> ## Documentation Index
> Fetch the complete documentation index at: https://wb-21fd5541-typedoc-upgrade.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# startTool

> TypeScript SDK reference

> **startTool**(`opts`): [`Tool`](../interfaces/tool)

Defined in: [src/genai/api.ts:110](https://github.com/wandb/weave/blob/8c5f077eb11c42b84000726ff20504abc728d3fb/sdks/node/src/genai/api.ts#L110)

Start a Tool span. Parent resolution (matches the design's "flat by
default, hierarchical if you nest"):

* If an LLM is active, the Tool nests under it.
* Otherwise, the Tool is a sibling under the current Turn.

Throws if neither a Turn nor an LLM is active.

## Parameters

### opts

[`ToolInit`](../interfaces/toolinit)

## Returns

[`Tool`](../interfaces/tool)

## Examples

```ts twoslash theme={null}
// @noErrors
weave.startTool({
  name: 'get_weather',
  args: JSON.stringify({city: 'Tokyo'}),
  toolCallId: 'call_t1',
});
```

```ts twoslash theme={null}
// @noErrors
weave.startTool({
  name: 'get_weather',
  args: JSON.stringify({city: 'Tokyo'}),
  toolCallId: 'call_t1',
  startTime: new Date('2026-05-29T10:00:00.800Z'),
});
```