Pydantic Cannot use model_copy(deep=True) on a model

The Fix

pip install pydantic==2.7.1

Based on closed pydantic/pydantic issue #9122 · 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

@@ -721,7 +721,7 @@ def __copy__(self: Model) -> Model:
         _object_setattr(m, '__pydantic_fields_set__', copy(self.__pydantic_fields_set__))
 
-        if self.__pydantic_private__ is None:
+        if not hasattr(self, '__pydantic_private__') or self.__pydantic_private__ is None:
             _object_setattr(m, '__pydantic_private__', None)

repro.py

pydantic-core version: 2.16.3
      pydantic-core build: profile=release pgo=true
             install path: /home/dave/axial/src/services/axial-api.git/venv/lib/python3.11/site-packages/pydantic
           python version: 3.11.8 (main, Feb 25 2024, 16:41:26) [GCC 9.4.0]
                 platform: Linux-5.15.0-1056-aws-x86_64-with-glibc2.31
         related packages: fastapi-0.110.0 typing_extensions-4.10.0 mypy-1.9.0
                   commit: unknown

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==2.7.1\nWhen NOT to use: Do not use this fix if the model does not define model_post_init.\n\n

Why This Fix Works in Production

Trigger: Cannot use model_copy(deep=True) on a model instance which was created using model_construct, when there is a model_post_init in the class hierarchy.
Mechanism: The model_copy method fails due to an unset __pydantic_private__ attribute when using model_construct
Why the fix works: Addresses an AttributeError that occurs when using model_copy on a model constructed with model_construct and having a model_post_init method. (first fixed release: 2.7.1).

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 3.11 in real deployments (not just unit tests).
The model_copy method fails due to an unset __pydantic_private__ attribute when using model_construct
Surfaces as: File "/home/dave/test_deepcopy.py", line 21, in <module>\n main()\n File "/home/dave/test_deepcopy.py", line 17, in main\n m.model_copy(deep=True)\n File…

Proof / Evidence

GitHub issue: #9122
Fix PR: https://github.com/pydantic/pydantic/pull/9168
First fixed release: 2.7.1
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.49

Discussion

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

Jump to Sources Open on GitHub

“Looks like a bug! Thanks for reporting this!”

@sydney-runkle · 2024-03-29 · source

“It occurs to me that maybe this simple patch would be sufficient to fix this”

@babygrimes · 2024-04-03 · source

“@babygrimes, Please open a PR, I'd be happy to review!”

@sydney-runkle · 2024-04-03 · source

Failure Signature (Search String)

Cannot use model_copy(deep=True) on a model instance which was created using model_construct, when there is a model_post_init in the class hierarchy.

Error Message

Stack trace

error.txt

Error Message
-------------
File "/home/dave/test_deepcopy.py", line 21, in <module>\n    main()\n  File "/home/dave/test_deepcopy.py", line 17, in main\n    m.model_copy(deep=True)\n  File "/home/dave/axial/src/services/axial-api.git/venv/lib/python3.11/site-packages/pydantic/main.py", line 266, in model_copy\n    copied = self.__deepcopy__() if deep else self.__copy__()\n             ^^^^^^^^^^^^^^^^^^^\n  File "/home/dave/axial/src/services/axial-api.git/venv/lib/python3.11/site-packages/pydantic/main.py", line 723, in __deepcopy__\n    if self.__pydantic_private__ is None:\n       ^^^^^^^^^^^^^^^^^^^^^^^^^\n  File "/home/dave/axial/src/services/axial-api.git/venv/lib/python3.11/site-packages/pydantic/main.py", line 764, in __getattr__\n    return super().__getattribute__(item)  # Raises AttributeError if appropriate\n           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Minimal Reproduction

repro.py

pydantic-core version: 2.16.3
      pydantic-core build: profile=release pgo=true
             install path: /home/dave/axial/src/services/axial-api.git/venv/lib/python3.11/site-packages/pydantic
           python version: 3.11.8 (main, Feb 25 2024, 16:41:26) [GCC 9.4.0]
                 platform: Linux-5.15.0-1056-aws-x86_64-with-glibc2.31
         related packages: fastapi-0.110.0 typing_extensions-4.10.0 mypy-1.9.0
                   commit: unknown

Environment

Python: 3.11
Pydantic: 2

What Broke

Users experience AttributeError when attempting to copy models constructed with model_construct.

Why It Broke

The model_copy method fails due to an unset __pydantic_private__ attribute when using model_construct

Fix Options (Details)

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

pip install pydantic==2.7.1

When NOT to use: Do not use this fix if the model does not define model_post_init.

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)

See OnceOnly SDK

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/9168

First fixed release: 2.7.1

Last verified: 2026-02-09. Validate in your environment.

When NOT to Use This Fix

Do not use this fix if the model does not define model_post_init.
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
2.7.1	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 Cannot use model_copy(deep=True) on a model instance which was created using (Fix)