Task details not available in the UI when using subflow output as input to a task ("Invalid TaskInputResponse" in browser console)

[fohrloop]

Bug summary

Compete MWE reproducing the issue

import time
from pathlib import Path

from prefect import flow, task


@flow
def mainflow() -> None:
    folder = Path(".") / "data"
    outfiles = fetch_data(folder)
    another_task(outfiles["a"])


@flow
def fetch_data(folder: Path) -> dict[str, Path]:
    sql_queries = ["a", "b", "c"]
    time.sleep(1)
    outfiles = {x: Path(x) for x in sql_queries}
    return outfiles


@task
def another_task(
    input_file: Path,
) -> None:
    time.sleep(1)


if __name__ == "__main__":
    mainflow()

This is what you see in the console

Workaround 1: Do not use subflows

If you change the fetch_data (subflow) in to a @task, everything works as expected:

import time
from pathlib import Path

from prefect import flow, task


@flow
def mainflow() -> None:
    folder = Path(".") / "data"
    outfiles = fetch_data(folder)
    another_task(outfiles["a"])


@task # This changed from @flow to @task
def fetch_data(folder: Path) -> dict[str, Path]:
    sql_queries = ["a", "b", "c"]
    time.sleep(1)
    outfiles = {x: Path(x) for x in sql_queries}
    return outfiles


@task
def another_task(
    input_file: Path,
) -> None:
    time.sleep(1)


if __name__ == "__main__":
    mainflow()

Workaround 2

If you don't use the output from the subflow as an argument to the task, everything works as expected:

import time
from pathlib import Path

from prefect import flow, task


@flow
def mainflow() -> None:
    folder = Path(".") / "data"
    _ = fetch_data(folder)
    another_task(folder / "input.txt") # This input changed


@flow
def fetch_data(folder: Path) -> dict[str, Path]:
    sql_queries = ["a", "b", "c"]
    time.sleep(1)
    outfiles = {x: Path(x) for x in sql_queries}
    return outfiles


@task
def another_task(
    input_file: Path,
) -> None:
    time.sleep(1)


if __name__ == "__main__":
    mainflow()

Version info

Version:             3.4.11
API version:         0.8.4
Python version:      3.12.10
Git commit:          3c98aa7f
Built:               Thu, Jul 31, 2025 08:45 PM
OS/Arch:             linux/x86_64
Profile:             local
Server type:         server
Pydantic version:    2.11.7

Additional context

No response