I prefer text. If the JSON had very specific keys that needed values then a form would be very useful. However, the function_call api takes essentially arbitrary json, which would be very difficult to edit with a form interface. You'd need buttons for adding nested items, editing keys, editing values. It would be a mess.

Imagine creating the following simple function call:

  [
    {
      "name": "get_current_weather",
      "description": "Get the current weather in a given location",
      "parameters": {
        "type": "object",
        "properties": {
          "location": {
            "type": "string",
            "description": "The city and state, e.g. San Francisco, CA"
          },
          "unit": {
            "type": "string",
            "enum": ["celsius", "fahrenheit"]
          }
        },
        "required": ["location"]
      }
    }
  ]