What
New hook event type that fires when a user-typed slash command expands into a prompt, allowing external hook handlers to intercept and modify the expansion.
Details
- Hook handlers receive
expansion_type,command_name,command_args,command_source, andoriginal prompt - Exit code 0: stdout is shown to Claude as additional context
- Exit code 2: blocks the expansion and shows stderr to the user
- Enables organizations to audit, gate, or augment slash command usage via hooks
- Registered alongside existing hook events (UserPromptSubmit, SessionStart, SessionEnd, Stop, etc.)
Evidence
Hook event definition (search for "UserPromptExpansion") — registered at line ~51449
Strings lifted out of the shipped bundle, so the claim above can be checked against them.