Pydantic Exception in mypy plugin with type alias, root

The Fix

pip install pydantic==2.11.4

Based on closed pydantic/pydantic issue #11727 · PR/commit linked

Jump to Verify Open PR/Commit

@@ -5,7 +5,7 @@
 from collections.abc import Iterator
 from configparser import ConfigParser
-from typing import Any, Callable, cast
+from typing import Any, Callable

repro.py

from pydantic import (
    BaseModel,
    RootModel,
)


ListInputType = list['DataTypeModel']


class DataTypeModel(RootModel):
    root: ListInputType


class ColumnModel(BaseModel):
    type: DataTypeModel

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==2.11.4\nWhen NOT to use: This fix should not be applied if the code relies on the previous behavior of the mypy plugin.\n\n

Why This Fix Works in Production

Trigger: transformer.transform()
Mechanism: The mypy plugin crashes when expanding root types due to unsolved forward references
Why the fix works: Fixes a crash when expanding root type in the mypy plugin, addressing an issue introduced in Pydantic 2.11.0. (first fixed release: 2.11.4).

Why This Breaks in Prod

Shows up under Python 3.13 in real deployments (not just unit tests).
The mypy plugin crashes when expanding root types due to unsolved forward references
Surfaces as: Traceback (most recent call last):

Proof / Evidence

GitHub issue: #11727
Fix PR: https://github.com/pydantic/pydantic/pull/11735
First fixed release: 2.11.4
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.39

Discussion

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

Jump to Sources Open on GitHub

“Thanks for the report, this will be fixed in the next patch release.”

@Viicos · 2025-04-11 · confirmation · source

“Confirming that the issue is fixed from our perspective 🎉”

@sbrandtb · 2025-04-30 · source

Failure Signature (Search String)

transformer.transform()

Error Message

Stack trace

error.txt

Error Message
-------------
Traceback (most recent call last):
  File "mypy/semanal.py", line 7240, in accept
  File "mypy/nodes.py", line 1177, in accept
  File "mypy/semanal.py", line 1728, in visit_class_def
  File "mypy/semanal.py", line 1944, in analyze_class
  File "mypy/semanal.py", line 1991, in analyze_class_body_common
  File "mypy/semanal.py", line 2076, in apply_class_plugin_hooks
  File "/virtualenv/.venv/lib/python3.13/site-packages/pydantic/mypy.py", line 162, in _pydantic_model_class_maker_callback
    transformer.transform()
    ~~~~~~~~~~~~~~~~~~~~~^^
  File "/virtualenv/.venv/lib/python3.13/site-packages/pydantic/mypy.py", line 473, in transform
    self.add_model_construct_method(fields, config, is_settings, is_a_root_model)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/virtualenv/.venv/lib/python3.13/site-packages/pydantic/mypy.py", line 916, in add_model_construct_method
    args = self.get_field_arguments(
        fields,
    ...<5 lines>...
        is_root_model=is_root_model,
    )
  File "/virtualenv/.venv/lib/python3.13/site-packages/pydantic/mypy.py", line 1116, in get_field_arguments
    field.to_argument(
    ~~~~~~~~~~~~~~~~~^
        info,
        ^^^^^
    ...<6 lines>...
        is_root_model_root=is_root_model and field.name == 'root',
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/virt
... (truncated) ...

Minimal Reproduction

repro.py

from pydantic import (
    BaseModel,
    RootModel,
)


ListInputType = list['DataTypeModel']


class DataTypeModel(RootModel):
    root: ListInputType


class ColumnModel(BaseModel):
    type: DataTypeModel

Environment

Python: 3.13
Pydantic: 2

What Broke

Users experience crashes when running mypy with the pydantic plugin on specific models.

Why It Broke

The mypy plugin crashes when expanding root types due to unsolved forward references

Fix Options (Details)

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

pip install pydantic==2.11.4

When NOT to use: This fix should not be applied if the code relies on the previous behavior of the mypy plugin.

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/11735

First fixed release: 2.11.4

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

When NOT to Use This Fix

This fix should not be applied if the code relies on the previous behavior of the mypy plugin.

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
2.11.4	Fixed

Related Issues

No related fixes found.

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Related clusters: Configuration error Data consistency OpenAPI schema

Sources

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

Pydantic Exception in mypy plugin with type alias, root model and forward reference (Fix)