The Fix
pip install pydantic==2.11.0
Based on closed pydantic/pydantic issue #11622 · 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%.
@@ -106,6 +106,12 @@ class PydanticJsonSchemaWarning(UserWarning):
+NoDefault = object()
+"""A sentinel value used to indicate that no default value should be used when generating a JSON Schema
+for a core schema with a default value.
repro.py
from pydantic import BaseModel
from datetime import datetime
class Parameters(BaseModel):
today: str = Field(default_factory=lambda: datetime.now().strftime("%Y-%m-%d"))
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==2.11.0\nWhen NOT to use: Do not use this fix if you require strict adherence to the original schema behavior without defaults.\n\n
Why This Fix Works in Production
- Trigger: Support "default" in generated schema when default_factory is used
- Mechanism: The JSON schema generation did not include default values for fields using default_factory
- Why the fix works: Introduced a new method to simplify the customization of default value inclusion in JSON Schema generation, addressing the issue of missing default values when using default_factory. (first fixed release: 2.11.0).
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 3.12.9 in real deployments (not just unit tests).
- The JSON schema generation did not include default values for fields using default_factory
- Surfaces as: ImportError: cannot import name 'NoDefault' from 'pydantic.json_schema' (/Users/carlos.pegueros/miniforge3/envs/pydantictest/lib/python3.12/site-packages/pydantic/json_schema.py)
Proof / Evidence
- GitHub issue: #11622
- Fix PR: https://github.com/pydantic/pydantic/pull/11634
- Fix commit: https://github.com/pydantic/pydantic/commit/fcba83291a8fe7e1dcfde9bbcc8ea57f8ef322c0
- First fixed release: 2.11.0
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.95
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.75
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“Thanks for the info! I wasn't aware of that option, although I couldn't run your code”
“This can be done by defining a custom GenerateJsonSchema subclass, as described in the documentation”
“> Thanks for the info! I wasn't aware of that option, although I couldn't run your code”
Failure Signature (Search String)
- Support "default" in generated schema when default_factory is used
Error Message
Stack trace
error.txt
Error Message
-------------
ImportError: cannot import name 'NoDefault' from 'pydantic.json_schema' (/Users/carlos.pegueros/miniforge3/envs/pydantictest/lib/python3.12/site-packages/pydantic/json_schema.py)
Minimal Reproduction
repro.py
from pydantic import BaseModel
from datetime import datetime
class Parameters(BaseModel):
today: str = Field(default_factory=lambda: datetime.now().strftime("%Y-%m-%d"))
Environment
- Python: 3.12.9
What Broke
API clients received incorrect schema indicating mandatory fields without defaults, causing confusion.
Why It Broke
The JSON schema generation did not include default values for fields using default_factory
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
pip install pydantic==2.11.0
When NOT to use: Do not use this fix if you require strict adherence to the original schema behavior without defaults.
Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.
Option D — Guard side-effects with OnceOnly Guardrail for side-effects
Mitigate duplicate external side-effects under retries/timeouts/agent loops by gating the operation before calling external systems.
- Place OnceOnly between your code/agent and real side-effects (Stripe, emails, CRM, APIs).
- Use a stable key per side-effect (e.g., customer_id + action + idempotency_key).
- Fail-safe: configure fail-open vs fail-closed based on blast radius and spend risk.
Show example snippet (optional)
onceonly.py
from onceonly import OnceOnly
import os
once = OnceOnly(api_key=os.environ["ONCEONLY_API_KEY"], fail_open=True)
# Stable idempotency key per real side-effect.
# Use a request id / job id / webhook delivery id / Stripe event id, etc.
event_id = "evt_..." # replace
key = f"stripe:webhook:{event_id}"
res = once.check_lock(key=key, ttl=3600)
if res.duplicate:
return {"status": "already_processed"}
# Safe to execute the side-effect exactly once.
handle_event(event_id)
When NOT to use: Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.
Fix reference: https://github.com/pydantic/pydantic/pull/11634
First fixed release: 2.11.0
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- Do not use this fix if you require strict adherence to the original schema behavior without defaults.
- Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.
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.
- Add a TLS smoke test that performs a real handshake in CI (include CA bundle validation and hostname checks).
- Alert on handshake failures by error string and endpoint to catch cert/CA changes quickly.
Version Compatibility Table
| Version | Status |
|---|---|
| 2.11.0 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.