Building automated booking reminders with Vercel Cron Jobs and Resend

One of the most requested features in any booking platform is simple:
"Send me a reminder before my appointment."

It sounds trivial. But when you start thinking about the infrastructure —
scheduled jobs, email queues, timezone handling — it gets complex fast.

Here's how I built automated booking reminders in Next.js 15 using
Vercel Cron Jobs and Resend, with zero additional infrastructure.

---

The problem with scheduled tasks in Next.js

Next.js is a request-driven framework. It handles HTTP requests — it doesn't
natively run background jobs on a schedule.

Traditional solutions involve:

• A separate cron server (more infrastructure to manage)
• A queue service like BullMQ or AWS SQS (overkill for most projects)
• A third-party scheduler service (another paid dependency)

Vercel Cron Jobs solve this cleanly. They're available on all plans
including Hobby, configured in a single JSON file, and trigger your
existing API routes on a schedule.

---

Setting up Vercel Cron Jobs

Create a vercel.json in the root of your project:

{
  "crons": [
    {
      "path": "/api/cron/reminders",
      "schedule": "0 8 * * *"
    }
  ]
}

0 8 * * * runs every day at 8:00 AM UTC. That's it — no additional
setup, no dashboard configuration beyond deploying your app.

---

Securing the endpoint

Vercel sends requests to your cron endpoint from their infrastructure.
You need to verify these requests come from Vercel and not from a random
external caller.

The standard approach: a shared secret in an Authorization header.

export async function GET(request: NextRequest) {
  const authHeader = request.headers.get('authorization')

  if (authHeader !== `Bearer ${process.env.CRON_SECRET}`) {
    return NextResponse.json({ error: 'Unauthorized' }, { status: 401 })
  }

  // proceed with the job
}

Set CRON_SECRET in your Vercel environment variables. Generate a
secure value with:

node -e "console.log(require('crypto').randomBytes(32).toString('hex'))"

---

Finding tomorrow's bookings

The reminder logic is straightforward: find all confirmed bookings
that start tomorrow, then send an email for each one.

const tomorrow = new Date()
tomorrow.setDate(tomorrow.getDate() + 1)
tomorrow.setHours(0, 0, 0, 0)

const tomorrowEnd = new Date(tomorrow)
tomorrowEnd.setHours(23, 59, 59, 999)

const upcomingBookings = await db
  .select()
  .from(bookings)
  .where(
    and(
      eq(bookings.status, 'confirmed'),
      gte(bookings.startTime, tomorrow),
      lte(bookings.startTime, tomorrowEnd)
    )
  )

Two things worth noting:

Only confirmed bookings. Pending bookings (waiting for payment
confirmation) are excluded. You don't want to remind someone about
an appointment that isn't actually confirmed yet.

Midnight boundaries. Setting setHours(0, 0, 0, 0) and
setHours(23, 59, 59, 999) ensures you capture the full day
regardless of what time the cron runs.

---

Sending the reminder email with Resend

For each booking, we fetch the related service, resource, and tenant
data, then send the reminder:

for (const booking of upcomingBookings) {
  try {
    const [service] = await db.select().from(services)
      .where(eq(services.id, booking.serviceId)).limit(1)

    const [resource] = await db.select().from(resources)
      .where(eq(resources.id, booking.resourceId)).limit(1)

    const [tenant] = await db.select().from(tenants)
      .where(eq(tenants.id, booking.tenantId)).limit(1)

    if (!service || !resource || !tenant) continue

    const html = await render(
      BookingReminderEmail({
        customerName: booking.customerName,
        businessName: tenant.name,
        serviceName: service.name,
        resourceName: resource.name,
        date: formatDate(booking.startTime),
        startTime: formatTime(booking.startTime),
        endTime: formatTime(booking.endTime),
      })
    )

    await resend.emails.send({
      from: process.env.RESEND_FROM_EMAIL!,
      to: booking.customerEmail,
      subject: `Reminder: your appointment tomorrow at ${tenant.name}`,
      html,
    })

    sent++
  } catch (error) {
    console.error(`Failed to send reminder for booking ${booking.id}:`, error)
    failed++
  }
}

return NextResponse.json({ success: true, sent, failed, total: upcomingBookings.length })

The try/catch per booking is intentional — if one email fails, the
loop continues and sends the remaining reminders. A single Resend
error shouldn't block everyone else's reminders.

---

The email template

Using React Email for the reminder template keeps things consistent
with the rest of the email system:

export function BookingReminderEmail({
  customerName,
  businessName,
  serviceName,
  resourceName,
  date,
  startTime,
  endTime,
}: BookingReminderEmailProps) {
  return (
    <Html>
      <Preview>Reminder: your appointment tomorrow at {businessName}</Preview>
      <Body>
        <Container>
          <Heading>Reminder: appointment tomorrow 🗓️</Heading>
          <Text>
            Hi {customerName}, this is a reminder for your appointment tomorrow.
          </Text>
          <Section>
            <Text><strong>Service:</strong> {serviceName}</Text>
            <Text><strong>With:</strong> {resourceName}</Text>
            <Text><strong>Date:</strong> {date}</Text>
            <Text><strong>Time:</strong> {startTime} – {endTime}</Text>
          </Section>
        </Container>
      </Body>
    </Html>
  )
}

---

Testing locally

Vercel Cron Jobs only trigger in production. For local development,
call the endpoint manually with the cron secret:

PowerShell:

$secret = (Get-Content .env.local | Select-String "CRON_SECRET").ToString().Split("=")[1]
Invoke-RestMethod -Uri "http://localhost:3000/api/cron/reminders" `
  -Headers @{Authorization="Bearer $secret"}

curl:

curl -H "Authorization: Bearer YOUR_CRON_SECRET" \
  http://localhost:3000/api/cron/reminders

The endpoint returns the count of sent and failed emails — useful
for verifying everything worked correctly.

---

What I learned

Vercel Cron Jobs are underrated. For most booking or scheduling
use cases, they're all you need. No queue infrastructure, no worker
processes, no additional cost.

Fail per item, not per batch. The try/catch inside the loop
pattern is important. A single failed email shouldn't abort the
entire reminder run.

Test with real data. Insert a booking with startTime = tomorrow
directly in your database and verify the email arrives before deploying.
Cron bugs are annoying to debug in production.

---

This reminder system is part of SlotKit — a production-ready
booking SaaS template for agencies built on Next.js 15, Supabase,
Stripe, and Resend.

If you're building booking functionality for clients and tired of
rebuilding the same infrastructure: slotkit.dev

Happy to answer questions in the comments.