Fixing 504 Gateway Timeout on Vercel Serverless Functions

You’ve launched a new feature that pulls data from a complex database query or an external AI model. It worked perfectly during testing. But the moment you go to production, users start seeing a blank white screen and a red "504 Gateway Timeout" in the console.

This is the "Execution Wall." Serverless functions (like those on Vercel) are designed to be ephemeral and fast. They are not intended for long-running processes. When your function takes too long to respond, Vercel’s gateway cuts the connection to protect its infrastructure, leaving your users stranded.

For developers building with the "Vibe Stack," this is a critical production roadblock. AI tools are great at writing the logic for a complex data scraper or a PDF generator, but they don't know that Vercel has a hard 10-second limit on free plans. You end up with code that is "correct" but "undeployable."

In this guide, we’ll show you how to diagnose the root cause of your 504 errors, explain Vercel's hard limits, and provide the architectural patterns (like Streaming and Edge Functions) to keep your app responsive.

Vercel Timeout Limits: Hobby vs. Pro

Before you debug your code, check your plan. Vercel enforces strict limits on how long a function can run.

• Hobby Plan: 10 seconds. No exceptions. If your code takes 10.1 seconds, your user gets a 504.
• Pro Plan: Defaults to 15 seconds, but can be increased up to 300 seconds (5 minutes).
• Enterprise: Can be increased up to 900 seconds.

If your function is consistently hitting the 10-second mark on a free plan, you have two choices: upgrade to Pro or refactor your architecture.

Diagnosing the Root Cause: Function vs. Backend?

A 504 doesn't tell you what timed out, only that something did. You need to follow the data path.

Step 1: Check Vercel Logs

Go to your Vercel Dashboard -> Logs. Look for the failed request. Vercel will often explicitly state: "Function exceeded its execution timeout."

Step 2: Isolate the Bottleneck

Add "Timing Logs" to your function to see where the time is being spent.

export async function POST(req: Request) {
  console.time("db_query");
  await db.query();
  console.timeEnd("db_query");

  console.time("external_api");
  await fetch("https://ai-model.com");
  console.timeEnd("external_api");
}

Common Trigger 1: Long-running Supabase/Postgres Queries

If your timing logs show that the database is taking 8+ seconds, you have a query optimization problem.

The Fix:

1. Add Indexes: Ensure the columns you are filtering by are indexed.
2. Optimize Joins: AI-generated SQL often includes unnecessary joins that multiply the execution time.
3. Connection Pooling: If you are hitting "Connection Refused" errors along with timeouts, check our Supabase connection refused guide.

Common Trigger 2: Chaining External API Calls

If your function calls Claude, then waits for the result to call Midjourney, then waits again to upload to S3, you are "chaining" latency. Each step adds up, and you will quickly blow past 10 seconds.

The Fix: Use Promise.all() to run independent API calls in parallel.

// ❌ WRONG: Sequential (Slow)
const user = await getUser();
const posts = await getPosts();

// ✅ CORRECT: Parallel (Fast)
const [user, posts] = await Promise.all([getUser(), getPosts()]);

Common Trigger 3: Heavy Image Processing or File Generation

Generating PDFs or resizing images is CPU-intensive. Serverless functions have limited CPU power, making these tasks much slower than they would be on your high-end local machine.

The Fix: Streaming Responses and Background Jobs

If you cannot make the process faster, you must change how the user experiences it.

1. HTTP Streaming

Next.js 14 supports Streaming. Instead of waiting for the whole function to finish, you can send "chunks" of data to the browser as they are ready. This keeps the connection alive and stops the 504 timeout.

2. Edge Functions

If your task is simple but needs to be fast, move it to the Edge. Edge functions have a 30-second timeout (on all plans!) and run closer to your user. However, they don't support all Node.js libraries (like fs or canvas).

3. Background Jobs

For tasks that legitimately take minutes (like generating a 100-page report), do not use a serverless function response.

1. Frontend sends request to API.
2. API starts a "Job" (using a tool like Inngest or Upstash) and returns 202 Accepted immediately.
3. Frontend shows a "Processing..." spinner and polls for the result.

FlowQL: Performance Tuning for Mission-Critical Vercel Deployments

Timeouts are the "last 20%" of cloud deployment where theoretical code meets the cold reality of infrastructure limits. AI assistants are remarkably bad at predicting timeouts because they don't know the latency of your specific database or the cold-start time of your serverless environment.

At FlowQL, we help companies scale their Vercel applications past the "Gateway Timeout." We provide the senior performance engineering to audit your API routes, optimize your database queries, and implement robust background job patterns.

If your production site is timing out and you're losing users to 504 errors, it's time for an expert performance audit.

Conclusion

A 504 Gateway Timeout is a message from Vercel that your code is asking for too much time. By identifying the bottleneck—whether it’s a slow DB, a chained API, or a CPU-heavy task—you can choose the right architectural fix to keep your app running smoothly.

Your Action Plan:

1. Check your Vercel logs to confirm it's an execution timeout.
2. Use timing logs to find the specific slow line of code.
3. If under 10s, optimize the query or API call.
4. If over 10s, move to Background Jobs or Streaming.

Don't let a timeout kill your launch. [Book a session with FlowQL] and let’s get your Vercel functions optimized for speed.

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FAQ: Vercel 504 Timeout Errors

Q: Can I increase the timeout on the Hobby plan? A: No. The 10-second limit is a hard cap for all free accounts. To get more time, you must upgrade to Pro.

Q: Does 'use client' affect 504 errors? A: No. 504 errors happen on the server. use client components run in the browser and will only time out if they are waiting for a slow server response.

Q: What is the difference between a 504 and a 500 error? A: A 500 error means your code crashed (syntax error, null pointer). A 504 error means your code was still running, but it took too long and the server gave up.