Copilot CLI Extensions Revamp: Custom Slash Commands and Full Extensibility

The Extension System Grew Up When I first wrote about Copilot CLI extensions, the story was about a powerful but undocumented feature you had to reverse-engineer from SDK type definitions. It was there, it worked, and almost nobody knew about it. That was March 14, 2026. In the nine days since, GitHub has shipped features that turn extensions from a power-user secret into a genuinely first-class extensibility platform: Custom slash commands are now part of the SDK contract via slashCommands in joinSession() UI elicitation dialogs let extensions show structured input forms to users The /extensions command lets you manage everything without leaving your session Multi-language SDK support — Node.js, Python, Go, and .NET — means you can write extension logic in whatever you're already good at This isn't an incremental update. It's the extension system becoming what it always should have been. What Changed: The Short Version If you've read my original extensions guide and just want to know