Pydantic `KeyError` happening during schema cleaning with

The Fix

pip install pydantic==1.10.20

Based on closed pydantic/pydantic issue #10279 · PR/commit linked

Jump to Verify Open PR/Commit

@@ -817,8 +817,12 @@ def __class_getitem__(
                 submodel = _generics.create_generic_submodel(model_name, origin, args, params)
 
-                # Update cache
-                _generics.set_cached_generic_type(cls, typevar_values, submodel, origin, args)
+                # Cache the generated model *only* if not in the process of parametrizing

repro.py

from typing import Generic, TypeVar

from pydantic import BaseModel

T = TypeVar('T')

class Model1(BaseModel, Generic[T]):
    model2: 'Model2[T]'

S = TypeVar('S')

class Model2(BaseModel, Generic[S]):
    model1: Model1[S]

Model1[str].model_rebuild()
Model2[str].model_rebuild()

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==1.10.20\nWhen NOT to use: This fix should not be applied if the model relies on caching for performance in a non-parametrizing context.\n\n

Why This Fix Works in Production

Trigger: `KeyError` happening during schema cleaning with generic models
Mechanism: KeyError occurs during schema cleaning due to caching parametrized models while parametrizing another model
Why the fix works: Fixes a KeyError occurring during schema cleaning with generic models by avoiding caching parametrized models while parametrizing another model. (first fixed release: 1.10.20).

Why This Breaks in Prod

KeyError occurs during schema cleaning due to caching parametrized models while parametrizing another model
Surfaces as: `KeyError` happening during schema cleaning with generic models

Proof / Evidence

GitHub issue: #10279
Fix PR: https://github.com/pydantic/pydantic/pull/10704
First fixed release: 1.10.20
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.64

Discussion

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

Jump to Sources Open on GitHub

“@Viicos, Great analysis above. Almost felt like I was live debugging :). Re the potential fix - seems reasonable enough, though I'm not as familiar…”

@sydney-runkle · 2024-09-11 · source

“> Re the potential fix - seems reasonable enough, though I'm not as familiar with the context here as you are”

@Viicos · 2024-09-30 · source

“Here is what happens in the first example: Before calling Model<1|2>[str].model_rebuild(), a huge amount of recursive calls are being made to generate the core schema…”

@Viicos · 2024-09-11 · source

Failure Signature (Search String)

`KeyError` happening during schema cleaning with generic models

Error Message

Stack trace

error.txt

Error Message
-------------
`KeyError` happening during schema cleaning with generic models

Minimal Reproduction

repro.py

from typing import Generic, TypeVar

from pydantic import BaseModel

T = TypeVar('T')

class Model1(BaseModel, Generic[T]):
    model2: 'Model2[T]'

S = TypeVar('S')

class Model2(BaseModel, Generic[S]):
    model1: Model1[S]

Model1[str].model_rebuild()
Model2[str].model_rebuild()

Environment

Pydantic: 2

What Broke

The application encounters a KeyError, leading to failures in schema validation and model instantiation.

Why It Broke

KeyError occurs during schema cleaning due to caching parametrized models while parametrizing another model

Fix Options (Details)

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

pip install pydantic==1.10.20

When NOT to use: This fix should not be applied if the model relies on caching for performance in a non-parametrizing context.

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/10704

First fixed release: 1.10.20

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

When NOT to Use This Fix

This fix should not be applied if the model relies on caching for performance in a non-parametrizing context.

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
1.10.20	Fixed

Related Issues

No related fixes found.

Cluster: pydantic:openapi-schema Pydantic hub Pydantic best practices All hubs All clusters

Related clusters: Configuration error Data consistency Serialization

Sources

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

Pydantic `KeyError` happening during schema cleaning with generic models (Fix)