Pydantic pydantic_core._pydantic

The Fix

pip install pydantic==2.10.0

Based on closed pydantic/pydantic issue #8984 · 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

@@ -522,6 +522,25 @@ except PydanticUserError as exc_info:
 ```
 
+## Circular reference schema {#circular-reference-schema}
+
+This error is raised when a circular reference is found that would otherwise result in an infinite recursion.

repro.py

from pydantic import BaseModel
from typing import Sequence

type MySeq[T] = Sequence[T]
type MyIntSeq = MySeq[int]

class MyModel(BaseModel):
    my_int_seq: MyIntSeq

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==2.10.0\nWhen NOT to use: This fix should not be applied if the type aliases do not reference each other.\n\n

Why This Fix Works in Production

Trigger: class MyModel(BaseModel):
Mechanism: A circular reference in type aliases causes Pydantic to fail during schema validation
Why the fix works: Fixes an error when using type aliases referencing other type aliases in Pydantic. (first fixed release: 2.10.0).

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

Shows up under Python 3.12 in real deployments (not just unit tests).
A circular reference in type aliases causes Pydantic to fail during schema validation
Surfaces as: Traceback (most recent call last):

Proof / Evidence

GitHub issue: #8984
Fix PR: https://github.com/pydantic/pydantic/pull/10809
First fixed release: 2.10.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.27

Discussion

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

Jump to Sources Open on GitHub

“I believe this is a bug in our handling of nested type aliases. Definitely intend to fix it, not sure how complicated it will be..”

@dmontagu · 2024-03-12 · source

“Maybe I'm being dumb, but I don't think your code is valid python.”

@samuelcolvin · 2024-03-11 · source

“Why do you think that? Python 3.12 will execute the code if I use a dataclass instead of BaseModel, and pyright finds no issue with…”

@multimeric · 2024-03-11 · source

“https://docs.python.org/3/library/typing.html#type-aliases - type statement new in 3.12”

@davidhewitt · 2024-03-12 · source

Failure Signature (Search String)

class MyModel(BaseModel):

Error Message

Stack trace

error.txt

Error Message
-------------
Traceback (most recent call last):
  File "/Users/milton.m/Programming/cyton-solver/backend/test_model.py", line 7, in <module>
    class MyModel(BaseModel):
  File "/Users/milton.m/Programming/cyton-solver/venv/lib/python3.12/site-packages/pydantic/_internal/_model_construction.py", line 183, in __new__
    complete_model_class(
  File "/Users/milton.m/Programming/cyton-solver/venv/lib/python3.12/site-packages/pydantic/_internal/_model_construction.py", line 535, in complete_model_class
    cls.__pydantic_validator__ = create_schema_validator(
                                 ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/milton.m/Programming/cyton-solver/venv/lib/python3.12/site-packages/pydantic/plugin/_schema_validator.py", line 49, in create_schema_validator
    return SchemaValidator(schema, config)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pydantic_core._pydantic_core.SchemaError: Definitions error: definition `__main__.MySeq:4304815984[int:4316993544]` was never filled

Stack trace

error.txt

Error Message
-------------
causes

Traceback (most recent call last):
  File "/path/to/proj/.venv/lib64/python3.12/site-packages/pydantic/type_adapter.py", line 217, in __init__
    validator = _getattr_no_parents(type, '__pydantic_validator__')
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/path/to/proj/.venv/lib64/python3.12/site-packages/pydantic/type_adapter.py", line 98, in _getattr_no_parents
    raise AttributeError(attribute)
AttributeError: __pydantic_validator__

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/path/to/proj/main.py", line 14, in <module>
    adapter = TypeAdapter(JSONs)
              ^^^^^^^^^^^^^^^^^^
  File "/path/to/proj/.venv/lib64/python3.12/site-packages/pydantic/type_adapter.py", line 222, in __init__
    validator = create_schema_validator(
                ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/path/to/proj/.venv/lib64/python3.12/site-packages/pydantic/plugin/_schema_validator.py", line 49, in create_schema_validator
    return SchemaValidator(schema, config)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pydantic_core._pydantic_core.SchemaError: Definitions error: definition `__main__.JSON:1234` was never filled

If I replace `type MyJSONAlias = JSON` by `MyJSONAlias = JSON`, then everything works. So I guess this is related to  Python 3.12 `type` aliases and their lazy evaluation?

He
... (truncated) ...

Minimal Reproduction

repro.py

from pydantic import BaseModel
from typing import Sequence

type MySeq[T] = Sequence[T]
type MyIntSeq = MySeq[int]

class MyModel(BaseModel):
    my_int_seq: MyIntSeq

Environment

Python: 3.12
Pydantic: 2

What Broke

Users encounter SchemaError when using nested type aliases in Pydantic models.

Why It Broke

A circular reference in type aliases causes Pydantic to fail during schema validation

Fix Options (Details)

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

pip install pydantic==2.10.0

When NOT to use: This fix should not be applied if the type aliases do not reference each other.

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/10809

First fixed release: 2.10.0

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

When NOT to Use This Fix

This fix should not be applied if the type aliases do not reference each other.

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?

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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
2.10.0	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 pydantic_core._pydantic_core.SchemaError: Definitions error: definition `XYZ` was never (Fix)