tool

function tool<I, O>(definition: ToolDefinition<I, O>): ToolDefinition<I, O>;

Identity helper that keeps a tool’s schema type, so its handler’s arguments are typed for real.

tools is an array, and TypeScript does not infer a generic per array element — every entry falls back to ToolDefinition’s default and the handler receives any. That has always been true here; Zod 3 hid it because its ZodTypeAny default WAS any, so nothing complained.

Wrapping one definition restores the inference, because the generic is then captured at the call:

tools: [
  tool({
    name: 'search',
    description: 'Search.',
    input: z.object({ query: z.string() }),
    async handler(args) {
      return args.query.toUpperCase(); // typed, not `any`
    },
  }),
]

Purely a type-level device: it returns its argument untouched, costs nothing at runtime, and is opt-in per tool.

Type Parameters

Type Parameter
I extends ZodType<unknown, unknown, $ZodTypeInternals<unknown, unknown>>
O extends ZodType<unknown, unknown, $ZodTypeInternals<unknown, unknown>>

Parameters

Parameter	Type	Description
definition	ToolDefinition<I, O>	The tool definition.

Returns

ToolDefinition<I, O>

The same definition, with its schema types preserved.