Credits

Credits

Per-call metering using a credit balance stored on the users table (shared DB). Atomic deduction ensures the balance never goes below zero without a race condition.

Architecture

• Balance stored in users.credits (shared Drizzle DB, integer column)
• Deduction uses UPDATE WHERE credits > 0 RETURNING, single statement, no race
• Free credits granted on signup via onUserCreated in src/server/auth-hooks.ts, wired into the better-auth user.create.after hook in src/server/auth.ts
• Top-up via Stripe one-time payment (mode: 'payment')

Key files

File	Purpose
src/features/credits/server/credits.server.ts	getCredits, deductCredit, addCredits
src/features/credits/server/credits.queries.ts	getCreditsFn server function
src/features/billing/server/stripe-checkout.server.ts	handleCreditsPurchase grants credits on checkout.session.completed
config.ts	credits.enabled, freeCreditsOnSignup, topUpPriceId, creditsPerTopUp

Server functions

Function	Method	Description
getCreditsFn	GET	Current user's credit balance
buyCreditsCheckoutFn	POST	Create Stripe checkout session for top-up

Gating a mutation

deductCredit operates on the shared DB, not per-user SQLite. Call it inside withWriteLock alongside the write it guards:

import { deductCredit } from '@/features/credits/server/credits.server';
import { db } from '@/lib/db';

return withWriteLock(user.id, async () => {
  const userDb = getUserDb(user.id);
  const deduct = await deductCredit(db, user.id);
  if (!deduct.ok) return err(ERROR_CODES.NO_CREDITS, 'No credits remaining');

  // ... metered operation ...
  return ok(result);
});

deductCredit is server-only, import directly, not via barrel (import protection blocks it in client bundles).

Refunding a failed metered call

If credit was deducted but the metered operation never actually ran (e.g. the downstream API failed to even start), use refundCredit(db, userId) to give the credit back, same file, same import rule as deductCredit:

import { deductCredit, refundCredit } from '@/features/credits/server/credits.server';
import { db } from '@/lib/db';

return withWriteLock(user.id, async () => {
  const deduct = await deductCredit(db, user.id);
  if (!deduct.ok) return err(ERROR_CODES.NO_CREDITS, 'No credits remaining');

  try {
    const result = await startMeteredOperation();
    return ok(result);
  } catch (e) {
    await refundCredit(db, user.id);
    throw e;
  }
});

Reference implementation: src/routes/api/v1/ai-chat.ts refunds the credit when the OpenRouter stream fails to start. Only refund on a genuine "never happened" failure, don't refund for a partial/degraded response the user still received value from.

Configuration

// config.ts
credits: {
  enabled: true,
  freeCreditsOnSignup: 10,
  topUpPriceId: 'price_xxx',   // VITE_STRIPE_CREDITS_PRICE_ID
  creditsPerTopUp: 100
}

Set enabled: false to disable the entire credits system. When disabled, the credits balance, settings page section, and top-up flow are all hidden.

Top-up flow

1. User clicks top-up on /dashboard/settings/credits
2. buyCreditsCheckoutFn creates a Stripe mode: 'payment' session with metadata.userId and metadata.amount
3. On checkout.session.completed, handleCreditsPurchase in stripe-checkout.server.ts grants the credits inside a db.transaction: it dedups on session.payment_intent against the purchases table (insert a marker row) before incrementing users.credits, so a redelivered webhook can't double-grant. See docs/warpkit/patterns/webhook-replay.md for the full pattern.

Free credits on signup

config.credits.freeCreditsOnSignup credits are added when a new account is created, via onUserCreated in src/server/auth-hooks.ts (called from the user.create.after hook in src/server/auth.ts).