and login URLs on your site, or embed our JavaScript widget. Users click to sign up or sign in, and we handle all the authentication flow for you.

3

User Gets Redirected

After users sign up or log in, we redirect them back to your callback URL with a secure token. This token contains all the authentication information you need.

4

Verify & Create User

Make one simple GET request with the token to get the user's information and authentication status. Important: Each token can only be exchanged once for security, but the response includes a long-lived access token for the user's session. Use this data to create or log in the user in your site. We provide complete user data including password digests and algorithms, so you can easily migrate authentication back to your own site whenever you want.

The complete flow in action

Here's exactly what happens when a user signs up through your site

1. User clicks your signup link

<a href="https://signin.fast/@your-site/signup">Sign Up</a>

2. User gets redirected back to your callback URL

https://yoursite.com/callback?token=abc123xyz789

3. Your site verifies the token and gets user data

curl "https://signin.fast/@your-site/verify?token=abc123xyz789"

Returns complete user information including email, name, photo, and password hash

4. User is now logged into your site

✓ Authentication complete - user can access your site

Security: Each callback token works only once. The API response includes a long_lived_token for ongoing session management.

View Full Documentation Try It Now