Pydantic Reconcile behavioral differences between

The Fix

pip install pydantic==1.10.15

Based on closed pydantic/pydantic issue #9049 · 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

@@ -431,9 +431,28 @@ print(m.model_dump())  # note: the password field is not included
     when adding sensitive information like secrets as fields of subclasses.
 
-### Serializing with duck-typing
+### Serializing with duck-typing 🦆

repro.py

from pydantic import BaseModel, ConfigDict, TypeAdapter, SerializeAsAny

class Parent(BaseModel):
    x: int

class Other(BaseModel):
    y: str

    model_config = ConfigDict(extra='allow')

ta = TypeAdapter(Parent)
other = Other(x=1, y='hello')
print(ta.dump_python(other))
#> {}

print(ta.dump_python(other, serialize_as_any=False))
#> {}

print(ta.dump_python(other, serialize_as_any=True))
#> {'y': 'hello', 'x': 1}

ta = TypeAdapter(SerializeAsAny[Parent])
other = Other(x=1, y='hello')
print(ta.dump_python(other))
#> {'y': 'hello', 'x': 1}
# note: if extra='ignore', the default was set, we'd get {'y': 'hello'}

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==1.10.15\nWhen NOT to use: This fix is not suitable if strict type enforcement is required during serialization.\n\nOption C — Workaround\nthat we considered was having the runtime flag named `duck_type_serialization`, but we ended up deciding not to go with this approach, as it's confusing for users to have "duck type serialization" and "serialize as any" as separate concepts. See [this comment](https://github.com/pydantic/pydantic-core/pull/1194#issuecomment-1997944022) and surrounding comments for more detail.\nWhen NOT to use: This fix is not suitable if strict type enforcement is required during serialization.\n\n

Why This Fix Works in Production

Trigger: The following test fails with warnings for passing str where int expected:
Mechanism: Behavioral differences exist between `SerializeAsAny` annotation and `serialize_as_any` runtime flag
Why the fix works: Supports the `serialize_as_any` runtime setting, addressing behavioral differences in serialization warnings. (first fixed release: 1.10.15).

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

Behavioral differences exist between `SerializeAsAny` annotation and `serialize_as_any` runtime flag
Production symptom (often without a traceback): The following test fails with warnings for passing str where int expected:

Proof / Evidence

GitHub issue: #9049
Fix PR: https://github.com/pydantic/pydantic/pull/8830
First fixed release: 1.10.15
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.75
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.60

Discussion

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

Jump to Sources Open on GitHub

“Ah, here's another thing to note, though I'm not sure if this merits a change:”

@sydney-runkle · 2024-03-19 · source

“Here's a case with unrelated models where the annotation usage vs runtime flag usage doesn't result in a behavioral discrepancy. This was one we were…”

@sydney-runkle · 2024-03-19 · source

“https://github.com/pydantic/pydantic-core/pull/1478 will make the runtime flag about as permissive as it can possibly be.”

@davidhewitt · 2025-05-21 · source

Failure Signature (Search String)

The following test fails with warnings for passing str where int expected:
serializer = SchemaSerializer(core_schema.list_schema(core_schema.int_schema()))

Copy-friendly signature

signature.txt

Failure Signature
-----------------
The following test fails with warnings for passing str where int expected:
serializer = SchemaSerializer(core_schema.list_schema(core_schema.int_schema()))

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
The following test fails with warnings for passing str where int expected:
serializer = SchemaSerializer(core_schema.list_schema(core_schema.int_schema()))

Minimal Reproduction

repro.py

from pydantic import BaseModel, ConfigDict, TypeAdapter, SerializeAsAny

class Parent(BaseModel):
    x: int

class Other(BaseModel):
    y: str

    model_config = ConfigDict(extra='allow')

ta = TypeAdapter(Parent)
other = Other(x=1, y='hello')
print(ta.dump_python(other))
#> {}

print(ta.dump_python(other, serialize_as_any=False))
#> {}

print(ta.dump_python(other, serialize_as_any=True))
#> {'y': 'hello', 'x': 1}

ta = TypeAdapter(SerializeAsAny[Parent])
other = Other(x=1, y='hello')
print(ta.dump_python(other))
#> {'y': 'hello', 'x': 1}
# note: if extra='ignore', the default was set, we'd get {'y': 'hello'}

What Broke

Serialization warnings are inconsistently raised, leading to unexpected behavior in production.

Why It Broke

Behavioral differences exist between `SerializeAsAny` annotation and `serialize_as_any` runtime flag

Fix Options (Details)

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

pip install pydantic==1.10.15

When NOT to use: This fix is not suitable if strict type enforcement is required during serialization.

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

Option C — Workaround Temporary workaround

that we considered was having the runtime flag named `duck_type_serialization`, but we ended up deciding not to go with this approach, as it's confusing for users to have "duck type serialization" and "serialize as any" as separate concepts. See [this comment](https://github.com/pydantic/pydantic-core/pull/1194#issuecomment-1997944022) and surrounding comments for more detail.

When NOT to use: This fix is not suitable if strict type enforcement is required during serialization.

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/8830

First fixed release: 1.10.15

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

When NOT to Use This Fix

This fix is not suitable if strict type enforcement is required during serialization.

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
1.10.15	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 Reconcile behavioral differences between `SerializeAsAny` annotation and (Fix)