Pydantic `FieldInfo` not expanding parameterization (Fix)

The Fix

pip install pydantic==2.12.5

Based on closed pydantic/pydantic issue #12450 · 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

@@ -216,7 +216,7 @@ def update_field_from_config(config_wrapper: ConfigWrapper, field_name: str, fie
     cls: type[BaseModel],
     config_wrapper: ConfigWrapper,
-    ns_resolver: NsResolver | None,
+    ns_resolver: NsResolver,
     *,

repro.py

from typing import Annotated

from pydantic import BaseModel, Field

class Model[T](BaseModel):
    t: T

M = Model[Annotated[int, Field(gt=1)]]

M.model_fields['t']
#> FieldInfo(annotation=Annotated[int, FieldInfo(annotation=NoneType, required=True, metadata=[Gt(gt=1)])], required=True)

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 pydantic==2.12.5\nWhen NOT to use: Do not use this fix if the model relies on non-parameterized behavior.\n\n

Why This Fix Works in Production

Trigger: Metadata propagation inconsistently fails between generic and regular models, causing unexpected behavior.
Mechanism: FieldInfo from Annotated arguments isn't being expanded when used in a parameterized GenericModel
Why the fix works: Fixes the issue where FieldInfo from Annotated arguments isn't being expanded when used in a parameterized GenericModel, ensuring consistent metadata propagation. (first fixed release: 2.12.5).

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

FieldInfo from Annotated arguments isn't being expanded when used in a parameterized GenericModel
Production symptom (often without a traceback): Metadata propagation inconsistently fails between generic and regular models, causing unexpected behavior.

Proof / Evidence

GitHub issue: #12450
Fix PR: https://github.com/pydantic/pydantic/pull/12463
First fixed release: 2.12.5
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.67

Discussion

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

Jump to Sources Open on GitHub

“@August26 Feel free to give it a shot, I'll note that it might be not trivial.”

@Viicos · 2025-10-27 · source

“Hi 👋 I’ve reproduced this issue and verified that the root cause is that FieldInfo from Annotated arguments isn’t being expanded when used in a…”

@August26 · 2025-10-26 · source

Failure Signature (Search String)

Metadata propagation inconsistently fails between generic and regular models, causing unexpected behavior.

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Metadata propagation inconsistently fails between generic and regular models, causing unexpected behavior.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Metadata propagation inconsistently fails between generic and regular models, causing unexpected behavior.

Minimal Reproduction

repro.py

from typing import Annotated

from pydantic import BaseModel, Field

class Model[T](BaseModel):
    t: T

M = Model[Annotated[int, Field(gt=1)]]

M.model_fields['t']
#> FieldInfo(annotation=Annotated[int, FieldInfo(annotation=NoneType, required=True, metadata=[Gt(gt=1)])], required=True)

What Broke

Metadata propagation inconsistently fails between generic and regular models, causing unexpected behavior.

Why It Broke

FieldInfo from Annotated arguments isn't being expanded when used in a parameterized GenericModel

Fix Options (Details)

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

pip install pydantic==2.12.5

When NOT to use: Do not use this fix if the model relies on non-parameterized behavior.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Fix reference: https://github.com/pydantic/pydantic/pull/12463

First fixed release: 2.12.5

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

When NOT to Use This Fix

Do not use this fix if the model relies on non-parameterized behavior.

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 CI check that diffs key outputs after upgrades (OpenAPI schema snapshots, JSON payload shapes, CLI output).
Upgrade behind a canary and run integration tests against the canary before 100% rollout.

Version Compatibility Table

Version	Status
2.12.5	Fixed

Related Issues

No related fixes found.

Cluster: pydantic:configuration-error Pydantic hub Pydantic best practices All hubs All clusters

Related clusters: Data consistency OpenAPI schema Serialization

Sources

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