Pydantic `Optional[Decimal]` with field constraints error:

The Fix

pip install pydantic==1.10.19

Based on closed pydantic/pydantic issue #10498 · 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

@@ -201,7 +201,7 @@ def apply_known_metadata(annotation: Any, schema: CoreSchema) -> CoreSchema | No
     import annotated_types as at
 
-    from ._validators import forbid_inf_nan_check, get_constraint_validator
+    from ._validators import NUMERIC_VALIDATOR_LOOKUP, forbid_inf_nan_check

repro.py

import decimal
from pydantic import BaseModel, Field, BeforeValidator
from typing import Annotated

DecimalAnnotation = Annotated[decimal.Decimal, BeforeValidator(lambda v: v)]


class ExampleSchema(BaseModel):
    example_value: DecimalAnnotation | None = Field(max_digits=10, decimal_places=4)

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==1.10.19\nWhen NOT to use: This fix should not be used if the application relies on previous behavior of Pydantic 2.8.\n\n

Why This Fix Works in Production

Trigger: `Optional[Decimal]` with field constraints error: Unable to apply constraint 'max_digits' to schema of type 'function-after'
Mechanism: The Pydantic library fails to apply 'max_digits' and 'decimal_places' constraints to Decimal fields when using annotations
Why the fix works: Adds Python validators for decimal constraints, specifically addressing issues with applying 'max_digits' and 'decimal_places' constraints to annotated fields in Pydantic. (first fixed release: 1.10.19).

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 Pydantic library fails to apply 'max_digits' and 'decimal_places' constraints to Decimal fields when using annotations
Production symptom (often without a traceback): `Optional[Decimal]` with field constraints error: Unable to apply constraint 'max_digits' to schema of type 'function-after'

Proof / Evidence

GitHub issue: #10498
Fix PR: https://github.com/pydantic/pydantic/pull/10506
First fixed release: 1.10.19
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.60

Discussion

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

Jump to Sources Open on GitHub

“@JoelRaymann, Could you please send a reproducible example (with imports, etc)? Happy to look into this for you.”

@sydney-runkle · 2024-09-26 · source

“@sydney-runkle So based on your suggestion, I should do this right: Am I right? or am I missing something?”

@JoelRaymann · 2024-09-27 · source

“Ok”

@JoelRaymann · 2024-09-26 · source

“Re 1: Because of https://github.com/pydantic/pydantic/blob/728d3d9f41c480d35c1188f56cc360bae93f0543/pydantic/fields.py#L371”

@sydney-runkle · 2024-09-27 · source

Failure Signature (Search String)

`Optional[Decimal]` with field constraints error: Unable to apply constraint 'max_digits' to schema of type 'function-after'
Still, nothing works. I am still getting the `RuntimeError: Unable to apply constraint 'max_digits' to schema of type 'function-after'`.

Copy-friendly signature

signature.txt

Failure Signature
-----------------
`Optional[Decimal]` with field constraints error: Unable to apply constraint 'max_digits' to schema of type 'function-after'
Still, nothing works. I am still getting the `RuntimeError: Unable to apply constraint 'max_digits' to schema of type 'function-after'`.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
`Optional[Decimal]` with field constraints error: Unable to apply constraint 'max_digits' to schema of type 'function-after'
Still, nothing works. I am still getting the `RuntimeError: Unable to apply constraint 'max_digits' to schema of type 'function-after'`.

Minimal Reproduction

repro.py

import decimal
from pydantic import BaseModel, Field, BeforeValidator
from typing import Annotated

DecimalAnnotation = Annotated[decimal.Decimal, BeforeValidator(lambda v: v)]


class ExampleSchema(BaseModel):
    example_value: DecimalAnnotation | None = Field(max_digits=10, decimal_places=4)

Environment

Pydantic: 2

Why It Broke

The Pydantic library fails to apply 'max_digits' and 'decimal_places' constraints to Decimal fields when using annotations

Fix Options (Details)

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

pip install pydantic==1.10.19

When NOT to use: This fix should not be used if the application relies on previous behavior of Pydantic 2.8.

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/10506

First fixed release: 1.10.19

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

When NOT to Use This Fix

This fix should not be used if the application relies on previous behavior of Pydantic 2.8.

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
1.10.19	Fixed

Related Issues

No related fixes found.

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Related clusters: Data consistency OpenAPI schema Serialization

Sources

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

Pydantic `Optional[Decimal]` with field constraints error: Unable to apply constraint 'max_digits' (Fix)