Although you are able to set custom fields through the XML API, there are two quirks I ran across simultaneously.
- If any of the custom fields you're trying to set contain invalid values, none of them will save, but the case will be created or the changes to standard case fields will (depending on new vs edit). If you are requesting the custom data be returned using the cols argument on the API call, it will return the values that you passed in with the query, even though they didn't save. If you need to check that the data actually saved, you will need to do a second search query to check the case values.
- A custom field of type "Number" actually means type "Integer". Storing a floating point value is invalid. Combine with quirk one for best results.
