Specifying units seems to be unreliable; I tried adding a description to the set_thermostat temperature:

    "temperature": {
      "type": "number",
      "description": "degrees Fahrenheit"
    },

Set the living room temperature to 70 degrees Celsius

    {
      "function_calls": [
        {
          "name": "set_thermostat",
          "arguments": {
            "room": "living room",
            "temperature": 70,
            "mode": "cool"
          }
        }
      ],
      "confidence": 0.6045
    }

Set the living room temperature to 70 degrees Fahrenheit

    {
      "function_calls": [
        {
          "name": "set_thermostat",
          "arguments": {
            "room": "living room",
            "temperature": 70,
            "mode": "heat"
          }
        }
      ],
      "confidence": 0.4536
    }

Set the living room temperature to 70 degrees

    {
      "function_calls": [
        {
          "name": "set_thermostat",
          "arguments": {
            "room": "living room",
            "temperature": 70
          }
        }
      ],
      "confidence": 0.8517
    }

Trying "in degrees Fahrenheit" for the tool description had similarly counterintuitive confidences.

Edit: to be clear, the counterintuitive behavior is that the confidence ended up higher for the wrong units.