Fly.io 502 Bad Gateway

How to fix it

Try these in order — the most common fix is first.

1. 1. Read the logs

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   1. Run fly logs while reproducing the request
   2. Look for stack traces or exit codes
   Did this fix it? Yes No

2. 2. Increase the grace period if it happens right after deploy

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   1. Add or raise grace_period on your health check (start at 10s; Rails, Django and big JVM apps may need 15 to 30s)
   2. Run fly deploy
   Did this fix it? Yes No

3. 3. Rule out OOM for intermittent 502s

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   1. Run fly machine status <machine-id>
   2. Inspect memory in fly ssh console with free or htop
   3. Run fly scale memory 512 or larger
   Did this fix it? Yes No

Symptoms

Browser or curl returns 502 Bad Gateway, either right after a deploy or intermittently.

Why it happens

• App crashes mid-request
• Wrong listen port
• App not ready after deploy
• Machine killed for running out of memory

This guide also covers

• fly.io 502 bad gateway
• fly.io 502 after deploy
• fly.io intermittent 502

Other Fly.io errors

• Fly.io: The app is not listening on the expected address and will not be reachable by fly-proxy
• Fly.io 503 Service Unavailable
• Fly.io Health Checks Failing
• Fly.io Machines Stopping or Not Starting (Auto-Stop)
• Fly.io Out of Memory (OOM Killed)
• Fly.io Deploy Failed: Waiting for Depot Builder, Registry 401, Too Many Requests
• Fly.io Certificate Not Issuing or Custom Domain Not Working
• How to Cancel Fly.io: Delete an App, Stop Billing and Delete Your Account