Rack::Timeout::RequestTimeoutException Ruby / Rails Origin: Application & API Last updated 5 Nov 2026

Rails: Rack::Timeout::RequestTimeoutException: Request ran for 15001ms, exceeding the 15000ms timeout (Heroku H12)

A request took longer than rack-timeout's limit (or Heroku's 30 s router timeout), so it was aborted.

Exact message you may see Rack::Timeout::RequestTimeoutException: Request ran for 15001ms, exceeding the 15000ms timeoutRequest waited too long in queue Rack::TimeoutHeroku H12 Request timeout
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Seen on: Ruby

Meaning

Find slow code: N+1 queries, slow external APIs, big exports. Move heavy work to background jobs (Sidekiq/ActiveJob), add indexes, caches and timeouts on HTTP clients. Tune RACK_TIMEOUT_SERVICE_TIMEOUT only after fixing causes. Heroku H12 means the router gave up after 30 s.

Common causes

  • N+1 queries or missing DB indexes
  • Slow third-party API without timeouts
  • Long exports/reports in a request
  • Cold boot/memory pressure
  • Too few workers/threads causing queueing

⚡ Quick fix

  1. Profile with rack-mini-profiler/APM; fix N+1 (includes)
  2. Use background jobs for long tasks
  3. Set client timeouts (Faraday/Net::HTTP)

Detailed fix by platform

Rails

  1. config.middleware.insert_before Rack::Runtime, Rack::Timeout, service_timeout: 20

How to diagnose

  1. Cause — DB/API/CPU?
  2. Queue — Request queueing?
  3. Fix — Background job?

🔧 Still not fixed?

Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:

Where this error sits

Origin: Application & API. Errors that are often confused with or caused by this one (click to open):

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