# start

URL: /business/nouns/experiment/start · Id: `verb:Experiment.start` · Call: `await $.Experiment.start({ … })` (SDK) or the `do` tool at `https://api.sb/mcp`. Inputs says what to pass, Outputs what returns, When it waits who approves.

> Start opens an Experiment, splits traffic across its variants, and begins measuring its primary and guardrail Metrics.

Start opens an [Experiment](/business/nouns/experiment), splits traffic across its variants, and begins measuring its primary and guardrail [Metrics](/business/nouns/metric).

## Inputs
| Input | Type | Required | Description |
| --- | --- | --- | --- |
| `experiment` | [Experiment](/business/nouns/experiment) | Yes | The Experiment to open. |

## Outputs
| Output | Type | Required | Description |
| --- | --- | --- | --- |
| `experiment` | [Experiment](/business/nouns/experiment) | Yes | The running Experiment. |

## Effects
- Emits [`Experiment.started`](/business/events/Experiment.started).
- Performed by the [Strategist](/agents/founding/strategist).

## Refuses when
| Check | Error | Fix | Then call |
| --- | --- | --- | --- |
| The input does not match the schema of the operation. | [`invalid-input`](/code/api/errors/invalid-input) | Read the schema of the operation, then send every required field with the right type. | [Experiment.start](/business/nouns/experiment/start) |
| The key is valid, but its scopes do not allow this operation. | [`forbidden`](/code/api/errors/forbidden) | Use a key whose scopes cover the operation, or ask an owner of the namespace to grant them. | [Experiment.start](/business/nouns/experiment/start) |
| The record is not in a state that allows this Verb, so the call is refused. | [`refused`](/code/api/errors/refused) | Read the record, run the Verb that moves it to an allowed state, then retry. | [Experiment.start](/business/nouns/experiment/start) |

## When it waits
| Gate | Default |
| --- | --- |
| Decision class | Low: two-way, nothing at stake |
| [Delay window](/systems/trust-and-governance/controls/one-way-windows) | None: it runs once review passes. |
| Limit | The call [Limit](/business/nouns/limit) of the [Permission](/business/nouns/permission) it runs under. |
| Over the Limit | It waits `pending` for an [Approval](/business/nouns/approval) from the [Founder](/agents/founding/founder). Approved, it runs; refused, it ends `refused` with [`limit-exceeded`](/code/api/errors/limit-exceeded); unanswered by its deadline, it ends `canceled`. |

## Example

**Ask**

```text
Start the latest experiment.
```

**API**

```bash
curl -X POST https://api.sb/patentpad.com/experiments/experiment_1/start \
  -H "Authorization: Bearer $STARTUP_BUILDER_API_KEY"
```

```json
{
  "api": "https://api.sb/patentpad.com",
  "experiment": {
    "$id": "https://api.sb/patentpad.com/experiments/experiment_1",
    "$type": "Experiment",
    "variants": {}
  },
  "action": {
    "$id": "https://api.sb/patentpad.com/actions/action_1",
    "operation": "Experiment.start",
    "status": "done"
  },
  "links": {
    "self": "https://api.sb/patentpad.com/actions/action_1"
  }
}
```

**SDK**

```ts
import { $ } from 'startup-builder'

const experiment = await $.Experiment.start({ experiment: 'experiment_1' })
```

**CLI**

```bash
npx startup-builder do Experiment.start --json '{"experiment":"experiment_1"}'
```

**MCP**

```json
{
  "name": "do",
  "arguments": {
    "code": "return await $.Experiment.start({ experiment: 'experiment_1' })"
  }
}
```

Guides that call it: [Spend $200 to test faster](/start/guides/spend-200-to-test-faster), [Test two onboarding emails](/workflows/guides/test-two-onboarding-emails), [Test a new way of answering customers](/agents/test-new-support-instructions).