Polar + Better Auth Integration

A Better Auth plugin for integrating Polar payments and subscriptions into your authentication flow.

Note: Fetch complete documentation index at: https://polar.sh/docs/llms.txt

Features

• Automatic Customer Creation: Syncs signup users to Polar customers.
• Sync Deletion: Deletes Polar customer when user is deleted.
• Reference System: Associates purchases with organizations/users.
• Plugins: Checkout, Usage (Billing), Webhooks, and Customer Portal.

Installation

npm install better-auth @polar-sh/better-auth @polar-sh/sdk
# or
yarn add better-auth @polar-sh/better-auth @polar-sh/sdk
# or
pnpm add better-auth @polar-sh/better-auth @polar-sh/sdk

Server Configuration

Initialize the Polar client and add the plugin to your Better Auth configuration.

Environment Variables

POLAR_ACCESS_TOKEN=polar_oat_...
POLAR_WEBHOOK_SECRET=...

Full Server Setup

import { betterAuth } from "better-auth";
import {
  polar,
  checkout,
  portal,
  usage,
  webhooks,
} from "@polar-sh/better-auth";
import { Polar } from "@polar-sh/sdk";

const polarClient = new Polar({
  accessToken: process.env.POLAR_ACCESS_TOKEN,
  server: "sandbox", // Use 'sandbox' for testing, defaults to 'production'
});

const auth = betterAuth({
  plugins: [
    polar({
      client: polarClient,
      createCustomerOnSignUp: true, // Auto-create Polar customer
      // Optional: Custom metadata for new customers
      getCustomerCreateParams: ({ user }, request) => ({
        metadata: { source: "better-auth" },
      }),
      use: [
        // 1. Checkout Plugin
        checkout({
          products: [{ productId: "prod_123", slug: "pro" }],
          successUrl: "/success?checkout_id={CHECKOUT_ID}",
          authenticatedUsersOnly: true,
          returnUrl: "https://myapp.com",
          theme: "dark", // 'light' or 'dark'
        }),

        // 2. Customer Portal Plugin
        portal({
          returnUrl: "https://myapp.com",
        }),

        // 3. Usage Billing Plugin
        usage(),

        // 4. Webhooks Plugin
        webhooks({
          secret: process.env.POLAR_WEBHOOK_SECRET,
          onOrderPaid: (payload) => console.log("💸 Order Paid:", payload),
          onCustomerStateChanged: (payload) =>
            console.log("👤 State Changed:", payload),
          onPayload: (payload) => console.log("📨 Other Event:", payload),
        }),
      ],
    }),
  ],
});

Server Options Dictionary

Option	Type	Description
client	Polar	Required. The Polar SDK instance.
createCustomerOnSignUp	boolean	Auto-create Polar customer on signup.
use	Plugin[]	Array of sub-plugins (checkout, portal, usage, webhooks).
getCustomerCreateParams	Function	Returns metadata/params for interaction.

---

Client Configuration

import { createAuthClient } from "better-auth/react";
import { polarClient } from "@polar-sh/better-auth";

export const authClient = createAuthClient({
  plugins: [polarClient()],
});

---

Plugins Summary

1. Checkout Plugin

Enables creating checkout sessions directly from the client.

Configuration:

• products: Array of { productId, slug }. Allows referencing products by slug.
• successUrl: Redirect URL after payment. Supports {CHECKOUT_ID}.
• authenticatedUsersOnly: true forces user login before checkout.
• returnUrl: Back button URL in checkout.
• theme: light or dark.

Client Usage:

await authClient.checkout({
  // Option A: Use slug defined in config
  slug: "pro",
  // Option B: Use direct Product ID
  products: ["prod_123"],
  // Optional: Link to an Organization (B2B)
  referenceId: "org_123",
});

2. Usage Plugin (Billing)

Handles Event Ingestion and Meter retrieval for Usage-Based Billing.

Client Usage:

A. Ingest Events:

await authClient.usage.ingestion({
  event: "ai_generation", // Must match Meter definition in Dashboard
  metadata: {
    tokens: 156,
    model: "gpt-4",
  },
});

Note: Automatically links event to the authenticated user.

B. List Meters:

const { data: meters } = await authClient.usage.meters.list({
  query: { page: 1, limit: 10 },
});
// Returns: consumed units, credited units, current balance

3. Portal Plugin

Manages customer access to the hosted Customer Portal.

Client Usage:

A. Open Portal:

// Redirects user to Polar Customer Portal
await authClient.customer.portal();

B. Get Customer State: Returns active subscriptions, granted benefits, and meter balances.

const { data: state } = await authClient.customer.state();

C. List Resources:

// List Active Subscriptions
const { data: subs } = await authClient.customer.subscriptions.list({
  query: { active: true },
});

// List Orders (Purchases)
const { data: orders } = await authClient.customer.orders.list();

// List Benefits
const { data: benefits } = await authClient.customer.benefits.list();

4. Webhooks Plugin

The webhooks plugin captures incoming events from your Polar organization.

Setup Steps:

1. Configure Endpoint: Go to Polar Dashboard > Webhooks and set endpoint to /api/auth/polar/webhooks.
2. Set Secret: Add POLAR_WEBHOOK_SECRET to your environment variables.
3. Add Plugin:

import { polar, webhooks } from "@polar-sh/better-auth";

const auth = betterAuth({
  plugins: [
    polar({
      client: polarClient,
      use: [
        webhooks({
          secret: process.env.POLAR_WEBHOOK_SECRET,
          // Handlers
          onCustomerStateChanged: (payload) => {
            console.log("Customer state changed:", payload);
          },
          onOrderPaid: (payload) => {
            console.log("Order paid:", payload);
          },
          // ... over 25+ handlers available
          onPayload: (payload) => {
            console.log("Catch-all event:", payload);
          },
        }),
      ],
    }),
  ],
});

Supported Events:

• onPayload (Catch-all)
• onCheckoutCreated, onCheckoutUpdated
• onOrderCreated, onOrderPaid, onOrderRefunded
• onRefundCreated, onRefundUpdated
• onSubscriptionCreated, onSubscriptionUpdated
• onSubscriptionActive, onSubscriptionCanceled, onSubscriptionRevoked
• onProductCreated, onProductUpdated
• onCustomerCreated, onCustomerUpdated, onCustomerDeleted, onCustomerStateChanged
• onBenefitCreated, onBenefitGrantCreated, onBenefitGrantRevoked

---

Common Workflows

Sync Customer Deletion

To delete the Polar customer when a user is deleted in your database:

const auth = betterAuth({
  user: {
    deleteUser: {
      enabled: true,
      afterDelete: async (user) => {
        await polarClient.customers.deleteExternal({
          externalId: user.id,
        });
      },
    },
  },
});

Check Organization Access

Check if a user has access via an Organization subscription (B2B):

const orgId = (await authClient.organization.list())?.data?.[0]?.id;

const { data: orders } = await authClient.customer.orders.list({
  query: {
    active: true,
    referenceId: orgId, // Filter by Org ID
  },
});

const hasAccess = orders.length > 0;