> ## Documentation Index
> Fetch the complete documentation index at: https://docs.agentcard.sh/llms.txt
> Use this file to discover all available pages before exploring further.

# buy_add_to_cart

> Add a product to the cart at a linked merchant

Adds a product to the cart at a linked merchant, using the product id from [buy\_search\_products](/companies/mcp/tools/buy_search_products).

<Note>Not advertised in `tools/list` by default — the [buy](/companies/mcp/tools/buy) tool covers this conversationally. Expert integrations can still call it by name.</Note>

## Parameters

| Parameter    | Type   | Required | Description                                |
| ------------ | ------ | -------- | ------------------------------------------ |
| `merchant`   | string | Yes      | The linked merchant slug.                  |
| `product_id` | string | Yes      | The product id from `buy_search_products`. |
| `quantity`   | number | No       | How many to add. Defaults to 1.            |

## Returns

The updated cart: a line per item with quantity, name, and price, then the subtotal, plus the total including fees, tax, and delivery when it differs. Failures return a short error message.

## Example

```json theme={null}
{ "name": "buy_add_to_cart", "arguments": { "merchant": "goodeggs", "product_id": "prod_84h2", "quantity": 2 } }
```