The Fix
pip install pydantic==2.11.4
Based on closed pydantic/pydantic issue #11819 · 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%.
@@ -631,23 +631,6 @@ except PydanticUserError as exc_info:
The fields definition syntax can be found in the [dynamic model creation](../concepts/models.md#dynamic-model-creation) documentation.
-
-## `create_model` config base {#create-model-config-base}
-
repro.py
from pydantic import create_model
class Config: ...
Model = create_model("Model", __config__=Config)
print(Model.model_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==2.11.4\nWhen NOT to use: This fix should not be used if class-based configuration is still required.\n\n
Why This Fix Works in Production
- Trigger: » uv run --with 'pydantic==2.11.4' repros/pydantic/configdict.py
- Mechanism: The `create_model` function no longer accepts class-based `Config` due to a breaking change
- Why the fix works: Allows config and bases to be specified together in `create_model()`, addressing the breaking change reported in issue #11819. (first fixed release: 2.11.4).
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 in real deployments (not just unit tests).
- The `create_model` function no longer accepts class-based `Config` due to a breaking change
- Surfaces as: » uv run --with 'pydantic==2.11.4' repros/pydantic/configdict.py
Proof / Evidence
- GitHub issue: #11819
- Fix PR: https://github.com/pydantic/pydantic/pull/11714
- First fixed release: 2.11.4
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.95
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.46
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“Thanks for the report. It is technically a breaking change but for something that wasn't supposed to work, and wasn't documented (the type hint of…”
Failure Signature (Search String)
- » uv run --with 'pydantic==2.11.4' repros/pydantic/configdict.py
Error Message
Stack trace
error.txt
Error Message
-------------
» uv run --with 'pydantic==2.11.4' repros/pydantic/configdict.py
Traceback (most recent call last):
File "/Users/nate/github.com/prefecthq/prefect/repros/pydantic/configdict.py", line 7, in <module>
Model = create_model("Model", __config__=Config)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/nate/Library/Caches/uv/archive-v0/a1MACwVa0TRCWn5WltviZ/lib/python3.12/site-packages/pydantic/main.py", line 1761, in create_model
return meta(
^^^^^
File "/Users/nate/Library/Caches/uv/archive-v0/a1MACwVa0TRCWn5WltviZ/lib/python3.12/site-packages/pydantic/_internal/_model_construction.py", line 110, in __new__
config_wrapper = ConfigWrapper.for_model(bases, namespace, kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/nate/Library/Caches/uv/archive-v0/a1MACwVa0TRCWn5WltviZ/lib/python3.12/site-packages/pydantic/_internal/_config.py", line 138, in for_model
config_new.update(config_from_namespace)
TypeError: 'type' object is not iterable
» uv run --with 'pydantic==2.11.3' repros/pydantic/configdict.py
{'properties': {}, 'title': 'Model', 'type': 'object'}
Minimal Reproduction
repro.py
from pydantic import create_model
class Config: ...
Model = create_model("Model", __config__=Config)
print(Model.model_json_schema())
Environment
- Python: 3.12
- Pydantic: 2
What Broke
Users experienced errors when attempting to use class-based configuration with `create_model`.
Why It Broke
The `create_model` function no longer accepts class-based `Config` due to a breaking change
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
pip install pydantic==2.11.4
When NOT to use: This fix should not be used if class-based configuration is still required.
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/11714
First fixed release: 2.11.4
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix should not be used if class-based configuration is still required.
- 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.
Version Compatibility Table
| Version | Status |
|---|---|
| 2.11.4 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.