Message editing and versioning plug-in

Key features

• Edit & resend — hover your own message, edit it in place (no modal, no new chat), and resend.
• Every version is a real fork session.
• Why are there no buttons on the very first message of a conversation?
• The plugin itself uses only harness public extension points (conversation.chat.assistant-actions / conversation.chat.user-actions slots, ctx.sessions.fork, session.prompt, ctx.sessions.open).
• A clean fork restarts from the turn before your question: history + edited question + new answer.