I've created a custom thermostat device using the following in my device.xml:

<Device type="thermostat" id="myThermoType">

I took this directly from the example custom thermostat in the sdk. The device.xml file in that plugin indicates that my custom thermostat should inherit, among other states, hvacCoolerIsOn and hvacHeaterIsOn. However, when I try to update either of these states I get errors:
Error device state key hvacCoolerIsOn not defined (ignoring update request)
Error device state key hvacHeaterIsOn not defined (ignoring update request)

After looking through the forum, the only discussion of this I've seen is about these states not appearing in older venstar thermostats, which wouldn't apply to me since my thermostat is a custom one. Is there a similar limitation on custom thermostats?

Hmmm. The Radio Thermostat plugin updates those states without issue - it's definitely not a limitation in the API.

Can you paste in the code where you do the update? Also, open your DB file and copy/paste in the entire device XML for the device you created.

That state is automatically added if (and only if) the device has the property ShowCoolHeatEquipmentStateUI set to True. The example plugin provides, in Devices.xml, a checkbox to set that property.

I thought there might be something!

Here is my <device> from device.xml with the new checkbox:

<Device type="thermostat" id="ISYThermostat">
<Name>ISY Thermostat</Name>
<ConfigUI>
<Field type="checkbox" id="ShowCoolHeatEquipmentStateUI" defaultValue="true" hidden="true">
<Label/>
<Description/>
</Field>
</ConfigUI>
</Device>

I still don't get the states. I create the device programatically, so maybe the device.xml doesn't get used in that case? Would it work to set device.pluginProps['ShowCoolHeatEquipmentStateUI'] = True in the device.create method?

update: this seems to work:

pDev = indigo.device.create(protocol=indigo.kProtocol.Plugin,
name=thermoName,
deviceTypeId=thermoType,
description=thermoDesc,
props={"ShowCoolHeatEquipmentStateUI":True},
folder=folderId)

Yeah! Thanks for pointing me in the right direction!

Yep, if you are creating it from code then you'll just want to set that prop value as you discovered.