Pydantic Mypy type checking not working properly for

The Fix

pip install pydantic==1.10.15

Based on closed pydantic/pydantic issue #9007 · 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

@@ -126,10 +126,6 @@ def plugin(version: str) -> type[Plugin]:
 
 
-class _DeferAnalysis(Exception):
-    pass
-

repro.py

from pydantic import BaseModel


class Foo(BaseModel):
    id: int | None


class Bar(BaseModel):
    foo: Foo | None


class Baz(Bar):
    name: str


b = Bar(foo={"id": 1})
z = Baz(foo={"id": 1}, name="test")


assert z.foo.id == 1  # needed an ignore before
assert b.foo.id == 1  # type: ignore[union-attr]

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==1.10.15\nWhen NOT to use: This fix should not be used if the mypy plugin is not configured correctly.\n\n

Why This Fix Works in Production

Trigger: assert z.foo.id == 1 # needed an ignore before
Mechanism: Fix handling of optionals in mypy plugin, resolving issues with optional type checks after upgrading to v2.6.4.
Why the fix works: Fix handling of optionals in mypy plugin, resolving issues with optional type checks after upgrading to v2.6.4. (first fixed release: 1.10.15).

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).
Production symptom (often without a traceback): assert z.foo.id == 1 # needed an ignore before

Proof / Evidence

GitHub issue: #9007
Fix PR: https://github.com/pydantic/pydantic/pull/9008
First fixed release: 1.10.15
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.75
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.65

Discussion

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

Jump to Sources Open on GitHub

“Awesome! We'll get #9008 across the line before the 2.7.0b1 release, so this should be fixed soon!”

@sydney-runkle · 2024-03-15 · confirmation · source

“Can you share your mypy config? Are you using no_strict_optional = True? Also, what version of mypy are you using?”

@dmontagu · 2024-03-13 · source

“I am using mypy v1.9.0 and I am actually using strict_optional=true. Here is the whole config:”

@pengelsman7 · 2024-03-14 · source

“Thank you for that info @pengelsman7, I was able to confirm locally that #9008 seems to fix your issue”

@dmontagu · 2024-03-14 · source

Failure Signature (Search String)

assert z.foo.id == 1 # needed an ignore before
assert b.foo.id == 1 # type: ignore[union-attr]

Copy-friendly signature

signature.txt

Failure Signature
-----------------
assert z.foo.id == 1  # needed an ignore before
assert b.foo.id == 1  # type: ignore[union-attr]

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
assert z.foo.id == 1  # needed an ignore before
assert b.foo.id == 1  # type: ignore[union-attr]

Minimal Reproduction

repro.py

from pydantic import BaseModel


class Foo(BaseModel):
    id: int | None


class Bar(BaseModel):
    foo: Foo | None


class Baz(Bar):
    name: str


b = Bar(foo={"id": 1})
z = Baz(foo={"id": 1}, name="test")


assert z.foo.id == 1  # needed an ignore before
assert b.foo.id == 1  # type: ignore[union-attr]

Environment

Python: 3.11
Pydantic: 2

What Broke

Users experienced incorrect type checking results when using optional attributes in models.

Fix Options (Details)

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

pip install pydantic==1.10.15

When NOT to use: This fix should not be used if the mypy plugin is not configured correctly.

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/9008

First fixed release: 1.10.15

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 configured correctly.

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
1.10.15	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 Mypy type checking not working properly for classes that inherit properties after v2.6.4 (Fix)