Pydantic Enable mypy plugin to support initializing

The Fix

pip install pydantic==2.10.5

Based on closed pydantic/pydantic issue #11203 · 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

@@ -4,7 +4,7 @@
 import sys
 from configparser import ConfigParser
-from typing import Any, Callable, Iterator
+from typing import Any, Callable, Iterator, cast

repro.py

from typing import Generic, TypeVar

from pydantic import BaseModel, RootModel

T = TypeVar('T')

class Maybe(RootModel[T | None], Generic[T]):
    @property
    def is_some(self) -> bool:
        return self.root is not None

    def get(self) -> T:
        assert self.root is not None
        return self.root
        
class Test(BaseModel):
    field: Maybe[int]
    
print(Test(field=1))
print(Test(field=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==2.10.5\nWhen NOT to use: This fix should not be used if the model's root type is not guaranteed to match the expected type.\n\n

Why This Fix Works in Production

Trigger: error: Argument "field" to "Test" has incompatible type "int"; expected "Maybe[int]" [arg-type] error: Argument "field" to "Test" has incompatible type…
Mechanism: The mypy plugin did not support initializing RootModel fields with values of the generic's type when init_typed=True
Why the fix works: Supports initializing root model fields with values of the `root` type in the mypy plugin, addressing type compatibility issues with mypy when using `init_typed=True`. (first fixed release: 2.10.5).

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

The mypy plugin did not support initializing RootModel fields with values of the generic's type when init_typed=True
Surfaces as: error: Argument "field" to "Test" has incompatible type "int"; expected "Maybe[int]" [arg-type]\nerror: Argument "field" to "Test" has incompatible type "None"; expected…

Proof / Evidence

GitHub issue: #11203
Fix PR: https://github.com/pydantic/pydantic/pull/11212
First fixed release: 2.10.5
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.66

Verified Execution

We executed the runnable minimal repro in a temporary environment and captured exit codes + logs.

Status: PASS
Ran: 2026-02-11T16:52:29Z
Package: pydantic
Fixed: 2.10.5
Mode: fixed_only
Outcome: ok

Logs

affected (exit=None)

fixed (exit=0)

field=Maybe[int](root=1)
field=Maybe[int](root=None)

Discussion

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

Jump to Sources Open on GitHub

“### Initial Checks - [X] I have searched Google & GitHub for similar requests and couldn't find anything - [X] I have read and followed the docs and still think this feature is missing ### Description Consider following code: This code runs”

Issue thread · issue description · source

Failure Signature (Search String)

error: Argument "field" to "Test" has incompatible type "int"; expected "Maybe[int]" [arg-type]\nerror: Argument "field" to "Test" has incompatible type "None"; expected

Error Message

Stack trace

error.txt

Error Message
-------------
error: Argument "field" to "Test" has incompatible type "int"; expected "Maybe[int]"  [arg-type]\nerror: Argument "field" to "Test" has incompatible type "None"; expected "Maybe[int]"  [arg-type]

Minimal Reproduction

repro.py

from typing import Generic, TypeVar

from pydantic import BaseModel, RootModel

T = TypeVar('T')

class Maybe(RootModel[T | None], Generic[T]):
    @property
    def is_some(self) -> bool:
        return self.root is not None

    def get(self) -> T:
        assert self.root is not None
        return self.root
        
class Test(BaseModel):
    field: Maybe[int]
    
print(Test(field=1))
print(Test(field=None))

What Broke

Mypy flags type compatibility errors when initializing RootModel fields with generic types.

Why It Broke

The mypy plugin did not support initializing RootModel fields with values of the generic's type when init_typed=True

Fix Options (Details)

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

pip install pydantic==2.10.5

When NOT to use: This fix should not be used if the model's root type is not guaranteed to match the expected type.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Option D — Guard side-effects with OnceOnly Guardrail for side-effects

Mitigate duplicate external side-effects under retries/timeouts/agent loops by gating the operation before calling external systems.

Place OnceOnly between your code/agent and real side-effects (Stripe, emails, CRM, APIs).
Use a stable key per side-effect (e.g., customer_id + action + idempotency_key).
Fail-safe: configure fail-open vs fail-closed based on blast radius and spend risk.

Show example snippet (optional)

onceonly.py

from onceonly import OnceOnly
import os

once = OnceOnly(api_key=os.environ["ONCEONLY_API_KEY"], fail_open=True)

# Stable idempotency key per real side-effect.
# Use a request id / job id / webhook delivery id / Stripe event id, etc.
event_id = "evt_..."  # replace
key = f"stripe:webhook:{event_id}"

res = once.check_lock(key=key, ttl=3600)
if res.duplicate:
    return {"status": "already_processed"}

# Safe to execute the side-effect exactly once.
handle_event(event_id)

See OnceOnly SDK

When NOT to use: Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.

Fix reference: https://github.com/pydantic/pydantic/pull/11212

First fixed release: 2.10.5

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

When NOT to Use This Fix

This fix should not be used if the model's root type is not guaranteed to match the expected type.
Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.

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
2.10.5	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 Enable mypy plugin to support initializing RootModel fields with values of the generic's (Fix)