What
The SDK is the kit that lets other programs drive Claude Code. Slash commands are commands typed with a leading /. When a slash command runs locally, its result message now carries a commandOutcome field next to commandRun. The outcome has one of these kinds, and can also carry an optional suggestion:
failed: set when the command wrote error output.unavailable_headless: set when the command cannot run in headless mode, meaning without the interactive screen.unknownrestart_required
The field is sent in the SDK stream and read back from local_command_outcome.
Why
A program using the SDK can now tell a failed or unavailable command apart from one that worked, without parsing the output text.