FastAPI Upcoming Pydantic 2.10 is going to break version

The Fix

pip install fastapi==0.128.4

Based on closed fastapi/fastapi issue #12901 · 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%.

Open PR/Commit

@@ -27,7 +27,8 @@
 # Reassign variable to make it reexported for mypy
 PYDANTIC_VERSION = P_VERSION
-PYDANTIC_V2 = PYDANTIC_VERSION.startswith("2.")
+PYDANTIC_VERSION_MINOR_TUPLE = tuple(int(x) for x in PYDANTIC_VERSION.split(".")[:2])
+PYDANTIC_V2 = PYDANTIC_VERSION_MINOR_TUPLE[0] == 2

fix.md

Option A — Upgrade to fixed release\npip install fastapi==0.128.4\nWhen NOT to use: Do not use if it changes public behavior or if the failure cannot be reproduced.\n\n

Why This Fix Works in Production

Trigger: Upcoming Pydantic 2.10 is going to break version comparison checks
Mechanism: Version comparison logic fails due to string comparison instead of tuple comparison
Why the fix works: Updated internal checks to support Pydantic 2.10 to prevent version comparison issues. (first fixed release: 0.128.4).

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

Dependency interaction matters here: Pydantic v2.10.
Version comparison logic fails due to string comparison instead of tuple comparison
Production symptom (often without a traceback): Upcoming Pydantic 2.10 is going to break version comparison checks

Proof / Evidence

GitHub issue: #12901
Fix PR: https://github.com/fastapi/fastapi/pull/12914
First fixed release: 0.128.4
Reproduced locally: No (not executed)
Last verified: 2026-02-08
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.72

Discussion

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

Jump to Sources Open on GitHub

“Thanks for the report @Viicos! I handled it in https://github.com/fastapi/fastapi/pull/12914, it will be released in the next hours. :nerd_face:”

@tiangolo · 2024-11-12 · confirmation · source

Failure Signature (Search String)

Upcoming Pydantic 2.10 is going to break version comparison checks

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Upcoming Pydantic 2.10 is going to break version comparison checks

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Upcoming Pydantic 2.10 is going to break version comparison checks

Environment

Pydantic: 2.10

Why It Broke

Version comparison logic fails due to string comparison instead of tuple comparison

Fix Options (Details)

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

pip install fastapi==0.128.4

When NOT to use: Do not use if it changes public behavior or if the failure cannot be reproduced.

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/12914

First fixed release: 0.128.4

Last verified: 2026-02-08. Validate in your environment.

When NOT to Use This Fix

Do not use if it changes public behavior or if the failure cannot be reproduced.

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.

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 Upcoming Pydantic 2.10 is going to break version comparison checks (Fix)