The Fix
Upgrade to version 0.18.0 or later.
Based on closed Kludex/starlette issue #1100 · PR/commit linked
Production note: Most teams hit this during upgrades or environment changes. Roll out with a canary and smoke critical endpoints (health, OpenAPI/docs) before 100%.
@@ -248,7 +248,7 @@ async def wrap(func: typing.Callable[[], typing.Coroutine]) -> None:
class FileResponse(Response):
- chunk_size = 4096
+ chunk_size = 64 * 1024
Option A — Upgrade to fixed release\nUpgrade to version 0.18.0 or later.\nWhen NOT to use: This fix is not suitable if a specific chunk size is required for performance tuning.\n\n
Why This Fix Works in Production
- Trigger: Hi, I found the same issue when I'm using FastAPI. Do you have a plan to change the default chunk size?
- Mechanism: The hardcoded chunk size in FileResponse limited file transfer speeds
- Why the fix works: The default chunk size for FileResponse has been changed from 4Kb to 64Kb to improve file transfer speeds. (first fixed release: 0.18.0).
- If left unfixed, the same config can fail only in production (env differences), causing startup failures or partial feature outages.
Why This Breaks in Prod
- The hardcoded chunk size in FileResponse limited file transfer speeds
- Production symptom (often without a traceback): Hi, I found the same issue when I'm using FastAPI. Do you have a plan to change the default chunk size?
Proof / Evidence
- GitHub issue: #1100
- Fix PR: https://github.com/kludex/starlette/pull/1345
- First fixed release: 0.18.0
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.95
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.78
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“Rather than making it configurable, I'd suggest we start by just improving the default. HTTPX uses 64k read sizes, which we selected based on actual…”
“Hi, I found the same issue when I'm using FastAPI. Do you have a plan to change the default chunk size?”
Failure Signature (Search String)
- Hi, I found the same issue when I'm using FastAPI. Do you have a plan to change the default chunk size?
Copy-friendly signature
Failure Signature
-----------------
Hi, I found the same issue when I'm using FastAPI. Do you have a plan to change the default chunk size?
Error Message
Signature-only (no traceback captured)
Error Message
-------------
Hi, I found the same issue when I'm using FastAPI. Do you have a plan to change the default chunk size?
What Broke
Users experienced slow file transfer speeds compared to other methods.
Why It Broke
The hardcoded chunk size in FileResponse limited file transfer speeds
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
Upgrade to version 0.18.0 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/1345
First fixed release: 0.18.0
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix is not suitable if a specific chunk size is required for performance tuning.
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 |
|---|---|
| 0.18.0 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.