Pydantic Data class with defer_build set to true causes the

The Fix

pip install pydantic==2.10.3

Based on closed pydantic/pydantic issue #10953 · 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

@@ -125,6 +125,8 @@ def __init__(__dataclass_self__: PydanticDataclass, *args: Any, **kwargs: Any) -
     cls.__pydantic_config__ = config_wrapper.config_dict  # type: ignore
 
+    set_dataclass_fields(cls, ns_resolver, config_wrapper=config_wrapper)
+
     if not _force_build and config_wrapper.defer_build:

repro.py

import typing

import pydantic


@pydantic.dataclasses.dataclass(kw_only=True, config=pydantic.config.ConfigDict(defer_build=True))
class MyObject:
    msg_id: str
    receipt_handle: str


@pydantic.validate_call
def do_work(input: MyObject|typing.Iterable[MyObject]|list[MyObject]):
    pass

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==2.10.3\nWhen NOT to use: This fix should not be applied if defer_build is not used.\n\n

Why This Fix Works in Production

Trigger: Data class with defer_build set to true causes the error AttributeError: type object '...' has no attribute '__pydantic_fields__'. Did you mean:…
Mechanism: The error occurs due to missing field initialization when defer_build is set to true
Why the fix works: Sets fields when `defer_build` is set on Pydantic dataclasses, aligning with Pydantic models where only the schema generation is deferred. (first fixed release: 2.10.3).

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 error occurs due to missing field initialization when defer_build is set to true
Surfaces as: Data class with defer_build set to true causes the error AttributeError: type object '...' has no attribute '__pydantic_fields__'. Did you mean: '__pydantic_config__'

Proof / Evidence

GitHub issue: #10953
Fix PR: https://github.com/pydantic/pydantic/pull/10984
First fixed release: 2.10.3
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.60

Verified Execution

We executed the runnable minimal repro in a temporary environment and captured exit codes + logs.

Status: PASS
Ran: 2026-02-11T16:52:29Z
Package: pydantic
Fixed: 2.10.3
Mode: fixed_only
Outcome: ok

Logs

affected (exit=None)

fixed (exit=0)

Discussion

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

Jump to Sources Open on GitHub

“### Initial Checks - [X] I confirm that I'm using Pydantic V2 ### Description Upgraded from 2.9.2 and the below code wll now fail unless I set **defer_build** to false ### Example Code ### Python, Pydantic & OS Version”

Issue thread · issue description · source

Failure Signature (Search String)

Data class with defer_build set to true causes the error AttributeError: type object '...' has no attribute '__pydantic_fields__'. Did you mean: '__pydantic_config__'

Error Message

Stack trace

error.txt

Error Message
-------------
Data class with defer_build set to true causes the error AttributeError: type object '...' has no attribute '__pydantic_fields__'. Did you mean: '__pydantic_config__'

Minimal Reproduction

repro.py

import typing

import pydantic


@pydantic.dataclasses.dataclass(kw_only=True, config=pydantic.config.ConfigDict(defer_build=True))
class MyObject:
    msg_id: str
    receipt_handle: str


@pydantic.validate_call
def do_work(input: MyObject|typing.Iterable[MyObject]|list[MyObject]):
    pass

Environment

Pydantic: 2

What Broke

Users experience AttributeError when using Pydantic dataclasses with defer_build set to true.

Why It Broke

The error occurs due to missing field initialization when defer_build is set to true

Fix Options (Details)

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

pip install pydantic==2.10.3

When NOT to use: This fix should not be applied if defer_build is not used.

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/10984

First fixed release: 2.10.3

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

When NOT to Use This Fix

This fix should not be applied if defer_build is not used.

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
2.10.3	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 Data class with defer_build set to true causes the error AttributeError: type object '...' (Fix)