Pydantic `KeyError: '__main__.Model1:95063711754448'` (Fix)

The Fix

pip install pydantic==1.10.18

Based on closed pydantic/pydantic issue #9319 · 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

@@ -212,6 +212,7 @@ def _handle_other_schemas(self, schema: core_schema.CoreSchema, f: Walk) -> core
         schema: core_schema.CoreSchema | None = ser_schema.get('schema', None)
         if schema is not None:
+            ser_schema = ser_schema.copy()
             ser_schema['schema'] = self.walk(schema, f)  # type: ignore
         return_schema: core_schema.CoreSchema | None = ser_schema.get('return_schema', None)

repro.py

from collections.abc import Sequence
from pydantic import PlainValidator, BaseModel
from typing import Annotated


class Model1(BaseModel, frozen=True):
    ...


def parse_model1(x: object) -> Model1:
    return Model1()


class Model2(BaseModel, frozen=True):
    model1:  Annotated[Model1, PlainValidator(parse_model1)]


class Model3(BaseModel, frozen=True):
    model2: Sequence[Model2]


class Model4(BaseModel, frozen=True):
    model2: Model2

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 serialization schema behavior is expected to change.\n\nOption C — Workaround\nfor specific code base?\nWhen NOT to use: This fix should not be applied if the serialization schema behavior is expected to change.\n\n

Why This Fix Works in Production

Trigger: class Model4(BaseModel, frozen=True):
Mechanism: The core schema simplification fails when serialization schemas are involved, leading to a KeyError
Why the fix works: Fix core schema simplification when serialization schemas are involved in specific scenarios, addressing the KeyError issue. (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 3.12 in real deployments (not just unit tests).
The core schema simplification fails when serialization schemas are involved, leading to a KeyError
Surfaces as: Traceback (most recent call last):

Proof / Evidence

GitHub issue: #9319
Fix PR: https://github.com/pydantic/pydantic/pull/10155
First fixed release: 1.10.18
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.34

Discussion

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

Jump to Sources Open on GitHub

“Hmph, definitely a bug. Thanks for the report. Will attempt to fix for 2.7.2”

@sydney-runkle · 2024-04-25 · source

“@jherbel We have a similar problem but we are not using Sequence, I'm wondering if you have any hints on what is the issue here…”

@cielecki · 2024-05-13 · source

“Hi @cielecki, by chance, I discovered yesterday that list instead of Sequence works in the scenario above. Maybe this helps, though you wrote that you…”

@jherbel · 2024-05-14 · source

“@jherbel thanks I decided to go with just going with "Any" in few places in order to make it work in the short term ...”

@cielecki · 2024-05-18 · source

Failure Signature (Search String)

class Model4(BaseModel, frozen=True):

Error Message

Stack trace

error.txt

Error Message
-------------
Traceback (most recent call last):
  File "/home/joerg/tmp/pydantic_key_error.py", line 22, in <module>
    class Model4(BaseModel, frozen=True):
  File "/home/joerg/.pyenv/versions/3.12.1/lib/python3.12/site-packages/pydantic/_internal/_model_construction.py", line 202, in __new__
    complete_model_class(
  File "/home/joerg/.pyenv/versions/3.12.1/lib/python3.12/site-packages/pydantic/_internal/_model_construction.py", line 549, in complete_model_class
    schema = gen_schema.clean_schema(schema)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/joerg/.pyenv/versions/3.12.1/lib/python3.12/site-packages/pydantic/_internal/_generate_schema.py", line 435, in clean_schema
    schema = simplify_schema_references(schema)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/joerg/.pyenv/versions/3.12.1/lib/python3.12/site-packages/pydantic/_internal/_core_utils.py", line 463, in simplify_schema_references
    schema = walk_core_schema(schema, count_refs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/joerg/.pyenv/versions/3.12.1/lib/python3.12/site-packages/pydantic/_internal/_core_utils.py", line 414, in walk_core_schema
    return f(schema.copy(), _dispatch)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/joerg/.pyenv/versions/3.12.1/lib/python3.12/site-packages/pydantic/_internal/_core_utils.py", line 459, in count_refs
    recurse(definiti
... (truncated) ...

Minimal Reproduction

repro.py

from collections.abc import Sequence
from pydantic import PlainValidator, BaseModel
from typing import Annotated


class Model1(BaseModel, frozen=True):
    ...


def parse_model1(x: object) -> Model1:
    return Model1()


class Model2(BaseModel, frozen=True):
    model1:  Annotated[Model1, PlainValidator(parse_model1)]


class Model3(BaseModel, frozen=True):
    model2: Sequence[Model2]


class Model4(BaseModel, frozen=True):
    model2: Model2

Environment

Python: 3.12
Pydantic: 2

What Broke

Users experience crashes with KeyError when defining models with serialization schemas.

Why It Broke

The core schema simplification fails when serialization schemas are involved, leading to a KeyError

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 serialization schema behavior is expected to change.

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

Option C — Workaround Temporary workaround

for specific code base?

When NOT to use: This fix should not be applied if the serialization schema behavior is expected to change.

Use only if you cannot change versions today. Treat this as a stopgap and remove once upgraded.

Fix reference: https://github.com/pydantic/pydantic/pull/10155

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 serialization schema behavior is expected to change.

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.