Why does a custom field I just created not show in the Update Contact list or the variable picker?

When a custom field is added, for example when the AI assistant creates it for you, some places in the workflow builder do not know about it until the page is refreshed. This includes the field dropdown in the Update Contact step mappings and the insert variable picker.

To fix it, refresh the browser page, then search for the field again. It will appear in the list.