better-auth: onUserUpdate sends email on every profile change, causing Polar API rejection

[yorickvandervis]

Description

The onUserUpdate hook in packages/polar-betterauth/src/hooks/customer.ts fires on every user profile update and unconditionally sends email + name to customers.updateExternal, even when neither field changed.

When a user updates a non-Polar field (e.g. username, avatar, custom profile flags), the Polar API rejects the call with:

A customer with this email address already exists.

The error is caught and only logged (doesn't block the update), but it's noisy and unnecessary.

Reproduction

1. Configure createCustomerOnSignUp: true
2. Sign up a user (Polar customer is created)
3. Update any non-email field on the user profile (e.g. username)
4. Observe the error in logs:

ERROR [Better Auth]: Polar customer update failed. Error: API error occurred:
{"detail":[{"loc":["body","email"],"msg":"A customer with this email address already exists.","type":"value_error"}]}

Root cause

onUserUpdate (source):

await options.client.customers.updateExternal({
  externalId: user.id,
  customerUpdateExternalID: {
    email: user.email,  // always sent, even if unchanged
    name: user.name,
  },
});

Two issues:

1. The hook sends email on every update regardless of what actually changed
2. The Polar API (PATCH /v1/customers/external/{id}) appears to run a uniqueness check on email that matches the customer's own existing email, rejecting an idempotent update

Suggested fix

The user.update.after hook in better-auth receives both the updated user and context. Ideally the hook should:

• Only send fields that actually changed to Polar, or
• Skip the Polar API call entirely when neither email nor name changed

This is similar to the fix applied in #161 for onBeforeUserCreate, where a check-before-create guard was added.

Environment

• @polar-sh/better-auth: latest
• @polar-sh/sdk: latest
• better-auth: latest
• Polar environment: sandbox (also reproducible in production)