Vercel: 504 timeout on serverless functions [2026 fix]

Serverless functions exceed 10s timeout or cold start delays. Reduce function execution time or increase timeout in vercel.json config.

Vercel: 504 timeout on serverless functions [2026 fix]

TL;DR

Cause: Your serverless function exceeded Vercel's 10-second default timeout (or cold start exceeded limits).

Fix: Add "maxDuration": 60 to your function's vercel.json config and optimize payload processing.

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Real Console Error Messages

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

``` 504: DEADLINE_EXCEEDED Deadline exceeded before the function completed execution ```

``` FATAL ERROR: CALL_AND_RETRY_LAST Allocation failed - JavaScript heap out of memory at processRequest (file:///var/task/index.js:1:1) ```

``` Error: connect ETIMEDOUT 10.0.0.1:5432 at TCPConnectWrap.afterConnect [as oncomplete] (net.js:1141:10) at Function.Module._load (internal/modules/commonRequires:js:485:15) ```

``` {"errorCode":"FUNCTION_INVOCATION_TIMEOUT","duration":10042} ```

``` WARNING in /var/task/node_modules: Critical dependency warning [Function] Duration: 10000.45 ms, Billed Duration: 10001 ms, Memory Size: 1024 MB ```

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The Problem: Before & After Code

❌ BROKEN CODE

api/slow-endpoint.js ```javascript export default async function handler(req, res) { // Database query without connection pooling const db = await connectToPostgres(); const users = await db.query('SELECT * FROM users WHERE ...'); // Synchronous file operations (blocks execution) const fileData = fs.readFileSync('/mnt/data/large-file.json'); const processed = JSON.parse(fileData); // Long processing loop for (let i = 0; i < 1000000; i++) { processed[i] = expensive_calculation(processed[i]); } res.json(processed); } ```

vercel.json (missing timeout config) ```json { "buildCommand": "npm run build", "outputDirectory": ".next" } ```

✅ FIXED CODE

api/slow-endpoint.js ```javascript import { Pool } from 'pg';

// Connection pool (reused across invocations) const pool = new Pool({ connectionString: process.env.DATABASE_URL, max: 2 });

export default async function handler(req, res) { try { // Use connection pooling + timeout const client = await pool.connect(); const result = await Promise.race([ client.query('SELECT * FROM users LIMIT 100'), new Promise((_, reject) => setTimeout(() => reject(new Error('Query timeout')), 5000) ) ]); client.release(); // Stream large files instead of loading entire file const processedData = await streamProcessFile('/mnt/data/large-file.json'); res.status(200).json(processedData); } catch (error) { console.error('Function error:', error); res.status(500).json({ error: error.message }); } }

async function streamProcessFile(filePath) { const stream = fs.createReadStream(filePath, { encoding: 'utf-8' }); // Process in chunks rather than all at once return new Promise((resolve) => { let data = ''; stream.on('data', chunk => { data += chunk.substring(0, 1000); // Limit processing }); stream.on('end', () => resolve(JSON.parse(data))); }); } ```

vercel.json (with proper timeout + config) ```json { "buildCommand": "npm run build", "outputDirectory": ".next", "functions": { "api/**/*.js": { "maxDuration": 60, "memory": 3008 } }, "env": { "DATABASE_POOL_SIZE": "2" } } ```

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Why This Matters

Vercel's default serverless timeout is 10 seconds on hobby plans and 60 seconds maximum on Pro plans. Common culprits:

Note on version behavior: As of 2026, Vercel's timeout behavior hasn't changed significantly since v3, but Pro plan limits may vary. Check your deployment plan in [Vercel Dashboard](https://vercel.com/dashboard).

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Still broken? Check these too

1. [Cold start optimization](/?guide=cold-starts) — Reduce bundle size and dependency count. Minify and tree-shake unused code. Enable [Prerender](https://vercel.com/docs/functions/supported-languages) for static routes.

2. Memory exhaustion — Increase "memory" in vercel.json (up to 3008 MB). Check for memory leaks in loops or cached objects that aren't freed.

3. [Environment variables loading slowly](/?guide=env-secrets) — Verify DATABASE_URL and secrets are properly configured in project settings. Large .env files can add startup delay.

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External Resources

📖 Official Vercel Documentation:

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Quick Checklist

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Found a different variation? Drop it in the comments — Include your exact error message, vercel.json config, and what fixed it for you.