Pydantic Root models are sometimes inferred as unions by

The Fix

pip install pydantic==2.11.2

Based on closed pydantic/pydantic issue #11675 · PR/commit linked

Jump to Verify Open PR/Commit

@@ -290,7 +290,7 @@ def to_argument(
         strict = model_strict if self.strict is None else self.strict
         if typed or strict:
-            type_annotation = self.expand_type(current_info, api)
+            type_annotation = self.expand_type(current_info, api, include_root_type=True)
         else:

repro.py

from typing import reveal_type

from pydantic import RootModel, BaseModel


class A(RootModel[list[int]]):
    pass


class B(BaseModel):
    a: A


class C(B):
    pass


b = B(a=A([1, 2, 3]))
c = C(a=A([1, 2, 3]))

reveal_type(b.a)  # A (expected)
reveal_type(c.a)  # A | list[int] (unexpected, should be A)

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==2.11.2\nWhen NOT to use: Do not apply this fix if the mypy plugin is not being used with Pydantic.\n\n

Why This Fix Works in Production

Trigger: reveal_type(c.a) # A | list[int] (unexpected, should be A)
Mechanism: Root models were incorrectly inferred as unions by mypy when using the Pydantic plugin
Why the fix works: Fixes an issue where root models were incorrectly inferred as unions by mypy when using the Pydantic plugin. (first fixed release: 2.11.2).

Production impact:

If left unfixed, this can cause silent data inconsistencies that propagate (bad cache entries, incorrect downstream decisions).

Why This Breaks in Prod

Root models were incorrectly inferred as unions by mypy when using the Pydantic plugin
Production symptom (often without a traceback): reveal_type(c.a) # A | list[int] (unexpected, should be A)

Proof / Evidence

GitHub issue: #11675
Fix PR: https://github.com/pydantic/pydantic/pull/11676
First fixed release: 2.11.2
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.61

Discussion

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

Jump to Sources Open on GitHub

“### Initial Checks - [x] I confirm that I'm using Pydantic V2 ### Description Maybe related to #11657. In the example below, Pyright 1.1.398 deduces A for both b.a and c.a, while mypy (with the Pydantic plugin) deduces the union type for c.”

Issue thread · issue description · source

Failure Signature (Search String)

reveal_type(c.a) # A | list[int] (unexpected, should be A)

Copy-friendly signature

signature.txt

Failure Signature
-----------------
reveal_type(c.a)  # A | list[int] (unexpected, should be A)

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
reveal_type(c.a)  # A | list[int] (unexpected, should be A)

Minimal Reproduction

repro.py

from typing import reveal_type

from pydantic import RootModel, BaseModel


class A(RootModel[list[int]]):
    pass


class B(BaseModel):
    a: A


class C(B):
    pass


b = B(a=A([1, 2, 3]))
c = C(a=A([1, 2, 3]))

reveal_type(b.a)  # A (expected)
reveal_type(c.a)  # A | list[int] (unexpected, should be A)

Environment

Pydantic: 2

What Broke

Mypy incorrectly deduces union types, leading to type errors in code.

Why It Broke

Root models were incorrectly inferred as unions by mypy when using the Pydantic plugin

Fix Options (Details)

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

pip install pydantic==2.11.2

When NOT to use: Do not apply this fix if the mypy plugin is not being used with Pydantic.

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/11676

First fixed release: 2.11.2

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

When NOT to Use This Fix

Do not apply this fix if the mypy plugin is not being used with Pydantic.

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
2.11.2	Fixed

Related Issues

No related fixes found.

Cluster: pydantic:data-consistency Pydantic hub Pydantic best practices All hubs All clusters

Related clusters: Configuration error OpenAPI schema Serialization

Sources

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

Pydantic Root models are sometimes inferred as unions by mypy when using the mypy plugin (Fix)