When I search on a custom field I get no hits

You can use a legacy action set or setup a ton of rules in decision diamond. It’s highlighted here:

Since you’re using the API, you can just send an HTTP POST with the contactId and then use a PHP script to check the field + run an action set if empty.

FYI- when searching, there’s a ‘Data Exists’ search option under Misc Criteria BUT it only works with standard fields, so wouldn’t really help in this case… :sleepy: