API Error: Too many concurrent requests (concurrency limit reached)
You have more requests in flight at once than the API allows for your key — even if your requests-per-minute rate is fine.
Too many concurrent requestsconcurrent request limitconcurrency limitsimultaneous requests Meaning
Many APIs limit concurrency separately from rate: e.g. “at most 10 simultaneous requests”. Long-running calls (exports, AI generations, uploads) hold a slot until they finish, so a burst of parallel jobs can hit the ceiling. It is usually returned as 429 with a message about concurrent or simultaneous requests; Shopify, Vimeo, AI APIs and many others do this.
Common causes
- Unbounded Promise.all / thread pool sending everything at once
- Several servers or workers sharing one API key
- Slow requests holding slots longer than expected
- Retries piling up on top of in-flight requests
⚡ Quick fix
- Cap parallelism with a queue or semaphore (e.g. 3–5 at a time)
- Retry with exponential backoff and jitter after the slot frees
- Spread workers across keys only if the provider allows it
Detailed fix by platform
Node.js
- javascript
import pLimit from 'p-limit'; const limit = pLimit(4); await Promise.all(items.map(i => limit(() => callApi(i))));
Python
- python
sem = asyncio.Semaphore(4) async def call(i): async with sem: return await client.post(url, json=i)
How to diagnose
- In-flight — How many requests are open at peak?
- Duration — Which calls take longest?
- Sharing — How many processes use this key?
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Last updated 15 Oct 2026