Building an AI SaaS in a Weekend: Auth, Billing, and Usage Tracking

The first version of every AI SaaS product looks the same: a chat box, a submit button, and a direct API call. No auth. No billing. No usage tracking. Just vibes.

The second version is where most projects die -- because wiring up auth, payments, and a dashboard from scratch takes 2-3 weeks of work that has nothing to do with your actual product.

Here's how to go from idea to deployed AI SaaS in a weekend, with the full stack pre-configured.

The Stack

Before I get into implementation, here's what I use and why:

Layer	Choice	Why
Framework	Next.js 14 App Router	Server components + API routes in one project
Auth	NextAuth v5	Google + GitHub OAuth, sessions, DB integration
Payments	Stripe	Best docs, best DX, webhook handling is straightforward
AI	Claude API / OpenAI	Both pre-wired, switch with one env var
Database	Prisma + PostgreSQL	Type-safe ORM, excellent migration tooling
UI	Tailwind + shadcn/ui	Fast, consistent, accessible by default
Deployment	Vercel	Zero-config for Next.js, handles env vars, preview deploys

Day 1: Auth + Database (4-6 hours)

Database Schema

Start with your user model. Everything else connects to it.

// prisma/schema.prisma
generator client {
  provider = "prisma-client-js"
}

datasource db {
  provider = "postgresql"
  url      = env("DATABASE_URL")
}

model User {
  id            String    @id @default(cuid())
  name          String?
  email         String    @unique
  emailVerified DateTime?
  image         String?
  createdAt     DateTime  @default(now())
  updatedAt     DateTime  @updatedAt

  // Stripe
  stripeCustomerId       String?   @unique
  stripeSubscriptionId   String?   @unique
  stripePriceId          String?
  stripeCurrentPeriodEnd DateTime?

  // Usage tracking
  tokensUsed    Int       @default(0)
  tokensLimit   Int       @default(50000)

  accounts Account[]
  sessions Session[]
  messages Message[]
}

// NextAuth required models
model Account { ... }
model Session { ... }
model VerificationToken { ... }

model Message {
  id        String   @id @default(cuid())
  userId    String
  role      String   // "user" | "assistant"
  content   String   @db.Text
  tokens    Int      @default(0)
  createdAt DateTime @default(now())

  user User @relation(fields: [userId], references: [id], onDelete: Cascade)
}

NextAuth Configuration

// src/lib/auth.ts
import NextAuth from "next-auth"
import { PrismaAdapter } from "@auth/prisma-adapter"
import GoogleProvider from "next-auth/providers/google"
import { db } from "@/lib/db"

export const { handlers, auth, signIn, signOut } = NextAuth({
  adapter: PrismaAdapter(db),
  providers: [
    GoogleProvider({
      clientId: process.env.GOOGLE_CLIENT_ID!,
      clientSecret: process.env.GOOGLE_CLIENT_SECRET!,
    }),
  ],
  callbacks: {
    session({ session, user }) {
      session.user.id = user.id
      return session
    },
  },
})

Auth Middleware

// middleware.ts
import { auth } from "@/lib/auth"

export default auth((req) => {
  const isLoggedIn = !!req.auth
  const isOnDashboard = req.nextUrl.pathname.startsWith("/dashboard")

  if (isOnDashboard && !isLoggedIn) {
    return Response.redirect(new URL("/login", req.nextUrl))
  }
})

export const config = {
  matcher: ["/((?!api|_next/static|_next/image|favicon.ico).*)"],
}

Day 1 Afternoon: The AI Route

// src/app/api/chat/route.ts
import { NextRequest, NextResponse } from "next/server"
import { auth } from "@/lib/auth"
import Anthropic from "@anthropic-ai/sdk"
import { db } from "@/lib/db"

const claude = new Anthropic()

export async function POST(req: NextRequest) {
  const session = await auth()
  if (!session?.user?.id) {
    return NextResponse.json({ error: "Unauthorized" }, { status: 401 })
  }

  const user = await db.user.findUnique({
    where: { id: session.user.id },
    select: { tokensUsed: true, tokensLimit: true, stripePriceId: true },
  })

  if (!user) {
    return NextResponse.json({ error: "User not found" }, { status: 404 })
  }

  // Check token budget
  if (user.tokensUsed >= user.tokensLimit) {
    return NextResponse.json(
      { error: "Token limit reached. Upgrade your plan to continue." },
      { status: 402 }
    )
  }

  const { messages } = await req.json()

  const response = await claude.messages.create({
    model: "claude-sonnet-4-6",
    max_tokens: 1024,
    messages,
  })

  const tokensUsed = response.usage.input_tokens + response.usage.output_tokens

  // Track usage
  await db.user.update({
    where: { id: session.user.id },
    data: { tokensUsed: { increment: tokensUsed } },
  })

  // Log message
  await db.message.create({
    data: {
      userId: session.user.id,
      role: "assistant",
      content: response.content[0].type === "text" ? response.content[0].text : "",
      tokens: tokensUsed,
    },
  })

  return NextResponse.json({
    content: response.content[0].type === "text" ? response.content[0].text : "",
    usage: { tokensUsed, tokensRemaining: user.tokensLimit - user.tokensUsed - tokensUsed },
  })
}

Day 2: Stripe Integration (3-4 hours)

