Pydantic Unexpected KeyError occured when calling

The Fix

pip install pydantic==2.6.0

Based on closed pydantic/pydantic issue #8633 · 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

@@ -334,6 +334,7 @@ def inspect_namespace(  # noqa C901
             isinstance(value, type)
             and value.__module__ == namespace['__module__']
+            and '__qualname__' in namespace
             and value.__qualname__.startswith(namespace['__qualname__'])
         ):

repro.py

from pydantic import create_model, BaseModel


class A:
    pass

class StaticModel(BaseModel):
    a_cls: type[A] = A  # this works fine


DynamicModel = create_model("DynamicModel", a_cls=(type[A], A))
"""
Traceback (most recent call last):
  File "path\to\test.py", line 8, in <module>
    DynamicModel = create_model("DynamicModel", a_cls=(type[A], A))
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "path\to\.venv\Lib\site-packages\pydantic\main.py", line 1490, in create_model
    return meta(
           ^^^^^
  File "path\to\.venv\Lib\site-packages\pydantic\_internal\_model_construction.py", line 92, in __new__
    private_attributes = inspect_namespace(
                         ^^^^^^^^^^^^^^^^^^
  File "path\to\.venv\Lib\site-packages\pydantic\_internal\_model_construction.py", line 337, in inspect_namespace
    and value.__qualname__.startswith(namespace['__qualname__'])
                                      ~~~~~~~~~^^^^^^^^^^^^^^^^
KeyError: '__qualname__'
"""

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==2.6.0\nWhen NOT to use: This fix should not be used if the model's namespace is intentionally designed without a '__qualname__'.\n\n

Why This Fix Works in Production

Trigger: from pydantic import create_model, BaseModel
Mechanism: The model's namespace lacks a '__qualname__', causing a KeyError when adding a type-type field
Why the fix works: Adds a check on the existence of `__qualname__` in the namespace to avoid unexpected KeyError when setting a type-type default value in `create_model`. (first fixed release: 2.6.0).

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

The model's namespace lacks a '__qualname__', causing a KeyError when adding a type-type field
Surfaces as: from pydantic import create_model, BaseModel

Proof / Evidence

GitHub issue: #8633
Fix PR: https://github.com/pydantic/pydantic/pull/8642
First fixed release: 2.6.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.34

Discussion

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

Jump to Sources Open on GitHub

“@anci3ntr0ck, Thanks for reporting this”

@sydney-runkle · 2024-01-25 · source

“@sydney-runkle Sure. Very glad to be a contributor to this great library 😆! I'll work on this soon 👌.”

@anci3ntr0ck · 2024-01-25 · source

Failure Signature (Search String)

from pydantic import create_model, BaseModel

Error Message

Stack trace

error.txt

Error Message
-------------
from pydantic import create_model, BaseModel


class A:
    pass

class StaticModel(BaseModel):
    a_cls: type[A] = A  # this works fine


DynamicModel = create_model("DynamicModel", a_cls=(type[A], A))
"""
Traceback (most recent call last):
  File "path\to\test.py", line 8, in <module>
    DynamicModel = create_model("DynamicModel", a_cls=(type[A], A))
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "path\to\.venv\Lib\site-packages\pydantic\main.py", line 1490, in create_model
    return meta(
           ^^^^^
  File "path\to\.venv\Lib\site-packages\pydantic\_internal\_model_construction.py", line 92, in __new__
    private_attributes = inspect_namespace(
                         ^^^^^^^^^^^^^^^^^^
  File "path\to\.venv\Lib\site-packages\pydantic\_internal\_model_construction.py", line 337, in inspect_namespace
    and value.__qualname__.startswith(namespace['__qualname__'])
                                      ~~~~~~~~~^^^^^^^^^^^^^^^^
KeyError: '__qualname__'
"""

Minimal Reproduction

repro.py

from pydantic import create_model, BaseModel


class A:
    pass

class StaticModel(BaseModel):
    a_cls: type[A] = A  # this works fine


DynamicModel = create_model("DynamicModel", a_cls=(type[A], A))
"""
Traceback (most recent call last):
  File "path\to\test.py", line 8, in <module>
    DynamicModel = create_model("DynamicModel", a_cls=(type[A], A))
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "path\to\.venv\Lib\site-packages\pydantic\main.py", line 1490, in create_model
    return meta(
           ^^^^^
  File "path\to\.venv\Lib\site-packages\pydantic\_internal\_model_construction.py", line 92, in __new__
    private_attributes = inspect_namespace(
                         ^^^^^^^^^^^^^^^^^^
  File "path\to\.venv\Lib\site-packages\pydantic\_internal\_model_construction.py", line 337, in inspect_namespace
    and value.__qualname__.startswith(namespace['__qualname__'])
                                      ~~~~~~~~~^^^^^^^^^^^^^^^^
KeyError: '__qualname__'
"""

Environment

Pydantic: 2

What Broke

KeyError occurs when creating models with type-type fields defined in the same module.

Why It Broke

The model's namespace lacks a '__qualname__', causing a KeyError when adding a type-type field

Fix Options (Details)

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

pip install pydantic==2.6.0

When NOT to use: This fix should not be used if the model's namespace is intentionally designed without a '__qualname__'.

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/8642

First fixed release: 2.6.0

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

When NOT to Use This Fix

This fix should not be used if the model's namespace is intentionally designed without a '__qualname__'.

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
2.6.0	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 Unexpected KeyError occured when calling `create_model` with a `type`-type field which is (Fix)