Pydantic Mypy error: incompatible definition of

The Fix

pip install pydantic==1.10.18

Based on closed pydantic/pydantic issue #9861 · 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

@@ -345,6 +345,7 @@ def to_argument(
         api: SemanticAnalyzerPluginInterface,
         force_typevars_invariant: bool,
+        is_root_model_root: bool,
     ) -> Argument:
         """Based on mypy.plugins.dataclasses.DataclassAttribute.to_argument."""

repro.py

from pydantic import BaseModel, RootModel

class Foo(BaseModel):
    pass


class Bar(Foo, RootModel):
    pass

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==1.10.18\nWhen NOT to use: This fix should not be applied if the inheritance structure is not intended to be compatible.\n\n

Why This Fix Works in Production

Trigger: Mypy error: incompatible definition of `model_construct()` in multiple inheritance from `BaseModel` and `RootModel`
Mechanism: The mypy plugin fails to handle multiple inheritance from BaseModel and RootModel correctly
Why the fix works: Prevents a type error for exotic BaseModel/RootModel inheritance, resolving the mypy error related to incompatible definitions. (first fixed release: 1.10.18).

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 3.8 in real deployments (not just unit tests).
The mypy plugin fails to handle multiple inheritance from BaseModel and RootModel correctly
Production symptom (often without a traceback): Mypy error: incompatible definition of `model_construct()` in multiple inheritance from `BaseModel` and `RootModel`

Proof / Evidence

GitHub issue: #9861
Fix PR: https://github.com/pydantic/pydantic/pull/9913
First fixed release: 1.10.18
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.67

Discussion

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

Jump to Sources Open on GitHub

“@brianmedigate it would be great if you could confirm whether #9913 resolves the issue for you”

@dmontagu · 2024-07-17 · confirmation · source

“I opened #9913 which I think may fix this but I'm concerned that the relevant code was necessary to resolve some other bug.. if there's…”

@dmontagu · 2024-07-17 · source

“Okay I realized that there was a reason the code did what it did, but I'm semi-optimistic I found a "real" fix.. see #9913 for…”

@dmontagu · 2024-07-17 · source

Failure Signature (Search String)

Mypy error: incompatible definition of `model_construct()` in multiple inheritance from `BaseModel` and `RootModel`
`test.py:7: error: Definition of "model_construct" in base class "Foo" is incompatible with definition in base class "RootModel" [misc]`

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Mypy error: incompatible definition of `model_construct()` in multiple inheritance from `BaseModel` and `RootModel`
`test.py:7: error: Definition of "model_construct" in base class "Foo" is incompatible with definition in base class "RootModel"  [misc]`

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Mypy error: incompatible definition of `model_construct()` in multiple inheritance from `BaseModel` and `RootModel`
`test.py:7: error: Definition of "model_construct" in base class "Foo" is incompatible with definition in base class "RootModel"  [misc]`

Minimal Reproduction

repro.py

from pydantic import BaseModel, RootModel

class Foo(BaseModel):
    pass


class Bar(Foo, RootModel):
    pass

Environment

Python: 3.8
Pydantic: 2

What Broke

Developers encounter mypy errors when using multiple inheritance with Pydantic models.

Why It Broke

The mypy plugin fails to handle multiple inheritance from BaseModel and RootModel correctly

Fix Options (Details)

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

pip install pydantic==1.10.18

When NOT to use: This fix should not be applied if the inheritance structure is not intended to be compatible.

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/9913

First fixed release: 1.10.18

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

When NOT to Use This Fix

This fix should not be applied if the inheritance structure is not intended to be compatible.

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
1.10.18	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 error: incompatible definition of `model_construct()` in multiple inheritance from (Fix)