Pydantic Add ability to exclude a type from serialization.

The Fix

pip install pydantic==2.11.8

Based on closed pydantic/pydantic issue #11843 · PR/commit linked

Jump to Verify Open PR/Commit

@@ -501,3 +501,49 @@ args, kwargs = val.validate_json('{"args": ["arg1"], "kwargs": {"extra": 1}}')
 #> ('arg1',) {'extra': 1}
 ```
+
+## `MISSING` sentinel
+

repro.py

from typing import Any

from pydantic import GetCoreSchemaHandler, GetJsonSchemaHandler
from pydantic.json_schema import SkipJsonSchema
from pydantic_core import core_schema


class _MissingSentinel:
    """Use this to have different behaviour between
        Source: https://github.com/pydantic/pydantic/discussions/9943#discussioncomment-10131374
    """
    _instance = None

    def __new__(cls):
        if not cls._instance:
            cls._instance = super().__new__(cls)
        return cls._instance

    def __repr__(self):
        return "<MISSING>"

    @classmethod
    def __get_pydantic_core_schema__(cls, source_type: Any, _: GetCoreSchemaHandler) -> core_schema.CoreSchema:
        return core_schema.is_instance_schema(
            cls=source_type,
            # Feature request: 
            # serialization=core_schema.exclude_ser_schema(),
            serialization=core_schema.to_string_ser_schema(),
        )

    @classmethod
    def __get_pydantic_json_schema__(cls, core_schema: core_schema.CoreSchema, handler: GetJsonSchemaHandler):
        return {}

# Feature Request: 
# MissingSentinel = SkipJsonSchema[ExcludeSerialization[_MissingSentinel]]
MissingSentinel = SkipJsonSchema[_MissingSentinel]
MISSING = _MissingSentinel()


class MyModel(BaseModel):
    nullable_field: str | None | MissingSentinel = MISSING

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==2.11.8\nWhen NOT to use: This fix should not be used if the model requires explicit tracking of unset fields.\n\n

Why This Fix Works in Production

Trigger: I have a 'missing' sentinel value. When the field is set to this sentinel I want it to be excluded from the model serialization.
Mechanism: The absence of a mechanism to exclude sentinel values from serialization in Pydantic models
Why the fix works: Introduces an experimental `MISSING` sentinel to exclude fields from serialization in Pydantic models. (first fixed release: 2.11.8).

Why This Breaks in Prod

The absence of a mechanism to exclude sentinel values from serialization in Pydantic models
Production symptom (often without a traceback): I have a 'missing' sentinel value. When the field is set to this sentinel I want it to be excluded from the model serialization.

Proof / Evidence

GitHub issue: #11843
Fix PR: https://github.com/pydantic/pydantic/pull/11883
First fixed release: 2.11.8
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.40

Discussion

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

Jump to Sources Open on GitHub

“A new MISSING sentinel was introduced in https://github.com/pydantic/pydantic/pull/11883 as an experimental feature, and available to be tested in the 2.12.0a1 release”

@Viicos · 2025-07-26 · source

Failure Signature (Search String)

I have a 'missing' sentinel value. When the field is set to this sentinel I want it to be excluded from the model serialization.
I'd propose adding a `core_schema.exclude_ser_schema()` and an `ExcludeSerialization` annotation.

Copy-friendly signature

signature.txt

Failure Signature
-----------------
I have a 'missing' sentinel value. When the field is set to this sentinel I want it to be excluded from the model serialization.
I'd propose adding a `core_schema.exclude_ser_schema()` and an `ExcludeSerialization` annotation.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
I have a 'missing' sentinel value. When the field is set to this sentinel I want it to be excluded from the model serialization.
I'd propose adding a `core_schema.exclude_ser_schema()` and an `ExcludeSerialization` annotation.

Minimal Reproduction

repro.py

from typing import Any

from pydantic import GetCoreSchemaHandler, GetJsonSchemaHandler
from pydantic.json_schema import SkipJsonSchema
from pydantic_core import core_schema


class _MissingSentinel:
    """Use this to have different behaviour between
        Source: https://github.com/pydantic/pydantic/discussions/9943#discussioncomment-10131374
    """
    _instance = None

    def __new__(cls):
        if not cls._instance:
            cls._instance = super().__new__(cls)
        return cls._instance

    def __repr__(self):
        return "<MISSING>"

    @classmethod
    def __get_pydantic_core_schema__(cls, source_type: Any, _: GetCoreSchemaHandler) -> core_schema.CoreSchema:
        return core_schema.is_instance_schema(
            cls=source_type,
            # Feature request: 
            # serialization=core_schema.exclude_ser_schema(),
            serialization=core_schema.to_string_ser_schema(),
        )

    @classmethod
    def __get_pydantic_json_schema__(cls, core_schema: core_schema.CoreSchema, handler: GetJsonSchemaHandler):
        return {}

# Feature Request: 
# MissingSentinel = SkipJsonSchema[ExcludeSerialization[_MissingSentinel]]
MissingSentinel = SkipJsonSchema[_MissingSentinel]
MISSING = _MissingSentinel()


class MyModel(BaseModel):
    nullable_field: str | None | MissingSentinel = MISSING

What Broke

Models with sentinel values included unwanted fields during serialization, leading to incorrect outputs.

Why It Broke

The absence of a mechanism to exclude sentinel values from serialization in Pydantic models

Fix Options (Details)

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

pip install pydantic==2.11.8

When NOT to use: This fix should not be used if the model requires explicit tracking of unset fields.

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/11883

First fixed release: 2.11.8

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

When NOT to Use This Fix

This fix should not be used if the model requires explicit tracking of unset fields.

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
2.11.8	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 Add ability to exclude a type from serialization. (Fix)