Letter.md lets people read, search, and review Markdown documents from ChatGPT. It can create traceable comments and reviewable edit suggestions, inspect document images, compare versions, and open the complete realtime collaborative editor. With an explicit user request and publication permission, it can publish a document or update its public page; publishing never changes the private source document automatically. Users can also save generated Markdown as a new unpublished document in a writable Letter.md workspace or create a derivative document. Access is limited to the workspaces and scopes approved through OAuth.