The Fix
pip install pydantic==2.12.5
Based on closed pydantic/pydantic issue #12521 · 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%.
@@ -20,6 +20,7 @@
from typing import TYPE_CHECKING, Any, Generic, TypeVar, overload
+from pydantic_core import MISSING
from typing_extensions import TypeAlias, TypeGuard, deprecated
# pydantic/_internal/_utils.py
from pydantic_core import MISSING
def smart_deepcopy(obj):
if obj is MISSING:
return obj
# rest of current logic...
Re-run the minimal reproduction on your broken version, then apply the fix and re-run.
Option A — Upgrade to fixed release\npip install pydantic==2.12.5\nWhen NOT to use: This fix should not be applied if the behavior of `__deepcopy__()` is expected to change.\n\n
Why This Fix Works in Production
- Trigger: TypeError: Cannot pickle 'Sentinel' object.
- Mechanism: Deep-copying the `MISSING` sentinel raises a TypeError due to its non-pickleable nature
- Why the fix works: Special-cases the `MISSING` sentinel in `smart_deepcopy()` to avoid TypeError during deep-copying. (first fixed release: 2.12.5).
- 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
- Deep-copying the `MISSING` sentinel raises a TypeError due to its non-pickleable nature
- Surfaces as: TypeError: Cannot pickle 'Sentinel' object.
Proof / Evidence
- GitHub issue: #12521
- Fix PR: https://github.com/pydantic/pydantic/pull/12522
- First fixed release: 2.12.5
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.85
- 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).
“A PR is welcomed but only to tweak smart_deepcopy(), I'd rather not change the current __deepcopy__() behavior”
“This is expected, as per the documentation, PEP 661 sentinels aren't pickleable. We can update smart_deepcopy() to account for this in the meanwhile, but this…”
“What do you think of my proposed tweak to smart_deepcopy()? And using smart_deepcopy() in .__deepcopy__()? @Viicos I'm happy to draft a PR :).”
Failure Signature (Search String)
- TypeError: Cannot pickle 'Sentinel' object.
Error Message
Stack trace
Error Message
-------------
TypeError: Cannot pickle 'Sentinel' object.
Minimal Reproduction
# pydantic/_internal/_utils.py
from pydantic_core import MISSING
def smart_deepcopy(obj):
if obj is MISSING:
return obj
# rest of current logic...
Environment
- Pydantic: 2
What Broke
Attempts to deepcopy a model with `MISSING` results in a crash, causing application failures.
Why It Broke
Deep-copying the `MISSING` sentinel raises a TypeError due to its non-pickleable nature
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
pip install pydantic==2.12.5
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/12522
First fixed release: 2.12.5
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix should not be applied if the behavior of `__deepcopy__()` is expected to change.
Verify Fix
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 |
|---|---|
| 2.12.5 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.