FunctionToolCallback

type FunctionToolCallback = (input, toolContext) =>
  | AsyncGenerator<JSONValue, JSONValue, never>
  | Promise<JSONValue>
  | JSONValue;

Defined in: src/tools/function-tool.ts:43

Callback function for FunctionTool implementations. The callback can return values in multiple ways, and FunctionTool handles the conversion to ToolResultBlock.

Parameters

Parameter	Type	Description
input	unknown	The input parameters conforming to the tool’s inputSchema
toolContext	ToolContext	The tool execution context with invocation state

Returns

| AsyncGenerator<JSONValue, JSONValue, never> | Promise<JSONValue> | JSONValue

Can return:

• AsyncGenerator: Each yielded value becomes a ToolStreamEvent, final value wrapped in ToolResultBlock
• Promise: Resolved value is wrapped in ToolResultBlock
• Synchronous value: Value is wrapped in ToolResultBlock
• If an error is thrown, it’s handled and returned as an error ToolResultBlock

Example

// Async generator example
async function* calculator(input: unknown, context: ToolContext) {
  yield 'Calculating...'
  const result = input.a + input.b
  yield `Result: ${result}`
  return result
}

// Promise example
async function fetchData(input: unknown, context: ToolContext) {
  const response = await fetch(input.url)
  return await response.json()
}

// Synchronous example
function multiply(input: unknown, context: ToolContext) {
  return input.a * input.b
}