Vercel: 504 timeout on serverless functions [2026 fix]

Serverless function exceeds 10s default timeout or memory limit. Increase timeout/memory in vercel.json or optimize function execution.

Vercel: 504 timeout on serverless functions [2026 fix]

TL;DR

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Exact Error Messages from Console

Here are real error signatures you'll see in Vercel logs:

``` Error: Task timed out after 10.00 seconds ```

``` 504 Gateway Timeout The request took too long to process and timed out. ```

``` FATAL ERROR: CALL_AND_RETRY_LAST Allocation failed - JavaScript heap out of memory ```

``` vercel[api]: ERROR Function timed out after 10s (duration: 10004ms) ```

``` ERROR: Function execution took 12456ms, exceeding timeout of 10000ms for plan: pro ```

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Code: Broken vs Fixed

❌ BROKEN: Default timeout, no optimization

```javascript // api/getData.js - TIMES OUT export default async function handler(req, res) { // No timeout configured - defaults to 10 seconds const data = await fetch('https://slow-api.example.com/data'); const json = await data.json(); // Multiple sequential database calls const user = await db.query('SELECT * FROM users WHERE id = ?', [req.query.id]); const posts = await db.query('SELECT * FROM posts WHERE user_id = ?', [user.id]); const comments = await db.query('SELECT * FROM comments WHERE post_id IN (?)', [posts.map(p => p.id)]); res.json({ user, posts, comments }); } ```

✅ FIXED: Timeout increased + optimized queries

vercel.json: ```json { "functions": { "api/getData.js": { "maxDuration": 30 } } } ```

api/getData.js: ```javascript export default async function handler(req, res) { // Parallel requests instead of sequential const [data, user, posts] = await Promise.all([ fetch('https://slow-api.example.com/data').then(r => r.json()), db.query('SELECT * FROM users WHERE id = ?', [req.query.id]), db.query('SELECT * FROM posts WHERE user_id = ? LIMIT 100', [req.query.id]) ]); // Single query with JOIN instead of multiple queries const comments = await db.query( 'SELECT c.* FROM comments c JOIN posts p ON c.post_id = p.id WHERE p.user_id = ? LIMIT 500', [req.query.id] ); res.json({ user, posts, comments }); } ```

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Step-by-Step Fix

Step 1: Check Your Plan Limits

If you're on Free tier and need >10s, upgrade to Pro.

Step 2: Set maxDuration in vercel.json

Note on version specificity: This vercel.json syntax works as of 2024-2026. Vercel occasionally adjusts configuration schema, so verify against [official Vercel config docs](https://vercel.com/docs/projects/project-configuration).

Step 3: Optimize Your Function Code

Problem patterns:

Step 4: Add Request Timeout to External Calls

const response = await fetch('https://api.example.com', { signal: controller.signal });

clearTimeout(timeoutId); ```

Step 5: Deploy and Monitor

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Still broken? Check these too

1. Database connection pooling: If using Postgres/MySQL, your connection might timeout before the function does. Use connection pooling (PgBouncer, Neon) or implement connection reuse with pooling: true in your connection string.

2. Cold start compounding: Function initialization takes 2-5 seconds on first invocation. Pre-warm functions via [cron jobs](/?guide=vercel-cron-jobs) or use Vercel's native Postgres to avoid external connection overhead.

3. Memory exhaustion: 1024MB default memory might be insufficient. Increase via vercel.json memory field, but note higher memory = faster CPU and higher cost. Monitor actual usage in logs before over-allocating.

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Related Guides

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Official Resources

🔗 [Vercel Serverless Function Configuration](https://vercel.com/docs/functions/serverless-functions/runtimes#function-payload-api-version)

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Found a different variation? Drop it in the comments—504s have many causes and your specific scenario helps others troubleshooting at 2am.