The Fix
pip install pydantic==1.10.14
Based on closed pydantic/pydantic issue #9093 · 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%.
@@ -1018,5 +1018,32 @@ except PydanticUserError as exc_info:
```
+## Cannot evaluate type annotation {#unevaluable-type-annotation}
+
+Because type annotations are evaluated *after* assignments, you might get unexpected results when using a type annotation name
repro.py
from __future__ import annotations
from pydantic import BaseModel, Field
class Aaa(BaseModel):
b: str = None
class X(BaseModel):
Aaa: Aaa = Field(None) # This crashes
# Aaa: "Aaa" = Field(None) # Likewise
# Aaa: Aaa = None # This is fine
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==1.10.14\nWhen NOT to use: This fix is not applicable when field names do not clash with type annotations.\n\n
Why This Fix Works in Production
- Trigger: class X(BaseModel):
- Mechanism: Using a default Field crashes when the type annotation references a class of the same name as the field
- Why the fix works: Explicitly raises an error if field names clash with types, addressing a specific bug where using a default Field crashes when the type annotation references a class of the same name as the field. (first fixed release: 1.10.14).
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 3.10 in real deployments (not just unit tests).
- Using a default Field crashes when the type annotation references a class of the same name as the field
- Surfaces as: Traceback (most recent call last):
Proof / Evidence
- GitHub issue: #9093
- Fix PR: https://github.com/pydantic/pydantic/pull/8243
- First fixed release: 1.10.14
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.95
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.35
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“I'd say we should close it given we can't really do much here as said in the previous comments”
“@lmmx, Thanks for the detailed bug report. I know that @Viicos worked on a PR recently to disallow / warn for cases similar to yours…”
“> I use flake8 all the time (even though my pre-commit is in ruff now so cool to see the plans for that!) My plan…”
“@Viicos, do you think this still fits within the scope of v2.10?”
Failure Signature (Search String)
- class X(BaseModel):
Error Message
Stack trace
error.txt
Error Message
-------------
Traceback (most recent call last):
File "/home/louis/lab/tubeulator/pascalify/Journey.py", line 10, in <module>
class X(BaseModel):
File "/home/louis/miniconda3/envs/tubeulator/lib/python3.10/site-packages/pydantic/_internal/_model_construction.py", line 178, in __new__
set_model_fields(cls, bases, config_wrapper, types_namespace)
File "/home/louis/miniconda3/envs/tubeulator/lib/python3.10/site-packages/pydantic/_internal/_model_construction.py", line 452, in set_model_fields
fields, class_vars = collect_model_fields(cls, bases, config_wrapper, types_namespace, typevars_map=typevars_map)
File "/home/louis/miniconda3/envs/tubeulator/lib/python3.10/site-packages/pydantic/_internal/_fields.py", line 122, in collect_model_fields
type_hints = get_cls_type_hints_lenient(cls, types_namespace)
File "/home/louis/miniconda3/envs/tubeulator/lib/python3.10/site-packages/pydantic/_internal/_typing_extra.py", line 212, in get_cls_type_hints_lenient
hints[name] = eval_type_lenient(value, globalns, localns)
File "/home/louis/miniconda3/envs/tubeulator/lib/python3.10/site-packages/pydantic/_internal/_typing_extra.py", line 224, in eval_type_lenient
return eval_type_backport(value, globalns, localns)
File "/home/louis/miniconda3/envs/tubeulator/lib/python3.10/site-packages/pydantic/_internal/_typing_extra.py", line 240, in eval_type_backport
return typin
... (truncated) ...
Stack trace
error.txt
Error Message
-------------
>> from tubeulator.generated.Journey import Identifier
>> Identifier.model_validate({})
Identifier(Id=None, Name=None, Uri=None, FullName=None, Type=None, Crowding=None, RouteType=None, Status=None)
>> Identifier.model_validate({"crowding": {"passengerFlows": [], "trainLoadings": []}})
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/home/louis/miniconda3/envs/tubeulator/lib/python3.10/site-packages/pydantic/main.py", line 509, in model_validate
return cls.__pydantic_validator__.validate_python(
pydantic_core._pydantic_core.ValidationError: 1 validation error for Identifier
crowding
Input should be None [type=none_required, input_value={}, input_type=dict]
For further information visit https://errors.pydantic.dev/2.6/v/none_required
Minimal Reproduction
repro.py
from __future__ import annotations
from pydantic import BaseModel, Field
class Aaa(BaseModel):
b: str = None
class X(BaseModel):
Aaa: Aaa = Field(None) # This crashes
# Aaa: "Aaa" = Field(None) # Likewise
# Aaa: Aaa = None # This is fine
Environment
- Python: 3.10
- Pydantic: 2
What Broke
Application crashes during model initialization due to type evaluation errors.
Why It Broke
Using a default Field crashes when the type annotation references a class of the same name as the field
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
pip install pydantic==1.10.14
When NOT to use: This fix is not applicable when field names do not clash with type annotations.
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/8243
First fixed release: 1.10.14
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix is not applicable when field names do not clash with type annotations.
- 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 |
|---|---|
| 1.10.14 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.