The Fix
Upgrade to version 0.12.4 or later.
Based on closed pallets/flask issue #2736 · PR/commit linked
Production note: This usually shows up under retries/timeouts. Treat it as a side-effect risk until you can verify behavior with a canary + real traffic.
@@ -1923,7 +1923,7 @@ def make_response(self, rv):
# unpack tuple returns
- if isinstance(rv, (tuple, list)):
+ if isinstance(rv, tuple):
len_rv = len(rv)
Option A — Upgrade to fixed release\nUpgrade to version 0.12.4 or later.\nWhen NOT to use: This fix should not be used if the application relies on lists being treated as valid responses.\n\n
Why This Fix Works in Production
- Trigger: TypeError: The view function did not return a valid response tuple. The tuple must have the form (body, status, headers), (body, status), or (body, headers).
- Mechanism: The make_response function incorrectly checked for lists as valid response types
- Why the fix works: Removes the explicit check for list as a return value in make_response, allowing custom response classes to handle lists correctly. (first fixed release: 0.12.4).
- 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 make_response function incorrectly checked for lists as valid response types
- Surfaces as: TypeError: The view function did not return a valid response tuple. The tuple must have the form (body, status, headers), (body, status), or (body, headers).
Proof / Evidence
- GitHub issue: #2736
- Fix PR: https://github.com/pallets/flask/pull/2737
- First fixed release: 0.12.4
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.85
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.76
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“duplicate of #2726 Thinking about it, it seems fine to relax this and only assume tuples are responses”
“Thanks @davidism - that does make sense and if necessary I am prepared to update that for our internal systems as it makes plenty of…”
Failure Signature (Search String)
- TypeError: The view function did not return a valid response tuple. The tuple must have the form (body, status, headers), (body, status), or (body, headers).
Error Message
Stack trace
Error Message
-------------
TypeError: The view function did not return a valid response tuple. The tuple must have the form (body, status, headers), (body, status), or (body, headers).
What Broke
Returning a list from a view function resulted in a TypeError, breaking response handling.
Why It Broke
The make_response function incorrectly checked for lists as valid response types
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
Upgrade to version 0.12.4 or later.
Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.
Fix reference: https://github.com/pallets/flask/pull/2737
First fixed release: 0.12.4
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix should not be used if the application relies on lists being treated as valid responses.
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 |
|---|---|
| 0.12.4 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.