Pydantic v2.8 create_model breaking change to api

The Fix

pip install pydantic==1.10.18

Based on closed pydantic/pydantic issue #9901 · PR/commit linked

Production note: This usually shows up under retries/timeouts. Treat it as a side-effect risk until you can verify behavior with a canary + real traffic.

Jump to Verify Open PR/Commit

@@ -83,6 +83,7 @@ is desired, you can switch to `left-to-right` mode and change the order of your
 * Add more robust custom validation examples by @ChrisPappalardo in [#9468](https://github.com/pydantic/pydantic/pull/9468)
 * Fix ignored `strict` specification for `StringConstraint(strict=False)` by @vbmendes in [#9476](https://github.com/pydantic/pydantic/pull/9476)
+* **Breaking Change:** Use PEP 570 syntax by @Viicos in [#9479](https://github.com/pydantic/pydantic/pull/9479)
 * Use `Self` where possible by @Viicos in [#9479](https://github.com/pydantic/pydantic/pull/9479)
 * Do not alter `RootModel.model_construct` signature in the `mypy` plugin by @Viicos in [#9480](https://github.com/pydantic/pydantic/pull/9480)

repro.py

pydantic version: 2.7.4
        pydantic-core version: 2.18.4
          pydantic-core build: profile=release pgo=true
                 install path: C:\Users\fraser\.virtualenvs\render-services-IYdnVPWc\Lib\site-packages\pydantic
               python version: 3.8.10 (tags/v3.8.10:3d8993a, May  3 2021, 11:48:03) [MSC v.1928 64 bit (AMD64)]
                     platform: Windows-10-10.0.22631-SP0
             related packages: fastapi-0.111.0 mypy-1.8.0 typing_extensions-4.12.2
                       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==1.10.18\nWhen NOT to use: This fix should not be used if backward compatibility for __model_name is required.\n\n

Why This Fix Works in Production

Trigger: v2.8 create_model breaking change to api (__model_name->model_name), not in changelog
Mechanism: The change to create_model introduced a breaking change by altering the parameter name from __model_name to model_name
Why the fix works: Updated the changelog to reflect breaking changes regarding PEP 570, which affects the create_model function. (first fixed release: 1.10.18).

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

The change to create_model introduced a breaking change by altering the parameter name from __model_name to model_name
Production symptom (often without a traceback): v2.8 create_model breaking change to api (__model_name->model_name), not in changelog

Proof / Evidence

GitHub issue: #9901
Fix PR: https://github.com/pydantic/pydantic/pull/9903
First fixed release: 1.10.18
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.48

Discussion

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

Jump to Sources Open on GitHub

“The change added / to the signature, might I suggest also adding __model_name as a kw arg deprecated just for back compat - that should…”

@fraser-langton · 2024-07-16 · source

“> Do you have a use case where replacing create_model(__model_name=...) by create_model(...) isn't possible? Nup it's obviously a niche use case (but we had it)…”

@fraser-langton · 2024-07-16 · source

“@Viicos, thanks for opening the PR - I've approved. @fraser-langton, see https://github.com/pydantic/pydantic/issues/9810, which is a duplicate of this issue.”

@sydney-runkle · 2024-07-17 · source

“> The change added / to the signature, might I suggest also adding __model_name as a kw arg deprecated just for back compat - that…”

@Viicos · 2024-07-16 · source

Failure Signature (Search String)

v2.8 create_model breaking change to api (__model_name->model_name), not in changelog
Interesting to find this #1367 which originally changed model_name -> __model_name

Copy-friendly signature

signature.txt

Failure Signature
-----------------
v2.8 create_model breaking change to api (__model_name->model_name), not in changelog
Interesting to find this #1367 which originally changed model_name -> __model_name

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
v2.8 create_model breaking change to api (__model_name->model_name), not in changelog
Interesting to find this #1367 which originally changed model_name -> __model_name

Minimal Reproduction

repro.py

pydantic version: 2.7.4
        pydantic-core version: 2.18.4
          pydantic-core build: profile=release pgo=true
                 install path: C:\Users\fraser\.virtualenvs\render-services-IYdnVPWc\Lib\site-packages\pydantic
               python version: 3.8.10 (tags/v3.8.10:3d8993a, May  3 2021, 11:48:03) [MSC v.1928 64 bit (AMD64)]
                     platform: Windows-10-10.0.22631-SP0
             related packages: fastapi-0.111.0 mypy-1.8.0 typing_extensions-4.12.2
                       commit: unknown

Environment

Pydantic: 2

What Broke

Users experienced API errors due to the breaking change in the create_model function.

Why It Broke

The change to create_model introduced a breaking change by altering the parameter name from __model_name to model_name

Fix Options (Details)

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

pip install pydantic==1.10.18

When NOT to use: This fix should not be used if backward compatibility for __model_name is 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)

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/9903

First fixed release: 1.10.18

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

When NOT to Use This Fix

This fix should not be used if backward compatibility for __model_name is 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
1.10.18	Fixed

Related Issues

No related fixes found.

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Related clusters: Data consistency OpenAPI schema Serialization

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

Pydantic v2.8 create_model breaking change to api (__model_name->model_name), not in changelog (Fix)