Referral quickstart: Better Auth

Referrals in an app on Better Auth: the code kept in a cookie by your server, each new user reported as a lead from an after-hook on sign-up, 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.

<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 or the Express quickstart shows.

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

An after hook on the sign-up endpoints sees the new session and the request's cookies.

// auth.js
import { betterAuth } from "better-auth";
import { createAuthMiddleware } from "better-auth/api";
import { reportConversion } from "./referrals";

export const auth = betterAuth({
  // ...database, providers
  hooks: {
    after: createAuthMiddleware(async (ctx) => {
      if (!ctx.path.startsWith("/sign-up")) return;
      const user = ctx.context.newSession?.user;
      if (!user) return;
      const cookie = ctx.headers?.get("cookie") ?? "";
      const code = /(?:^|;\s*)bc_ref=([a-z0-9-]{3,40})/.exec(cookie)?.[1];
      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. For social sign-in, report the lead the first time a user is created instead (a database hook on user.create.after).

4. Record sales

Connect the billing system you take payments with on the Setup tab (see Connect your billing system), 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.