Pydantic `RootModel` from `dataclass` does not preserve

The Fix

pip install pydantic==2.2.1

Based on closed pydantic/pydantic issue #9645 · 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

@@ -900,7 +900,7 @@ except PydanticUserError as exc_info:
 ```
 
-## `config` is unused with TypeAdapter {#type-adapter-config-unused}
+## `config` is unused with `TypeAdapter` {#type-adapter-config-unused}

repro.py

# %%
import pydantic
from pydantic.dataclasses import dataclass

# %%
class A(pydantic.BaseModel):
    model_config = pydantic.ConfigDict(extra="allow")

    name: str

a = A(name="a", i=42)

# %%
a.model_extra
# {'i': 42}

# %%
a.model_dump()
# {'name': 'a', 'i': 42}

# %%
@dataclass(config=pydantic.ConfigDict(extra="allow"))
class B:
    name: str

b = B(name="a", i=42)

# %%
pydantic.RootModel[type(b)](b).model_extra
# None

# %%
pydantic.RootModel[type(b)](b).model_dump()
# {'name': 'a'}

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==2.2.1\nWhen NOT to use: This fix is not applicable if you need to preserve extra fields in RootModel.\n\n

Why This Fix Works in Production

Trigger: `RootModel` from `dataclass` does not preserve extra fields
Mechanism: RootModel does not preserve extra fields when created from a dataclass with extra='allow'
Why the fix works: Addresses the issue where `RootModel` does not preserve extra fields when created from a dataclass with `extra='allow'`. (first fixed release: 2.2.1).

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 3.9 in real deployments (not just unit tests).
RootModel does not preserve extra fields when created from a dataclass with extra='allow'
Production symptom (often without a traceback): `RootModel` from `dataclass` does not preserve extra fields

Proof / Evidence

GitHub issue: #9645
Fix PR: https://github.com/pydantic/pydantic/pull/6937
First fixed release: 2.2.1
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.60

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: 2.2.1
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

“Hmm, my first thought is that RootModel doesn't support the extra specification in config...”

@sydney-runkle · 2024-06-12 · source

“Tracking in https://github.com/pydantic/pydantic/issues/12557.”

@Viicos · 2025-11-20 · source

“I've been looking into this in my search for a good **pydantic-core** issue to get into and it seems to be intentional, not a bug”

@Stormageddon37 · 2025-10-03 · source

Failure Signature (Search String)

`RootModel` from `dataclass` does not preserve extra fields
Hmm, my first thought is that `RootModel` doesn't support the `extra` specification in `config`...

Copy-friendly signature

signature.txt

Failure Signature
-----------------
`RootModel` from `dataclass` does not preserve extra fields
Hmm, my first thought is that `RootModel` doesn't support the `extra` specification in `config`...

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
`RootModel` from `dataclass` does not preserve extra fields
Hmm, my first thought is that `RootModel` doesn't support the `extra` specification in `config`...

Minimal Reproduction

repro.py

# %%
import pydantic
from pydantic.dataclasses import dataclass

# %%
class A(pydantic.BaseModel):
    model_config = pydantic.ConfigDict(extra="allow")

    name: str

a = A(name="a", i=42)

# %%
a.model_extra
# {'i': 42}

# %%
a.model_dump()
# {'name': 'a', 'i': 42}

# %%
@dataclass(config=pydantic.ConfigDict(extra="allow"))
class B:
    name: str

b = B(name="a", i=42)

# %%
pydantic.RootModel[type(b)](b).model_extra
# None

# %%
pydantic.RootModel[type(b)](b).model_dump()
# {'name': 'a'}

Environment

Python: 3.9
Pydantic: 2

What Broke

Extra fields are lost during serialization, leading to incomplete data representation.

Why It Broke

RootModel does not preserve extra fields when created from a dataclass with extra='allow'

Fix Options (Details)

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

pip install pydantic==2.2.1

When NOT to use: This fix is not applicable if you need to preserve extra fields in RootModel.

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/6937

First fixed release: 2.2.1

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

When NOT to Use This Fix

This fix is not applicable if you need to preserve extra fields in RootModel.

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
2.2.1	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 `RootModel` from `dataclass` does not preserve extra fields (Fix)