FastAPI Regression between 0.113.0 and 0.114.0:

The Fix

pip install fastapi==0.128.5

Based on closed fastapi/fastapi issue #12240 · 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%.

Jump to Verify Open PR/Commit

@@ -902,8 +902,9 @@ async def process_fn(
         if value is not None:
             values[field.alias] = value
+    field_aliases = {field.alias for field in body_fields}
     for key, value in received_body.items():
-        if key not in values:

repro.py

grant_type: Annotated[
    Union[str, None],
    Form(),
    Doc(
        """
        The OAuth2 spec says it is required and MUST be the fixed string
        "password". Nevertheless, this dependency class is permissive and
        allows not passing it. If you want to enforce it, use instead the
        `OAuth2PasswordRequestFormStrict` dependency.
        """
    ),
] = "password",

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

fix.md

Option A — Upgrade to fixed release\npip install fastapi==0.128.5\nWhen NOT to use: Do not use this fix if strict validation of grant_type is required.\n\n

Why This Fix Works in Production

Trigger: Regression between 0.113.0 and 0.114.0: OAuth2PasswordRequestForm used to accept grant_type=""
Mechanism: A regression caused form values with empty strings to be interpreted as missing
Why the fix works: Fixed a regression that caused form values with empty strings to be interpreted as missing. (first fixed release: 0.128.5).

Production impact:

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

A regression caused form values with empty strings to be interpreted as missing
Production symptom (often without a traceback): Regression between 0.113.0 and 0.114.0: OAuth2PasswordRequestForm used to accept grant_type=""

Proof / Evidence

GitHub issue: #12240
Fix PR: https://github.com/fastapi/fastapi/pull/13537
First fixed release: 0.128.5
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.59

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub

“This was fixed by https://github.com/fastapi/fastapi/pull/13537 and available in FastAPI 0.123.2 🎉”

@YuriiMotov · 2025-12-03 · source

“@Kludex I don't know if this is the correct way, but we can change this code by replacing the form pattern with the default value.…”

@sinisaos · 2024-09-22 · source

“Is there any objection to this proposed fix?”

@rbubley · 2024-10-15 · source

“Investigating this report, I was able to trace the origin of the problem and opened a new discussion here https://github.com/fastapi/fastapi/discussions/12536 as it broadly concerns Forms”

@luzzodev · 2024-10-25 · source

Failure Signature (Search String)

Regression between 0.113.0 and 0.114.0: OAuth2PasswordRequestForm used to accept grant_type=""

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Regression between 0.113.0 and 0.114.0: OAuth2PasswordRequestForm used to accept grant_type=""
from fastapi import Depends, FastAPI, HTTPException, status

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Regression between 0.113.0 and 0.114.0: OAuth2PasswordRequestForm used to accept grant_type=""
from fastapi import Depends, FastAPI, HTTPException, status

Minimal Reproduction

repro.py

grant_type: Annotated[
    Union[str, None],
    Form(),
    Doc(
        """
        The OAuth2 spec says it is required and MUST be the fixed string
        "password". Nevertheless, this dependency class is permissive and
        allows not passing it. If you want to enforce it, use instead the
        `OAuth2PasswordRequestFormStrict` dependency.
        """
    ),
] = "password",

What Broke

OAuth2PasswordRequestForm fails to accept empty grant_type, causing authentication issues.

Why It Broke

A regression caused form values with empty strings to be interpreted as missing

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

pip install fastapi==0.128.5

When NOT to use: Do not use this fix if strict validation of grant_type is required.

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/13537

First fixed release: 0.128.5

Last verified: 2026-02-09. Validate in your environment.

When NOT to Use This Fix

Do not use this fix if strict validation of grant_type is required.

Verify Fix

verify

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

Add a TLS smoke test that performs a real handshake in CI (include CA bundle validation and hostname checks).
Alert on handshake failures by error string and endpoint to catch cert/CA changes quickly.

Version Compatibility Table

Version	Status
0.128.5	Fixed

Related Issues

No related fixes found.

Cluster: fastapi:configuration-error FastAPI hub FastAPI best practices All hubs All clusters

Related clusters: Data consistency Rate limit Race condition

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

FastAPI Regression between 0.113.0 and 0.114.0: OAuth2PasswordRequestForm used to accept (Fix)