Slash command

A slash command turns the composer into a command line without making it look like one. Typing a forward slash at a word boundary opens a menu anchored to the caret, each further character filters it, and choosing an entry runs the command. The crucial half of the definition is the second one: the typed characters are consumed rather than sent. A slash command is not a shorthand that expands into text, it is a command whose name happened to be typed where the message goes.

The trigger has to be careful, because the slash is a real character people type. The conventional rule is that it counts only at the start of a line or after whitespace, so a date, a fraction, or a path never opens the menu, and that any whitespace closes the query again. When the menu is open the arrow keys and Enter belong to it rather than to the composer, and Escape has to give them back without deleting what was typed. Get that last part wrong and the pattern becomes hostile: a reader who opened the menu by accident should be able to dismiss it and carry on writing the sentence they were in the middle of.

It is worth being precise about the neighbours, because they are all the same machinery pointed at different things. Mention autocomplete is the identical mechanism keyed to @, and the difference is what it produces: a mention leaves a token in the message, while a slash command leaves nothing and does something instead. A command palette is the global version, opened with a keystroke from anywhere and acting on the application rather than on the caret. Ordinary inline autocompletion is a third thing again, since it is completing what you were already writing rather than offering a list of actions. In a rich text editor the slash menu has largely replaced the block-insert toolbar, which is why so many of its entries are structures (heading, list, table) rather than verbs.

Two design details decide whether it feels good. Reserve the menu’s space by drawing it over the document rather than pushing the composer around, because a composer that jumps while someone is typing into it is unusable. And keep the entries short and stable: the whole value of the pattern is that a practised reader stops reading the menu, types three characters and Enter, and never looks. That only works if the ordering is predictable, so resist reordering by recency unless the list is long enough that scanning it was the cost in the first place.