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CallHandler invokes a client-side JavaScript function registered by the app developer. Where CallTool crosses the network to your server, CallHandler runs instantly in the browser — ideal for state transformations like constraint enforcement, computed fields, or domain-specific logic that doesn’t need server involvement. 
from prefab_ui.components import Slider
from prefab_ui.actions import CallHandler

Slider(name="infra", on_change=CallHandler("constrainBudget"))
Handlers are registered via js_actions on PrefabApp. See Custom Handlers for how to write and register them.

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Passing Arguments

Use arguments to pass extra data to the handler. Values support {{ }} interpolation, so you can pass client state at call time: 
from prefab_ui.components import Button
from prefab_ui.actions import CallHandler

Button("Recalculate", on_click=CallHandler(
    "recalculate",
    arguments={"mode": "aggressive", "total": "{{ budget }}"},
))
The handler receives these via ctx.arguments: 
// In js_actions
"recalculate": (ctx) => {
    const mode = ctx.arguments.mode;  // "aggressive"
    const total = ctx.arguments.total;  // resolved budget value
    // ... compute and return state updates
}

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Handling Results

The handler’s return value is available as $result in on_success callbacks, just like CallTool: 
from prefab_ui.actions import CallHandler, SetState, ShowToast
from prefab_ui.rx import RESULT

Button("Validate", on_click=CallHandler(
    "validate",
    on_success=ShowToast("Valid!", variant="success"),
    on_error=ShowToast("Validation failed", variant="error"),
))
If the handler throws an exception, on_error fires with the error message in $error.

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Combined with Other Actions

Chain CallHandler with other actions in a list: 
from prefab_ui.actions import CallHandler, SetState

Slider(name="infra", on_change=[
    SetState("dirty", True),
    CallHandler("constrainBudget"),
])

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API Reference

CallHandler Parameters

​
handler
str
required
Name of the registered handler function. Can be passed as a positional argument.
​
arguments
dict[str, Any] | None
default:"None"
Extra arguments passed to the handler via ctx.arguments. Values support {{ key }} interpolation.

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Protocol Reference

CallHandler
{
  "action": "callHandler",
  "handler": "string (required)",
  "arguments?": "object"
}
For the complete protocol schema, see CallHandler.