The Fix
Upgrade to version 0.13.5 or later.
Based on closed Kludex/starlette issue #819 · PR/commit linked
Production note: Watch p95/p99 latency and retry volume; timeouts can turn into retry storms and duplicate side-effects.
@@ -16,6 +16,7 @@ isort
pytest
pytest-cov
+pytest-asyncio
# Documentation
Option A — Upgrade to fixed release\nUpgrade to version 0.13.5 or later.\nWhen NOT to use: This fix is not suitable for synchronous file operations.\n\n
Why This Fix Works in Production
- Trigger: Increased latency during file read/write operations due to thread pool usage.
- Mechanism: The thread pool was causing performance overhead during file operations
- Why the fix works: The thread pool is no longer used for file operations when files are in memory, improving performance. (first fixed release: 0.13.5).
- If left unfixed, tail latency can spike under load and surface as timeouts/retries (amplifying incident impact).
Why This Breaks in Prod
- The thread pool was causing performance overhead during file operations
- Production symptom (often without a traceback): Increased latency during file read/write operations due to thread pool usage.
Proof / Evidence
- GitHub issue: #819
- Fix PR: https://github.com/kludex/starlette/pull/933
- First fixed release: 0.13.5
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.85
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.86
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“https://github.com/Tinche/aiofiles/issues/47 Someone has done a performance test here, maybe small file reading and writing should really give up using the thread pool?”
Failure Signature (Search String)
- Increased latency during file read/write operations due to thread pool usage.
Copy-friendly signature
Failure Signature
-----------------
Increased latency during file read/write operations due to thread pool usage.
Error Message
Signature-only (no traceback captured)
Error Message
-------------
Increased latency during file read/write operations due to thread pool usage.
What Broke
Increased latency during file read/write operations due to thread pool usage.
Why It Broke
The thread pool was causing performance overhead during file operations
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
Upgrade to version 0.13.5 or later.
Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.
Fix reference: https://github.com/kludex/starlette/pull/933
First fixed release: 0.13.5
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix is not suitable for synchronous file operations.
Did This Fix Work in Your Case?
Quick signal helps us prioritize which fixes to verify and improve.
Prevention
- Capture the exact failing error string in logs and tests so you can reproduce via a minimal script.
- Pin production dependencies and upgrade only with a reproducible test that hits the failing path.
Version Compatibility Table
| Version | Status |
|---|---|
| 0.13.5 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.