Pydantic `Field()` and `FieldInfo`/metadata ordering

The Fix

pip install pydantic==2.11.8

Based on closed pydantic/pydantic issue #11122 · 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.

Jump to Verify Open PR/Commit

@@ -326,18 +326,15 @@ def collect_model_fields(  # noqa: C901
                 # extend this to any descriptor in the future (by simply checking for
                 # `hasattr(assigned_value.default, '__get__')`).
-                assigned_value.default = assigned_value.default.__get__(None, cls)
-
-            # The `from_annotated_attribute()` call below mutates the assigned `Field()`, so make a copy:

repro.py

from pydantic import BaseModel, AfterValidator

def func1(v): pass
def func2(v): pass

class Model(BaseModel):
    a: Annotated[int, AfterValidator(func1), Field(gt=1), AfterValidator(func2)] = Field(...)

Model.model_fields['a'].metadata
#> [Gt(gt=1), AfterValidator(func=<function func2 at 0x7c2889945f80>)]
# Some metadata is lost?

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==2.11.8\nWhen NOT to use: This fix should not be used if backward compatibility with third-party libraries is a concern.\n\nOption C — Workaround\nin https://github.com/pydantic/pydantic/pull/9279/), but not all:\nWhen NOT to use: This fix should not be used if backward compatibility with third-party libraries is a concern.\n\n

Why This Fix Works in Production

Trigger: Here is a description of an alternative implementation (that doesn't take backwards compatibility into account):
Mechanism: The implementation of Field() and FieldInfo is inconsistent and not performant, leading to bugs
Why the fix works: Refactor the `FieldInfo` creation implementation to address inconsistencies and bugs related to the `Field()` function and `FieldInfo` logic. (first fixed release: 2.11.8).

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

The implementation of Field() and FieldInfo is inconsistent and not performant, leading to bugs
Production symptom (often without a traceback): Here is a description of an alternative implementation (that doesn't take backwards compatibility into account):

Proof / Evidence

GitHub issue: #11122
Fix PR: https://github.com/pydantic/pydantic/pull/11898
First fixed release: 2.11.8
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.63

Discussion

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

Jump to Sources Open on GitHub

“### Initial Checks - [X] I confirm that I'm using Pydantic V2 ### Description Closely related: https://github.com/pydantic/pydantic/issues/10036 The current Field() function and FieldInfo logic and implementation is currently quite messy an”

Issue thread · issue description · source

Failure Signature (Search String)

Here is a description of an alternative implementation (that doesn't take backwards compatibility into account):

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Here is a description of an alternative implementation (that doesn't take backwards compatibility into account):

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Here is a description of an alternative implementation (that doesn't take backwards compatibility into account):

Minimal Reproduction

repro.py

from pydantic import BaseModel, AfterValidator

def func1(v): pass
def func2(v): pass

class Model(BaseModel):
    a: Annotated[int, AfterValidator(func1), Field(gt=1), AfterValidator(func2)] = Field(...)

Model.model_fields['a'].metadata
#> [Gt(gt=1), AfterValidator(func=<function func2 at 0x7c2889945f80>)]
# Some metadata is lost?

Environment

Pydantic: 2

What Broke

Users experience unexpected behavior and bugs when using Field() and FieldInfo in models.

Why It Broke

The implementation of Field() and FieldInfo is inconsistent and not performant, leading to bugs

Fix Options (Details)

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

pip install pydantic==2.11.8

When NOT to use: This fix should not be used if backward compatibility with third-party libraries is a concern.

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

Option C — Workaround Temporary workaround

in https://github.com/pydantic/pydantic/pull/9279/), but not all:

When NOT to use: This fix should not be used if backward compatibility with third-party libraries is a concern.

Use only if you cannot change versions today. Treat this as a stopgap and remove once upgraded.

Option D — Guard side-effects with OnceOnly Guardrail for side-effects

Mitigate duplicate external side-effects under retries/timeouts/agent loops by gating the operation before calling external systems.

Place OnceOnly between your code/agent and real side-effects (Stripe, emails, CRM, APIs).
Use a stable key per side-effect (e.g., customer_id + action + idempotency_key).
Fail-safe: configure fail-open vs fail-closed based on blast radius and spend risk.

Show example snippet (optional)

onceonly.py

from onceonly import OnceOnly
import os

once = OnceOnly(api_key=os.environ["ONCEONLY_API_KEY"], fail_open=True)

# Stable idempotency key per real side-effect.
# Use a request id / job id / webhook delivery id / Stripe event id, etc.
event_id = "evt_..."  # replace
key = f"stripe:webhook:{event_id}"

res = once.check_lock(key=key, ttl=3600)
if res.duplicate:
    return {"status": "already_processed"}

# Safe to execute the side-effect exactly once.
handle_event(event_id)

See OnceOnly SDK

When NOT to use: Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.

Fix reference: https://github.com/pydantic/pydantic/pull/11898

First fixed release: 2.11.8

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

When NOT to Use This Fix

This fix should not be used if backward compatibility with third-party libraries is a concern.
Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.

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
2.11.8	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.

Pydantic `Field()` and `FieldInfo`/metadata ordering inconsistencies and bugs (Fix)