Pydantic Support async functions with `@validate

The Fix

pip install pydantic==2.9.2

Based on closed pydantic/pydantic issue #10370 · 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

@@ -3,6 +3,7 @@
 
 import functools
+import inspect
 from typing import TYPE_CHECKING, Any, Callable, TypeVar, overload

repro.py

import asyncio
import inspect

from pydantic import validate_call


@validate_call(validate_return=True)
async def delay_number(n: int) -> int:
    await asyncio.sleep(1)
    return n


if __name__ == "__main__":
    print(inspect.iscoroutinefunction(delay_number))

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==2.9.2\nWhen NOT to use: This fix should not be used if the decorator is not intended for async functions.\n\n

Why This Fix Works in Production

Trigger: When I use `async_to_sync`, I got a error ` UserWarning: async_to_sync was passed a non-async-marked callable`.
Mechanism: Ensures that the `@validate_call` decorator correctly identifies and works with async functions, allowing `inspect.iscoroutinefunction` to return True for decorated coroutines.
Why the fix works: Ensures that the `@validate_call` decorator correctly identifies and works with async functions, allowing `inspect.iscoroutinefunction` to return True for decorated coroutines. (first fixed release: 2.9.2).

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

Production symptom (often without a traceback): When I use `async_to_sync`, I got a error ` UserWarning: async_to_sync was passed a non-async-marked callable`.

Proof / Evidence

GitHub issue: #10370
Fix PR: https://github.com/pydantic/pydantic/pull/10374
First fixed release: 2.9.2
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.64

Discussion

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

Jump to Sources Open on GitHub

“Maybe pydantic._internal._validate_call.ValidateCallWrapper need some changes to adapt async function(coroutine)?”

@MovisLi · 2024-09-10 · source

“Yeah. I don't think it's likely that we'll support async functions in the short term, but we can leave this open as a potential feature.”

@sydney-runkle · 2024-09-10 · source

“Huh, I forgot that we offered support! That's great, thanks for the ref! I'd say we probably want to have the functions defined within this…”

@sydney-runkle · 2024-09-10 · source

“> Yeah”

@MovisLi · 2024-09-10 · source

Failure Signature (Search String)

When I use `async_to_sync`, I got a error ` UserWarning: async_to_sync was passed a non-async-marked callable`.
Maybe `pydantic._internal._validate_call.ValidateCallWrapper` need some changes to adapt async function(coroutine)?

Copy-friendly signature

signature.txt

Failure Signature
-----------------
When I use `async_to_sync`, I got a error ` UserWarning: async_to_sync was passed a non-async-marked callable`.
Maybe `pydantic._internal._validate_call.ValidateCallWrapper` need some changes to adapt async function(coroutine)?

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
When I use `async_to_sync`, I got a error ` UserWarning: async_to_sync was passed a non-async-marked callable`.
Maybe `pydantic._internal._validate_call.ValidateCallWrapper` need some changes to adapt async function(coroutine)?

Minimal Reproduction

repro.py

import asyncio
import inspect

from pydantic import validate_call


@validate_call(validate_return=True)
async def delay_number(n: int) -> int:
    await asyncio.sleep(1)
    return n


if __name__ == "__main__":
    print(inspect.iscoroutinefunction(delay_number))

Environment

Pydantic: 2

What Broke

Using async_to_sync with decorated async functions results in a UserWarning and incorrect behavior.

Fix Options (Details)

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

pip install pydantic==2.9.2

When NOT to use: This fix should not be used if the decorator is not intended for async functions.

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/10374

First fixed release: 2.9.2

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

When NOT to Use This Fix

This fix should not be used if the decorator is not intended for async functions.

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
2.9.2	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 Support async functions with `@validate_call` decorator (Fix)