Create Products and Prices in Stripe

// Run once: scripts/setup-stripe.ts
import Stripe from "stripe"

const stripe = new Stripe(process.env.STRIPE_SECRET_KEY!)

async function setup() {
  const product = await stripe.products.create({
    name: "AI SaaS Pro",
    description: "500k tokens/month",
  })

  const price = await stripe.prices.create({
    product: product.id,
    unit_amount: 2000, // $20/month
    currency: "usd",
    recurring: { interval: "month" },
  })

  console.log("Price ID:", price.id)
  // Add STRIPE_PRICE_ID to your .env
}

setup()

Checkout Session Route

// src/app/api/billing/checkout/route.ts
import { NextResponse } from "next/server"
import { auth } from "@/lib/auth"
import Stripe from "stripe"
import { db } from "@/lib/db"

const stripe = new Stripe(process.env.STRIPE_SECRET_KEY!)

export async function POST() {
  const session = await auth()
  if (!session?.user?.id) {
    return NextResponse.json({ error: "Unauthorized" }, { status: 401 })
  }

  const user = await db.user.findUnique({
    where: { id: session.user.id },
  })

  let customerId = user?.stripeCustomerId

  if (!customerId) {
    const customer = await stripe.customers.create({
      email: session.user.email!,
      name: session.user.name || undefined,
    })
    customerId = customer.id
    await db.user.update({
      where: { id: session.user.id },
      data: { stripeCustomerId: customerId },
    })
  }

  const checkoutSession = await stripe.checkout.sessions.create({
    customer: customerId,
    mode: "subscription",
    payment_method_types: ["card"],
    line_items: [{ price: process.env.STRIPE_PRICE_ID!, quantity: 1 }],
    success_url: `${process.env.NEXTAUTH_URL}/dashboard?upgraded=true`,
    cancel_url: `${process.env.NEXTAUTH_URL}/pricing`,
  })

  return NextResponse.json({ url: checkoutSession.url })
}

Webhook Handler

// src/app/api/billing/webhook/route.ts
import { NextRequest, NextResponse } from "next/server"
import Stripe from "stripe"
import { db } from "@/lib/db"

const stripe = new Stripe(process.env.STRIPE_SECRET_KEY!)

export async function POST(req: NextRequest) {
  const body = await req.text()
  const sig = req.headers.get("stripe-signature")!

  let event: Stripe.Event
  try {
    event = stripe.webhooks.constructEvent(body, sig, process.env.STRIPE_WEBHOOK_SECRET!)
  } catch (err) {
    return NextResponse.json({ error: "Invalid signature" }, { status: 400 })
  }

  const subscription = event.data.object as Stripe.Subscription

  if (event.type === "customer.subscription.created" ||
      event.type === "customer.subscription.updated") {
    await db.user.update({
      where: { stripeCustomerId: subscription.customer as string },
      data: {
        stripeSubscriptionId: subscription.id,
        stripePriceId: subscription.items.data[0].price.id,
        stripeCurrentPeriodEnd: new Date(subscription.current_period_end * 1000),
        tokensLimit: 500_000, // Upgrade limit on subscription
      },
    })
  }

  if (event.type === "customer.subscription.deleted") {
    await db.user.update({
      where: { stripeCustomerId: subscription.customer as string },
      data: {
        stripeSubscriptionId: null,
        stripePriceId: null,
        tokensLimit: 50_000, // Back to free limit
      },
    })
  }

  return NextResponse.json({ received: true })
}

Day 2 Afternoon: Dashboard (2-3 hours)

The dashboard needs: usage stats, upgrade CTA if on free plan, message history.

// src/app/dashboard/page.tsx
import { auth } from "@/lib/auth"
import { db } from "@/lib/db"
import { redirect } from "next/navigation"
import { UsageCard } from "@/components/usage-card"
import { ChatInterface } from "@/components/chat-interface"

export default async function DashboardPage() {
  const session = await auth()
  if (!session?.user?.id) redirect("/login")

  const user = await db.user.findUnique({
    where: { id: session.user.id },
    select: {
      tokensUsed: true,
      tokensLimit: true,
      stripePriceId: true,
      stripeCurrentPeriodEnd: true,
    },
  })

  const isPro = !!user?.stripePriceId
  const usagePercent = Math.round((user!.tokensUsed / user!.tokensLimit) * 100)

  return (
    <div className="max-w-4xl mx-auto p-6 space-y-6">
      <UsageCard
        tokensUsed={user!.tokensUsed}
        tokensLimit={user!.tokensLimit}
        usagePercent={usagePercent}
        isPro={isPro}
        renewsAt={user?.stripeCurrentPeriodEnd}
      />
      <ChatInterface />
    </div>
  )
}

Skip All of This

The setup I described takes a full weekend. Even experienced developers make mistakes the first time wiring up NextAuth, Prisma, and Stripe webhooks together.

The AI SaaS Starter Kit has all of this pre-configured and tested:

• Auth (NextAuth v5 + Prisma adapter)
• Stripe (subscriptions + webhooks + billing portal)
• AI routes (Claude + OpenAI, token tracking)
• Dashboard (usage stats, upgrade flow)
• Landing page
• Deployment config

53 files. All connected. Clone, add your API keys, deploy.

AI SaaS Starter Kit ($99) ->

The $99 pays for itself the first time you don't spend a weekend debugging Stripe webhooks.

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