The Fix
pip install pydantic==2.7.2
Based on closed pydantic/pydantic issue #9461 · 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%.
@@ -480,7 +480,8 @@ 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)
+ if not is_root_model:
+ self.add_model_construct_method(fields, config, is_settings)
repro.py
import pydantic
class MyRootModel(pydantic.RootModel[str]):
pass
if __name__ == '__main__':
reveal_type(MyRootModel)
reveal_type(MyRootModel.model_construct('yo'))
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==2.7.2\nWhen NOT to use: This fix should not be used if the behavior of `RootModel` is expected to change significantly.\n\n
Why This Fix Works in Production
- Trigger: error: Missing named argument "root" for "model_construct" of "MyRootModel" [call-arg] error: Argument 1 to "model_construct" of "MyRootModel" has…
- Mechanism: Adjusts the signature handling of `RootModel.model_construct` in the mypy plugin to avoid type checking issues.
- Why the fix works: Adjusts the signature handling of `RootModel.model_construct` in the mypy plugin to avoid type checking issues. (first fixed release: 2.7.2).
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
- Triggered by an upgrade/regression window: 1.2.0 breaks; 2.7.2 is the first fixed release.
- Surfaces as: error: Missing named argument "root" for "model_construct" of "MyRootModel" [call-arg]\nerror: Argument 1 to "model_construct" of "MyRootModel" has incompatible type "str";…
Proof / Evidence
- GitHub issue: #9461
- Fix PR: https://github.com/pydantic/pydantic/pull/9480
- First fixed release: 2.7.2
- Affected versions: 1.2.0
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.95
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.53
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“Ok great, at least we were able to understand where it was coming from.”
“I'm not able to reproduce, could you please throw a reveal_type(my_root_model) in your snippet to see the actual type?”
“mypy 1.2.0 is pretty old, do you get the same error on latest (currently 1.10.0)?”
“yes i actually tried this on 1.10 directly, after realizing i was on 1.2”
Failure Signature (Search String)
- error: Missing named argument "root" for "model_construct" of "MyRootModel" [call-arg]\nerror: Argument 1 to "model_construct" of "MyRootModel" has incompatible type "str";
Error Message
Stack trace
error.txt
Error Message
-------------
error: Missing named argument "root" for "model_construct" of "MyRootModel" [call-arg]\nerror: Argument 1 to "model_construct" of "MyRootModel" has incompatible type "str"; expected "Optional[Set[str]]" [arg-type]
Stack trace
error.txt
Error Message
-------------
error: Missing named argument "root" for "model_construct" of "MyRootModel" [call-arg]
Stack trace
error.txt
Error Message
-------------
error: Argument 1 to "model_construct" of "MyRootModel" has incompatible type "str"; expected "set[str] | None" [arg-type]
Minimal Reproduction
repro.py
import pydantic
class MyRootModel(pydantic.RootModel[str]):
pass
if __name__ == '__main__':
reveal_type(MyRootModel)
reveal_type(MyRootModel.model_construct('yo'))
Environment
- Pydantic: 2
What Broke
Type checking fails in mypy, leading to errors during development and testing.
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
pip install pydantic==2.7.2
When NOT to use: This fix should not be used if the behavior of `RootModel` is expected to change significantly.
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/9480
First fixed release: 2.7.2
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix should not be used if the behavior of `RootModel` is expected to change significantly.
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 |
|---|---|
| 1.2.0 | Broken |
| 2.7.2 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.