Pydantic Pydantic to JSON Schema translation: Write a field

The Fix

pip install pydantic==2.11.8

Based on closed pydantic/pydantic issue #9057 · 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

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

repro.py

import json
from typing import NotRequired

from pydantic import TypeAdapter
from typing_extensions import TypedDict


class Cell(TypedDict):
    name: str
    type: str
    organism: NotRequired[str]


ta = TypeAdapter(Cell)

print(json.dumps(ta.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==2.11.8\nWhen NOT to use: Do not use this fix if you need strict validation without optional fields.\n\n

Why This Fix Works in Production

Trigger: Pydantic to JSON Schema translation: Write a field with no default value that is not required in the JSON schema
Mechanism: The Pydantic model requires default values for fields to be excluded from JSON schema requirements
Why the fix works: Introduced an experimental `MISSING` sentinel to allow fields without default values to be excluded from JSON schema requirements. (first fixed release: 2.11.8).

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 Pydantic model requires default values for fields to be excluded from JSON schema requirements
Production symptom (often without a traceback): Pydantic to JSON Schema translation: Write a field with no default value that is not required in the JSON schema

Proof / Evidence

GitHub issue: #9057
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: 0.68

Discussion

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

Jump to Sources Open on GitHub

“#8394 is similar, but I'm referring here specifically to the JSON schema generator”

@mettaMeddl · 2024-03-19 · source

“Maybe TypedDict is what you need. **JSON Schema**:”

@dominik-schwabe · 2024-05-06 · source

“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)

Pydantic to JSON Schema translation: Write a field with no default value that is not required in the JSON schema
Consider the following schema:

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Pydantic to JSON Schema translation: Write a field with no default value that is not required in the JSON schema
Consider the following schema:

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Pydantic to JSON Schema translation: Write a field with no default value that is not required in the JSON schema
Consider the following schema:

Minimal Reproduction

repro.py

import json
from typing import NotRequired

from pydantic import TypeAdapter
from typing_extensions import TypedDict


class Cell(TypedDict):
    name: str
    type: str
    organism: NotRequired[str]


ta = TypeAdapter(Cell)

print(json.dumps(ta.json_schema()))

Environment

Pydantic: 2.6.4

What Broke

Users cannot create optional fields in JSON schema without default values, leading to incorrect form behavior.

Why It Broke

The Pydantic model requires default values for fields to be excluded from JSON schema requirements

Fix Options (Details)

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

pip install pydantic==2.11.8

When NOT to use: Do not use this fix if you need strict validation without optional 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

Do not use this fix if you need strict validation without optional 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: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 Pydantic to JSON Schema translation: Write a field with no default value that is not (Fix)