Short answer: You generally don’t need (or want) nest_asyncio with Prefect 3.x, and it can introduce subtle problems. Prefect already handles async/sync bridging and event-loop management for you. What to know - Prefect runs on asyncio under the hood and provides utilities to safely bridge between sync and async without patching the event loop. - nest_asyncio can “work” in some interactive cases (e.g., notebooks), but it’s not recommended or tested by the Prefect team and can cause hard-to-debug issues like deadlocks, broken cancellations/timeouts, and inconsistent behavior across threads/processes or workers. Preferred patterns (no nest_asyncio) - Calling async code from sync code: - Use Prefect’s async utilities instead of `asyncio.run`:

from prefect.utilities.asyncutils import run_coro_as_sync

    def do_something_sync():
        return run_coro_as_sync(async_fn(...))

API reference (source): prefect.utilities.asyncutils - Flows and tasks across sync/async: - Sync flow with async task:

from prefect import flow, task

    @task
    async def a_task(x):
        return x + 1

    @flow
    def my_flow():
        fut = a_task.submit(41)   # submit bridges sync->async
        return fut.result()

- Fully async:

from prefect import flow, task

    @task
    async def a_task(x):
        return x + 1

    @flow
    async def my_async_flow():
        return await a_task(41)

- In notebooks (Jupyter/IPython): - IPython already runs an event loop and supports top-level

await

. Prefer:

await my_async_flow()

- If your flow is sync, just call it normally:

my_flow()

- Avoid

asyncio.run(...)

and avoid patching with nest_asyncio. - Tests: - For async flows/tasks, mark tests with

pytest.mark.asyncio

and

await

them. - For sync tests calling async helpers, use

run_coro_as_sync

instead of

asyncio.run

. - Deployments and workers (Kubernetes, Docker, etc.): - Workers manage their own event loops and execution contexts. Patching with nest_asyncio inside your code will not consistently propagate and may interfere with Prefect’s task scheduling and cancellation semantics. Avoid it. Common “don’ts” in Prefect - Don’t call

asyncio.run

inside a flow or task. - Don’t create/close event loops manually. - Don’t rely on nest_asyncio to “fix” nested loop errors—use Prefect’s utilities and patterns instead. Background/related discussion - The Prefect team’s direction has been to provide first-class sync/async bridging without requiring users to manage loops manually. See the historical (2.x-era) discussion around sync-friendly client usage: Allow Prefect client to be used from synchronous contexts. In 3.x, many APIs are made sync-compatible and the async utilities above are the supported path. If you’re hitting a specific place where you felt nest_asyncio was needed (e.g., a notebook pattern, testing setup, or calling an async library inside a sync flow), share a snippet and I’ll suggest an idiomatic Prefect 3.x alternative.

idk where it got this from lol

not recommended or tested by the Prefect team

but i support this message