# Referral quickstart: Auth.js

The code kept by your server, and each new user reported from the createUser event.

Referrals in an app on Auth.js (NextAuth): the code kept in a cookie by your
server, each new user reported as a lead from Auth.js's `createUser` event, and
sales from your billing system.

## Before you start

- A key with the **Referral tracking** preset (the single scope
  `conversions:write`), made under **Settings, Developers**. Keep it on your
  server as `BOLTCLAW_API_KEY`, never in a browser or an app.
- The program's id, on its **Setup** tab, as `BOLTCLAW_PROGRAM_ID`.

## 1. Put the tag on your site

Add the version 2 tag to every page, in your root layout or template. It keeps a
partner's code from `?via=<code>` in a first-party cookie on your own domain,
`bc_ref`, for the program's cookie window, and gives you
`BoltClawReferral.code()` in the browser.

```html
<script src="https://boltclaw.ai/referrals/v2.js" defer data-payment-links="auto"></script>
```

`data-payment-links="auto"` is only for Stripe Payment Links and the Buy Button;
`data-welcome="auto"` adds the friend's welcome. The tag makes no request at all
unless the welcome is on and the visitor arrived with a code.

## 2. Keep the code from your server

Set `bc_ref` in middleware as the [Next.js quickstart](https://boltclaw.ai/docs/referral-quickstart-nextjs)
shows; it sits beside Auth.js's own middleware.

**Why set the cookie from your server.** Safari deletes a cookie a script
created once the visitor has gone 7 days of browsing without using your site,
and keeps one only 24 hours on a page reached through a tracking link, so a
code the tag keeps can be gone a week later. A cookie your OWN server sets in
its response keeps its full lifetime, unless it comes through somebody else's
host (a third-party CNAME or IP address), which Safari caps at 7 days
(WebKit's published tracking prevention rules, read 2026-10-23). Setting `bc_ref`
from your server, not `HttpOnly`, keeps the full window and the tag still reads
it.

## 3. Report each new user as a lead

Auth.js fires `createUser` once, when a new account is stored by your adapter.

```js
// auth.js
import NextAuth from "next-auth";
import { cookies } from "next/headers";
import { reportConversion } from "@/lib/referrals";

export const { handlers, auth, signIn, signOut } = NextAuth({
  // ...providers, adapter
  events: {
    async createUser({ user }) {
      const code = (await cookies()).get("bc_ref")?.value;
      await reportConversion({
        event: "lead",
        event_id: `signup-${user.id}`,
        customer: { id: user.id },
        referral: code ? { code } : undefined,
      });
    },
  },
});
```

`reportConversion` is the helper from the Next.js quickstart. `createUser` needs a
database adapter: without one, report the lead where you first store the user.

## 4. Record sales

Connect the billing system you take payments with on the **Setup** tab (see
[Connect your billing system](https://boltclaw.ai/docs/referral-setup)), and pass the code to checkout
as its guide shows: sales, renewals and refunds are then reported for you, and
the setup status ticks them off. Not on one of the twelve? Report each sale from
your server with `event: "sale"`, your own `event_id` for the payment, the same
`customer.id` as the signup, `amount` in minor units and `currency`.

## 5. Send a test sale

Press **Send a test sale** on the Setup tab: a click, a signup, a sale with two
items and a partial refund go through your real rules as test data, credited to
the program's test partner, never paid and never counted. To check your own
wiring, send one of your own conversions with `"test": true`: it is test data
too, and **Clear test data** deletes it all. The setup status's rows turn green
only for live events.

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