Pydantic Make opt-in fast build a global setting (Fix)

The Fix

pip install pydantic==1.10.18

Based on closed pydantic/pydantic issue #10076 · 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%.

Open PR/Commit

@@ -89,6 +89,7 @@ class ConfigWrapper:
     use_attribute_docstrings: bool
     cache_strings: bool | Literal['all', 'keys', 'none']
+    experimental_fast_build: bool
 
     def __init__(self, config: ConfigDict | dict[str, Any] | type[Any] | None, *, check: bool = True):

fix.md

Option A — Upgrade to fixed release\npip install pydantic==1.10.18\nWhen NOT to use: If this setting is set to `True`, model schemas for edge cases might be incorrect.\n\n

Why This Fix Works in Production

Trigger: - [ ] [Compatibility between releases](https://docs.pydantic.dev/changelog/)
Mechanism: Added an experimental setting to enable fast build globally for all models, improving performance significantly
Why the fix works: Added an experimental setting to enable fast build globally for all models, improving performance significantly. (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

Added an experimental setting to enable fast build globally for all models, improving performance significantly
Production symptom (often without a traceback): - [ ] [Compatibility between releases](https://docs.pydantic.dev/changelog/)

Proof / Evidence

GitHub issue: #10076
Fix PR: https://github.com/pydantic/pydantic/pull/10064
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: 0.71

Discussion

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

Jump to Sources Open on GitHub

“Subclassing all models would be tedious, but I think your suggestion of setting a global model config would work. Will give it a try when…”

@michaelgmiller1 · 2024-08-08 · confirmation · source

“Yeah, we'd recommend something like: Going to close this as resolved for now - we're planning on making this speedup apply in all cases, as…”

@sydney-runkle · 2024-08-08 · confirmation · source

“We discussed about defining an environment variable, but this would enable the flag for every model, even the ones you don't own defined in external…”

@Viicos · 2024-08-07 · source

“Of course we can still revisit (cc @sydney-runkle), but if it does work by globally mutating the BaseModel.model_config attribute, then I think we could keep…”

@Viicos · 2024-08-08 · source

Failure Signature (Search String)

- [ ] [Compatibility between releases](https://docs.pydantic.dev/changelog/)
- [ ] [Data validation/parsing](https://docs.pydantic.dev/concepts/models/#basic-model-usage)

Copy-friendly signature

signature.txt

Failure Signature
-----------------
- [ ] [Compatibility between releases](https://docs.pydantic.dev/changelog/)
- [ ] [Data validation/parsing](https://docs.pydantic.dev/concepts/models/#basic-model-usage)

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
- [ ] [Compatibility between releases](https://docs.pydantic.dev/changelog/)
- [ ] [Data validation/parsing](https://docs.pydantic.dev/concepts/models/#basic-model-usage)

What Broke

Applications cannot migrate without a global setting for fast build, causing delays.

Why It Broke

Added an experimental setting to enable fast build globally for all models, improving performance significantly

Fix Options (Details)

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

pip install pydantic==1.10.18

When NOT to use: If this setting is set to `True`, model schemas for edge cases might be incorrect.

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/10064

First fixed release: 1.10.18

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

When NOT to Use This Fix

If this setting is set to `True`, model schemas for edge cases might be incorrect.

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.

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.