Pydantic 2.8.0 unexpectedly change `create

The Fix

pip install pydantic==2.7.2

Based on closed pydantic/pydantic issue #9810 · 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

@@ -7,12 +7,26 @@
 import warnings
 from copy import copy, deepcopy
-from typing import Any, ClassVar, Dict, Generator, Literal, Set, Tuple, TypeVar, Union
+from typing import (
+    TYPE_CHECKING,

repro.py

def inject_default_from_instance(instance_w_defaults: BaseModel) -> Type[BaseModel]:
    model_w_defaults = create_model(
        __model_name=instance_w_defaults.__repr_name__() + "WithDefaults",
        __base__=type(instance_w_defaults),
        **{
            k: (
                instance_w_defaults.model_fields[k].annotation,
                FieldInfo.merge_field_infos(
                    instance_w_defaults.model_fields[k],
                    default=getattr(instance_w_defaults, k)
                )
            )
            for k in instance_w_defaults.model_fields.keys()
        }
    )
    return model_w_defaults

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==2.7.2\nWhen NOT to use: This fix is not applicable if the code relies on the previous argument naming convention.\n\n

Why This Fix Works in Production

Trigger: TypeError: create_model() missing 1 required positional argument: 'model_name'
Mechanism: The argument name for `create_model()` was changed from `__model_name` to `model_name`, causing compatibility issues
Why the fix works: The argument name for `create_model()` was changed from `__model_name` to `model_name`, which caused compatibility issues for users relying on the previous naming convention. (first fixed release: 2.7.2).

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 3.11 in real deployments (not just unit tests).
The argument name for `create_model()` was changed from `__model_name` to `model_name`, causing compatibility issues
Surfaces as: TypeError: create_model() missing 1 required positional argument: 'model_name'

Proof / Evidence

GitHub issue: #9810
Fix PR: https://github.com/pydantic/pydantic/pull/9479
First fixed release: 2.7.2
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.59

Discussion

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

Jump to Sources Open on GitHub

“@mudravrik, > After some reading on topic I got to this part of PEP-570 explaining, that __model_name is a naming convetion meaning, that it is…”

@sydney-runkle · 2024-07-01 · confirmation · source

“After some reading on topic I got to this part of PEP-570 explaining, that __model_name is a naming convetion meaning, that it is positional-only argument…”

@mudravrik · 2024-07-01 · source

Failure Signature (Search String)

TypeError: create_model() missing 1 required positional argument: 'model_name'

Error Message

Stack trace

error.txt

Error Message
-------------
TypeError: create_model() missing 1 required positional argument: 'model_name'

Minimal Reproduction

repro.py

def inject_default_from_instance(instance_w_defaults: BaseModel) -> Type[BaseModel]:
    model_w_defaults = create_model(
        __model_name=instance_w_defaults.__repr_name__() + "WithDefaults",
        __base__=type(instance_w_defaults),
        **{
            k: (
                instance_w_defaults.model_fields[k].annotation,
                FieldInfo.merge_field_infos(
                    instance_w_defaults.model_fields[k],
                    default=getattr(instance_w_defaults, k)
                )
            )
            for k in instance_w_defaults.model_fields.keys()
        }
    )
    return model_w_defaults

Environment

Python: 3.11
Pydantic: 2

What Broke

Pipelines failed during tests with TypeError due to the argument name change.

Why It Broke

The argument name for `create_model()` was changed from `__model_name` to `model_name`, causing compatibility issues

Fix Options (Details)

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

pip install pydantic==2.7.2

When NOT to use: This fix is not applicable if the code relies on the previous argument naming convention.

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/9479

First fixed release: 2.7.2

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

When NOT to Use This Fix

This fix is not applicable if the code relies on the previous argument naming convention.

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
2.7.2	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 2.8.0 unexpectedly change `create_model()`arg `__model_name` to `model_name` (Fix)