Pydantic Mypy crashes with 1.11.1 version and Pydantic

The Fix

pip install pydantic==1.10.16

Based on closed pydantic/pydantic issue #10129 · PR/commit linked

Production note: This usually shows up under retries/timeouts. Treat it as a side-effect risk until you can verify behavior with a canary + real traffic.

Jump to Verify Open PR/Commit

@@ -57,6 +57,7 @@
     TypeOfAny,
     TypeType,
+    TypeVarId,
     TypeVarType,
     UnionType,

repro.py

❯ mypy xxx/ --show-traceback
/home/omerfi/xxx/.venv/lib/python3.10/site-packages/pydantic/env_settings.py:23: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 1.11.1
Traceback (most recent call last):
  File "mypy/semanal.py", line 7092, in accept
  File "mypy/nodes.py", line 1183, in accept
  File "mypy/semanal.py", line 1700, in visit_class_def
  File "mypy/semanal.py", line 1891, in analyze_class
  File "mypy/semanal.py", line 1925, in analyze_class_body_common
  File "mypy/semanal.py", line 2010, in apply_class_plugin_hooks
  File "pydantic/mypy.py", line 154, in pydantic.mypy.PydanticPlugin._pydantic_model_class_maker_callback
  File "pydantic/mypy.py", line 317, in pydantic.mypy.PydanticModelTransformer.transform
  File "pydantic/mypy.py", line 497, in pydantic.mypy.PydanticModelTransformer.add_construct_method
  File "mypy/types.py", line 607, in __init__
TypeError: mypy.types.TypeVarId object expected; got int
/home/omerfi/xxx/.venv/lib/python3.10/site-packages/pydantic/env_settings.py:23: : note: use --pdb to drop into pdb

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==1.10.16\nWhen NOT to use: Do not use if it changes public behavior or if the failure cannot be reproduced.\n\n

Why This Fix Works in Production

Trigger: ❯ mypy xxx/ --show-traceback
Mechanism: The mypy plugin for Pydantic fails due to a change in the TypeVarType signature
Why the fix works: Fixes the mypy v1 plugin for an upcoming mypy release, addressing an internal error caused by a change in the TypeVarType signature. (first fixed release: 1.10.16).

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 3.10 in real deployments (not just unit tests).
The mypy plugin for Pydantic fails due to a change in the TypeVarType signature
Surfaces as: ❯ mypy xxx/ --show-traceback

Proof / Evidence

GitHub issue: #10129
Fix PR: https://github.com/pydantic/pydantic/pull/9586
First fixed release: 1.10.16
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.29

Discussion

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

Jump to Sources Open on GitHub

“This was fixed in Pydantic 1.10.16 (see https://github.com/pydantic/pydantic/pull/9586).”

@Viicos · 2024-08-14 · confirmation · source

Failure Signature (Search String)

❯ mypy xxx/ --show-traceback

Error Message

Stack trace

error.txt

Error Message
-------------
❯ mypy xxx/ --show-traceback
/home/omerfi/xxx/.venv/lib/python3.10/site-packages/pydantic/env_settings.py:23: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 1.11.1
Traceback (most recent call last):
  File "mypy/semanal.py", line 7092, in accept
  File "mypy/nodes.py", line 1183, in accept
  File "mypy/semanal.py", line 1700, in visit_class_def
  File "mypy/semanal.py", line 1891, in analyze_class
  File "mypy/semanal.py", line 1925, in analyze_class_body_common
  File "mypy/semanal.py", line 2010, in apply_class_plugin_hooks
  File "pydantic/mypy.py", line 154, in pydantic.mypy.PydanticPlugin._pydantic_model_class_maker_callback
  File "pydantic/mypy.py", line 317, in pydantic.mypy.PydanticModelTransformer.transform
  File "pydantic/mypy.py", line 497, in pydantic.mypy.PydanticModelTransformer.add_construct_method
  File "mypy/types.py", line 607, in __init__
TypeError: mypy.types.TypeVarId object expected; got int
/home/omerfi/xxx/.venv/lib/python3.10/site-packages/pydantic/env_settings.py:23: : note: use --pdb to drop into pdb

Minimal Reproduction

repro.py

❯ mypy xxx/ --show-traceback
/home/omerfi/xxx/.venv/lib/python3.10/site-packages/pydantic/env_settings.py:23: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 1.11.1
Traceback (most recent call last):
  File "mypy/semanal.py", line 7092, in accept
  File "mypy/nodes.py", line 1183, in accept
  File "mypy/semanal.py", line 1700, in visit_class_def
  File "mypy/semanal.py", line 1891, in analyze_class
  File "mypy/semanal.py", line 1925, in analyze_class_body_common
  File "mypy/semanal.py", line 2010, in apply_class_plugin_hooks
  File "pydantic/mypy.py", line 154, in pydantic.mypy.PydanticPlugin._pydantic_model_class_maker_callback
  File "pydantic/mypy.py", line 317, in pydantic.mypy.PydanticModelTransformer.transform
  File "pydantic/mypy.py", line 497, in pydantic.mypy.PydanticModelTransformer.add_construct_method
  File "mypy/types.py", line 607, in __init__
TypeError: mypy.types.TypeVarId object expected; got int
/home/omerfi/xxx/.venv/lib/python3.10/site-packages/pydantic/env_settings.py:23: : note: use --pdb to drop into pdb

Environment

Python: 3.10
Pydantic: 1.10.11

Why It Broke

The mypy plugin for Pydantic fails due to a change in the TypeVarType signature

Fix Options (Details)

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

pip install pydantic==1.10.16

When NOT to use: Do not use if it changes public behavior or if the failure cannot be reproduced.

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/9586

First fixed release: 1.10.16

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

When NOT to Use This Fix

Do not use if it changes public behavior or if the failure cannot be reproduced.

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
1.10.16	Fixed

Related Issues

No related fixes found.

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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 crashes with 1.11.1 version and Pydantic 1.10.11 (Fix)