Pydantic Base64Bytes field doesn't raises validation error

The Fix

pip install pydantic==1.10.19

Based on closed pydantic/pydantic issue #9251 · 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

@@ -2477,9 +2477,11 @@ def __get_pydantic_json_schema__(
 
     def __get_pydantic_core_schema__(self, source: type[Any], handler: GetCoreSchemaHandler) -> core_schema.CoreSchema:
+        schema = handler(source)
+        _check_annotated_type(schema['type'], 'bytes', self.__class__.__name__)
         return core_schema.with_info_after_validator_function(

repro.py

from pydantic import Base64Bytes, BaseModel, Field


class Test(BaseModel):
    data: Base64Bytes = Field(min_length=10)


test = Test(data=b"")  # no validation error

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 applied if backward compatibility with previous versions is a concern.\n\n

Why This Fix Works in Production

Trigger: Base64Bytes field doesn't raises validation error for min_length constraint
Mechanism: The Base64Bytes field did not properly enforce the min_length constraint during validation
Why the fix works: Fixes a bug where the Base64Bytes field did not raise a validation error for the min_length constraint. (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 Base64Bytes field did not properly enforce the min_length constraint during validation
Production symptom (often without a traceback): Base64Bytes field doesn't raises validation error for min_length constraint

Proof / Evidence

GitHub issue: #9251
Fix PR: https://github.com/pydantic/pydantic/pull/10584
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: 0.62

Discussion

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

Jump to Sources Open on GitHub

“@kirillklimenko, Thanks for reporting, definitely a bug! PRs welcome with a fix!”

@sydney-runkle · 2024-04-16 · source

“I would like to take a look at it! Where should I start looking at?”

@gitFire001 · 2024-07-02 · source

“I wanna to contribute too, but, idk how to debug it, it's my first time trying to submit anything in this project. Any suggest?”

@danbailo · 2024-07-06 · source

“I think this is related to encode, decode logic since even when converted Base64Bytes to Base64Str, there is no validation error too. We can avoid…”

@RektPunk · 2024-07-13 · source

Failure Signature (Search String)

Base64Bytes field doesn't raises validation error for min_length constraint
The validation should raise an error due to the minimum length constraint set for the `data` field in the `Test` model. However, no validation error is raised even though the

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Base64Bytes field doesn't raises validation error for min_length constraint
The validation should raise an error due to the minimum length constraint set for the `data` field in the `Test` model. However, no validation error is raised even though the `data` field is empty.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Base64Bytes field doesn't raises validation error for min_length constraint
The validation should raise an error due to the minimum length constraint set for the `data` field in the `Test` model. However, no validation error is raised even though the `data` field is empty.

Minimal Reproduction

repro.py

from pydantic import Base64Bytes, BaseModel, Field


class Test(BaseModel):
    data: Base64Bytes = Field(min_length=10)


test = Test(data=b"")  # no validation error

Environment

Pydantic: 2

What Broke

Validation errors were not raised for empty data, leading to incorrect model behavior.

Why It Broke

The Base64Bytes field did not properly enforce the min_length constraint during validation

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 applied if backward compatibility with previous versions is a concern.

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/10584

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 applied if backward compatibility with previous versions is a concern.

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.

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 Base64Bytes field doesn't raises validation error for min_length constraint (Fix)