Thanks for the clear description — that sounds like a UI rendering quirk with Union parameters made of “primitive” types (date + enum). The fact that the JSON input shows the correct enum selection but the control flips back to the date tab suggests the backend/schema and value are fine, but the UI isn’t retaining the selected variant’s visible state. A couple of quick questions to help us narrow it down: - Are you on Prefect 3.x (Core) and using Prefect Cloud or self-hosted Prefect Server? - If self-hosting, which UI version are you running? - Could you share a minimal example of your parameter type (enum definition + flow signature)? In the meantime, here are reliable patterns that avoid this UI edge case: Workaround A: Use a discriminated union of models - Instead of a union of primitives, wrap each option in a Pydantic model and add a discriminator. The UI tends to handle these unions more consistently.

from datetime import date
from enum import Enum
from typing import Union, Annotated
from pydantic import BaseModel, Field
from typing_extensions import Literal
from prefect import flow

class MyEnum(str, Enum):
    A = "A"
    B = "B"

class DateChoice(BaseModel):
    kind: Literal["date"]
    value: date

class EnumChoice(BaseModel):
    kind: Literal["enum"]
    value: MyEnum

Param = Annotated[Union[DateChoice, EnumChoice], Field(discriminator="kind")]

@flow
def my_flow(x: Param):
    print(x)

Workaround B: One model with two optional fields + validation - Show both inputs and validate that exactly one was provided.

from datetime import date
from enum import Enum
from pydantic import BaseModel, model_validator
from prefect import flow

class MyEnum(str, Enum):
    A = "A"
    B = "B"

class Input(BaseModel):
    enum_value: MyEnum | None = None
    run_date: date | None = None

    @model_validator(mode="after")
    def _one_of(self):
        if (self.enum_value is None) == (self.run_date is None):
            raise ValueError("Provide exactly one of enum_value or run_date")
        return self

@flow
def my_flow(x: Input):
    print(x)

Workaround C: Two separate parameters with runtime validation - Keep

run_date: date | None

and

enum_value: MyEnum | None

as separate params and assert that exactly one is set at the start of the flow. A couple of additional tips: - Setting a default value for the parameter (e.g., defaulting to the enum variant) can “anchor” the initially selected tab in some cases, though it may not fix the tab-flipping you’re seeing. - If you’re self-hosting, consider upgrading the UI to the latest version in case this has already been addressed. I wasn’t able to find an existing GitHub issue for this specific behavior in the UI. If you can share a minimal reproducer (your enum, flow signature, and how you deploy), I’m happy to file a bug in the UI repo on your behalf, or you can open one directly here: - Prefect UI repo: PrefectHQ/prefect-ui - Prefect Core repo (for reference): PrefectHQ/prefect If you prefer, drop a minimal code snippet here and your environment details, and I’ll package it up for the team.

Thanks for the details — that helps. Given you’re on Prefect 3 and self-hosting the UI 3.4.25, what you’re seeing is consistent with a UI quirk when rendering Union parameters made of “primitive” types (date + enum). The backend is fine (as you noticed from the JSON), but the UI isn’t retaining the selected variant’s visible state. Quick things to try - Reorder the union so the enum comes first. Sometimes the UI anchors to the first variant:

MyUnionType = Union[MyEnum, date]
# or with | syntax: MyUnionType = MyEnum | date

You already default to

MyEnum.VALUE_ONE

, so this change may help the UI default/rendering heuristics. More reliable workarounds If the above doesn’t stick, these patterns are much more stable in the UI: 1) Discriminated union of models (recommended) - Wrap each variant in a small model and add a discriminator; the UI tends to handle this well.

from datetime import date
from enum import Enum
from typing import Union, Annotated
from typing_extensions import Literal
from pydantic import BaseModel, Field
from prefect import flow

class MyEnum(str, Enum):
    VALUE_ONE = "VALUE_ONE"
    # ...

class DateChoice(BaseModel):
    kind: Literal["date"]
    value: date

class EnumChoice(BaseModel):
    kind: Literal["enum"]
    value: MyEnum

MyUnionType = Annotated[Union[DateChoice, EnumChoice], Field(discriminator="kind")]

@flow
def my_flow(input: MyUnionType = EnumChoice(kind="enum", value=MyEnum.VALUE_ONE)):
    ...

2) Single model with two optional fields + validation - Show both inputs and enforce that exactly one is provided.

from datetime import date
from enum import Enum
from pydantic import BaseModel, model_validator
from prefect import flow

class MyEnum(str, Enum):
    VALUE_ONE = "VALUE_ONE"
    # ...

class Input(BaseModel):
    enum_value: MyEnum | None = MyEnum.VALUE_ONE
    run_date: date | None = None

    @model_validator(mode="after")
    def _one_of(self):
        if (self.enum_value is None) == (self.run_date is None):
            raise ValueError("Provide exactly one of enum_value or run_date")
        return self

@flow
def my_flow(input: Input = Input()):
    ...

Upgrade suggestion - If you can, try upgrading the UI to the latest available version; there have been periodic fixes to form rendering. Since you’re self-hosted, updating just the UI container/image may be sufficient depending on your deployment. - Prefect UI repo: PrefectHQ/prefect-ui - Form building docs (helpful for shaping parameters the UI handles best): Form building in Prefect 3 If reordering the union doesn’t help and you’d prefer not to change the type, I can open a UI issue with your minimal example. If you’re up for it, please also share: - Browser and version - How you deploy the UI (image tag/Helm chart version) - A short GIF or screenshots showing the selection flipping I’m happy to file the bug on your behalf with that info.