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RESULT is a reactive reference to $result — the return value available inside on_success callbacks. When an action completes successfully, the framework captures its output and makes it available through this variable. 
from prefab_ui.components import Button
from prefab_ui.actions import SetState
from prefab_ui.actions.mcp import CallTool
from prefab_ui.rx import RESULT

Button(
    "Search",
    on_click=CallTool(
        "search",
        arguments={"q": "{{ query }}"},
        on_success=SetState("results", RESULT),
    ),
)

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Where It’s Available

RESULT is only meaningful inside on_success handlers. Outside that context, $result is undefined. Every action that supports callbacks (on_success / on_error) makes this variable available when the action succeeds. For CallTool, $result is the tool’s return value (parsed as JSON when possible). When the action includes unwrapResult: true (set automatically by callable references), the renderer extracts the value from a {"result": X} envelope before exposing it. For Fetch, it’s the parsed response body. You can use it with any action in the callback: 
from prefab_ui.actions import AppendState, SetState, ShowToast
from prefab_ui.actions.mcp import CallTool
from prefab_ui.rx import RESULT

CallTool(
    "create_item",
    arguments={"name": "{{ item_name }}"},
    on_success=[
        AppendState("items", RESULT),
        ShowToast("Created!", variant="success"),
    ],
)
$result is the success counterpart of $error: one is available in on_success, the other in on_error. Neither exists outside its callback scope.

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Import

from prefab_ui.rx import RESULT