Merges the contact source_contact_id into the contact in the path. This is irreversible, like deleting a contact: the source contact is deleted, the kept contact's empty fields are filled from it, its notes are appended, its sequences, tasks, tags, audience memberships, opportunity links, custom field values move to the kept contact, and its live sequences are stopped.

When live sequences would be stopped the request is rejected with 409 sequence_stop_confirmation_required unless confirm_sequence_stop is true. The merge runs in one transaction: on 422 merge_failed nothing was changed. Retrying a merge that already completed returns 404, because the source contact no longer exists.