List custom field definitions

The schema behind crm_contacts.custom_fields — which keys are valid, what type each expects, and which values a select accepts. Read this before writing custom_fields, or you are guessing against an unversioned blob.

Unpaginated: definitions number in the handful, and you always want them whole. Read-only over the API — changing a definition reshapes every contact already carrying the key, so it is a dashboard operation (Settings → Custom Fields).