{
  "tools": [
    {
      "name": "lookup_invoice",
      "description": "Retrieve an invoice by its invoice ID.",
      "parameters": {
        "type": "object",
        "properties": {
          "invoice_id": {
            "type": "string",
            "description": "The invoice identifier from the billing system."
          }
        },
        "required": [
          "invoice_key"
        ]
      }
    }
  ]
}