Endpoints for the home page and feed plus the full create, read, edit and delete cycle for posts and comments, with their form fields, topic enum, attachment_uids handling, pagination cursors and JSON payload shapes.
Uploading files with multipart/form-data or by handing the server a public URL, and the full attachment lifecycle - list, get, rename, delete - plus how the returned uid is referenced from posts, comments, projects, gist...
The direct-message endpoints: opening the inbox and a conversation, the read-marking side effect, sending a message with attachments, and the recipient lookup that feeds the composer.