Pydantic Referencing an external JSON Schema when

The Fix

pip install pydantic==1.10.18

Based on closed pydantic/pydantic issue #9783 · 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

@@ -2050,10 +2050,15 @@ def handle_ref_overrides(self, json_schema: JsonSchemaValue) -> JsonSchemaValue:
 
     def get_schema_from_definitions(self, json_ref: JsonRef) -> JsonSchemaValue | None:
-        def_ref = self.json_to_defs_refs[json_ref]
-        if def_ref in self._core_defs_invalid_for_json_schema:
-            raise self._core_defs_invalid_for_json_schema[def_ref]

repro.py

import typing
import pydantic


class Model(pydantic.BaseModel):
    config_jsonschema: typing.Annotated[
        dict,
        pydantic.WithJsonSchema(
            {"$ref": "https://json-schema.org/draft/2020-12/schema"}
        ),
    ]

Model.model_json_schema()

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: Do not use this fix if your application relies on internal definitions for JSON schemas.\n\n

Why This Fix Works in Production

Trigger: Referencing an external JSON Schema when generating a JSONSchema fails with a KeyError
Mechanism: A KeyError occurs when using WithJsonSchema with an external JSON Schema reference due to missing definitions
Why the fix works: Fixes a KeyError when using WithJsonSchema with an external JSON Schema reference. (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 3.11 in real deployments (not just unit tests).
A KeyError occurs when using WithJsonSchema with an external JSON Schema reference due to missing definitions
Surfaces as: Referencing an external JSON Schema when generating a JSONSchema fails with a KeyError

Proof / Evidence

GitHub issue: #9783
Fix PR: https://github.com/pydantic/pydantic/pull/9863
First fixed release: 1.10.18
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

“I don't know that this has to do with defs/refs simplifications. That's in the core schema, this is json schema. I'm guessing we can just…”

@adriangb · 2024-06-28 · source

“@reversefold, Thanks for reporting this. Looks like this is causing an issue in the defs / refs simplification process. @adriangb, Any ideas on how we…”

@sydney-runkle · 2024-06-28 · source

Failure Signature (Search String)

Referencing an external JSON Schema when generating a JSONSchema fails with a KeyError

Error Message

Stack trace

error.txt

Error Message
-------------
Referencing an external JSON Schema when generating a JSONSchema fails with a KeyError

Minimal Reproduction

repro.py

import typing
import pydantic


class Model(pydantic.BaseModel):
    config_jsonschema: typing.Annotated[
        dict,
        pydantic.WithJsonSchema(
            {"$ref": "https://json-schema.org/draft/2020-12/schema"}
        ),
    ]

Model.model_json_schema()

Environment

Python: 3.11
Pydantic: 2

What Broke

Users experience crashes when attempting to generate JSON schema for models with external references.

Why It Broke

A KeyError occurs when using WithJsonSchema with an external JSON Schema reference due to missing definitions

Fix Options (Details)

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

pip install pydantic==1.10.18

When NOT to use: Do not use this fix if your application relies on internal definitions for JSON schemas.

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/9863

First fixed release: 1.10.18

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

When NOT to Use This Fix

Do not use this fix if your application relies on internal definitions for JSON schemas.

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.

Pydantic Referencing an external JSON Schema when generating a JSONSchema fails with a KeyError (Fix)