Pydantic Mypy crashes when trying to parse a pydantic model

The Fix

pip install pydantic==2.6.1

Based on closed pydantic/pydantic issue #8665 · PR/commit linked

Jump to Verify Open PR/Commit

@@ -475,9 +475,9 @@ def transform(self) -> bool:
 
         is_settings = any(base.fullname == BASESETTINGS_FULLNAME for base in info.mro[:-1])
-        self.add_initializer(fields, config, is_settings, is_root_model)
-        self.add_model_construct_method(fields, config, is_settings)
         try:

repro.py

from pydantic import BaseModel


class MongoSettings(BaseModel):
    MONGO_PASSWORD: str | None

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==2.6.1\nWhen NOT to use: This fix should not be used if the mypy plugin is not required or if strict optional handling is needed.\n\n

Why This Fix Works in Production

Trigger: testo.py:4: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
Mechanism: The mypy plugin was not handling all scenarios leading to a _DeferAnalysis error
Why the fix works: Fixes a bug with the mypy plugin when using no_strict_optional, preventing mypy from crashing. (first fixed release: 2.6.1).

Why This Breaks in Prod

Shows up under Python 3.12 in real deployments (not just unit tests).
The mypy plugin was not handling all scenarios leading to a _DeferAnalysis error
Surfaces as: testo.py:4: error: INTERNAL ERROR -- Please try using mypy master on GitHub:

Proof / Evidence

GitHub issue: #8665
Fix PR: https://github.com/pydantic/pydantic/pull/8666
First fixed release: 2.6.1
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.39

Discussion

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

Jump to Sources Open on GitHub

“@kikones34 thanks for reporting, I've opened a PR that I believe fixes it — #8666. It would be great if you could confirm whether that…”

@dmontagu · 2024-01-29 · confirmation · source

“Yep, this was introduced in #7411 and it's obvious that the change I made there was not handling all possible scenarios under which the _DeferAnalysis…”

@dmontagu · 2024-01-29 · source

“@kikones34, Thanks for bringing this to our attention. We'll do a patch release tomorrow including @dmontagu's fix!”

@sydney-runkle · 2024-01-29 · source

Failure Signature (Search String)

testo.py:4: error: INTERNAL ERROR -- Please try using mypy master on GitHub:

Error Message

Stack trace

error.txt

Error Message
-------------
testo.py:4: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 1.8.0
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "mypy\semanal.py", line 6539, in accept
  File "mypy\nodes.py", line 1142, in accept
  File "mypy\semanal.py", line 1612, in visit_class_def
  File "mypy\semanal.py", line 1697, in analyze_class
  File "mypy\semanal.py", line 1731, in analyze_class_body_common
  File "mypy\semanal.py", line 1816, in apply_class_plugin_hooks
  File "C:\Users\arico\venv2\Lib\site-packages\pydantic\mypy.py", line 187, in _pydantic_model_class_maker_callback
    return transformer.transform()
           ^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\arico\venv2\Lib\site-packages\pydantic\mypy.py", line 478, in transform
    self.add_initializer(fields, config, is_settings, is_root_model)
  File "C:\Users\arico\venv2\Lib\site-packages\pydantic\mypy.py", line 856, in add_initializer
    args = self.get_field_arguments(
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\arico\venv2\Lib\site-packages\pydantic\mypy.py", line 1039, in get_field_arguments
    field.to_argument(
  File "C:\Users\arico\venv2\Lib\site-packages\pyd
... (truncated) ...

Minimal Reproduction

repro.py

from pydantic import BaseModel


class MongoSettings(BaseModel):
    MONGO_PASSWORD: str | None

Environment

Python: 3.12
Pydantic: 2

What Broke

Mypy crashes when parsing models with optional fields, causing development interruptions.

Why It Broke

The mypy plugin was not handling all scenarios leading to a _DeferAnalysis error

Fix Options (Details)

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

pip install pydantic==2.6.1

When NOT to use: This fix should not be used if the mypy plugin is not required or if strict optional handling is needed.

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/8666

First fixed release: 2.6.1

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

When NOT to Use This Fix

This fix should not be used if the mypy plugin is not required or if strict optional handling is needed.

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
2.6.1	Fixed

Related Issues

No related fixes found.

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Related clusters: Configuration error Data consistency OpenAPI schema

Sources

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Pydantic Mypy crashes when trying to parse a pydantic model with optional fields (Fix)