Pydantic Schema generation error when serialization schema

The Fix

pip install pydantic==1.10.19

Based on closed pydantic/pydantic issue #10443 · 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

@@ -458,6 +458,10 @@ def count_refs(s: core_schema.CoreSchema, recurse: Recurse) -> core_schema.CoreS
 
         current_recursion_ref_count[ref] += 1
+        if 'serialization' in s:
+            # Even though this is a `'definition-ref'` schema, there might
+            # be more references inside the serialization schema:

repro.py

from typing import Annotated

from pydantic import BaseModel, PlainSerializer

class Sub(BaseModel):
    pass

class Model(BaseModel):
    sub: Annotated[
        Sub,
        PlainSerializer(lambda v: v, return_type=Sub),
    ]
# pydantic_core._pydantic_core.SchemaError: Definitions error: definition `__main__.Sub:97954749220704` was never filled

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==1.10.19\nWhen NOT to use: Do not apply this fix if the serialization schema is expected to contain additional references.\n\n

Why This Fix Works in Production

Trigger: Schema generation error when serialization schema holds a reference
Mechanism: The schema generation error occurs due to unexpected serialization keys in the definition-ref schema
Why the fix works: Fix schema generation error when serialization schema holds references. (first fixed release: 1.10.19).

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

The schema generation error occurs due to unexpected serialization keys in the definition-ref schema
Production symptom (often without a traceback): Schema generation error when serialization schema holds a reference

Proof / Evidence

GitHub issue: #10443
Fix PR: https://github.com/pydantic/pydantic/pull/10444
First fixed release: 1.10.19
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.57

Discussion

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

Jump to Sources Open on GitHub

“### Initial Checks - [X] I confirm that I'm using Pydantic V2 ### Description The following code currently raises: The reason is that, before schema cleaning, the core schema looks like: Which is quite unusual: the 'definition-ref' schema t”

Issue thread · issue description · source

Failure Signature (Search String)

Schema generation error when serialization schema holds a reference
The reason is that, before schema cleaning, the core schema looks like:

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Schema generation error when serialization schema holds a reference
The reason is that, before schema cleaning, the core schema looks like:

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Schema generation error when serialization schema holds a reference
The reason is that, before schema cleaning, the core schema looks like:

Minimal Reproduction

repro.py

from typing import Annotated

from pydantic import BaseModel, PlainSerializer

class Sub(BaseModel):
    pass

class Model(BaseModel):
    sub: Annotated[
        Sub,
        PlainSerializer(lambda v: v, return_type=Sub),
    ]
# pydantic_core._pydantic_core.SchemaError: Definitions error: definition `__main__.Sub:97954749220704` was never filled

Environment

Pydantic: 2

What Broke

Users experience schema generation failures leading to application crashes during model serialization.

Why It Broke

The schema generation error occurs due to unexpected serialization keys in the definition-ref schema

Fix Options (Details)

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

pip install pydantic==1.10.19

When NOT to use: Do not apply this fix if the serialization schema is expected to contain additional references.

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/10444

First fixed release: 1.10.19

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

When NOT to Use This Fix

Do not apply this fix if the serialization schema is expected to contain additional references.

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
1.10.19	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 Schema generation error when serialization schema holds a reference (Fix)