The Fix
pip install fastapi==0.128.4
Based on closed fastapi/fastapi issue #13440 · 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%.
@@ -406,6 +406,68 @@ To exclude a query parameter from the generated OpenAPI schema (and thus, from t
{* ../../docs_src/query_params_str_validations/tutorial014_an_py310.py hl[10] *}
+## Custom Validation
+
+There could be cases where you need to do some **custom validation** that can't be done with the parameters shown above.
from typing import Annotated
from pydantic import AfterValidator
from fastapi import FastAPI
app = FastAPI()
def validator(v):
raise ValueError()
Ints = Annotated[list[int], AfterValidator(validator)]
@app.post("/")
def post(ints: Ints) -> None:
return None
Re-run the minimal reproduction on your broken version, then apply the fix and re-run.
Option A — Upgrade to fixed release\npip install fastapi==0.128.4\nWhen NOT to use: Do not use this fix if you require the previous behavior of returning 200 for invalid inputs.\n\n
Why This Fix Works in Production
- Trigger: Validations in `Annotated` like `AfterValidator` do not work in FastAPI 0.115.10
- Mechanism: Adds documentation examples and tests for custom validations using `Annotated` with `AfterValidator`, reverting a previous change that caused issues with validation in FastAPI 0.115.10.
- Why the fix works: Adds documentation examples and tests for custom validations using `Annotated` with `AfterValidator`, reverting a previous change that caused issues with validation in FastAPI 0.115.10. (first fixed release: 0.128.4).
- 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
- Production symptom (often without a traceback): Validations in `Annotated` like `AfterValidator` do not work in FastAPI 0.115.10
Proof / Evidence
- GitHub issue: #13440
- Fix PR: https://github.com/fastapi/fastapi/pull/13442
- First fixed release: 0.128.4
- Reproduced locally: No (not executed)
- Last verified: 2026-02-08
- Confidence: 0.85
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.61
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“This was handled in https://github.com/fastapi/fastapi/pull/13442 That reverted the PR that introduced the problem, and added docs for these specific use cases, including source examples and…”
Failure Signature (Search String)
- Validations in `Annotated` like `AfterValidator` do not work in FastAPI 0.115.10
- raise ValueError()
Copy-friendly signature
Failure Signature
-----------------
Validations in `Annotated` like `AfterValidator` do not work in FastAPI 0.115.10
raise ValueError()
Error Message
Signature-only (no traceback captured)
Error Message
-------------
Validations in `Annotated` like `AfterValidator` do not work in FastAPI 0.115.10
raise ValueError()
Minimal Reproduction
from typing import Annotated
from pydantic import AfterValidator
from fastapi import FastAPI
app = FastAPI()
def validator(v):
raise ValueError()
Ints = Annotated[list[int], AfterValidator(validator)]
@app.post("/")
def post(ints: Ints) -> None:
return None
What Broke
Requests to endpoints using `Annotated` returned 200 instead of 422 for validation errors.
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
pip install fastapi==0.128.4
Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.
Fix reference: https://github.com/fastapi/fastapi/pull/13442
First fixed release: 0.128.4
Last verified: 2026-02-08. Validate in your environment.
When NOT to Use This Fix
- Do not use this fix if you require the previous behavior of returning 200 for invalid inputs.
Verify Fix
Re-run the minimal reproduction on your broken version, then apply the fix and re-run.
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 |
|---|---|
| 0.128.4 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.