Instant input-source switching for macOS, Caps Lock, rerouted to a spare key at the HID layer.
A single Swift Package, builds with the swift CLI alone, no Xcode project required.

What it does

macOS can switch input sources with Caps Lock, but that path carries a deliberate delay (it has to distinguish a tap from engaging Caps Lock). The classic fix is to remap Caps Lock to a spare function key and bind "Select next source in Input menu" to it, usually via Karabiner-Elements, which runs a virtual keyboard driver and daemons for the privilege.

Keystone does the remap the way hidutil does: it books a UserKeyMapping entry into macOS's own HID event system, where the kernel rewrites Caps Lock → F19. No virtual driver, no event tap, no Accessibility or Input Monitoring permission, no process of Keystone's ever sees a keystroke. The app itself just sits in the menu bar and re-asserts the mapping when the system would forget it (wake from sleep, a keyboard reconnecting).

The ⌘ and ⌥ keys, either side, can switch too. Each key has its own action (next input source, or one specific source) and its own trigger: switch on a lone release, keeping the key a modifier so every shortcut still works, or switch the instant it goes down, giving up its modifier role. The instant kind is rerouted in the kernel like Caps Lock, but to nothing: no function key exists, so no other app or system shortcut can ever see the press. Keystone watches the physical key below the remap, at the HID device layer, and switches through the Text Input Source API. That needs the Input Monitoring permission and stops during secure input (password fields, Terminal's Secure Keyboard Entry). Caps Lock can take that path as well, when a specific source is wanted; the system shortcut path is kept for one key at a time because that is where its permission-free, secure-input-proof behavior comes from, and it is the only path with a function key.

Setup

1. If Karabiner-Elements was doing this job, uninstall it first (its settings window → Uninstall), quitting is not enough, its daemons keep remapping.
2. Launch Keystone and follow the onboarding.
3. In System Settings › Keyboard › Keyboard Shortcuts… › Input Sources, record "Select next source in Input menu" by pressing Caps Lock (it already types F19).

Settings

• General: launch at login, hide the menu bar icon, updates.
• Remapping: the master switch, then one section per key (Caps Lock, left/right ⌘, left/right ⌥): action (off, system shortcut, next input source, or a specific source), how a modifier switches (on release keeping shortcuts, or instantly on press), the function key for the system-shortcut key (F13–F20), and hold-for-real-Caps-Lock.

Development

swift run                # menu bar app, dev build
./Scripts/dev.sh         # rebuild-and-relaunch loop
./Scripts/test.sh        # unit tests
./Scripts/bundle.sh      # standalone build/Keystone.app
swift run Keystone --language ko --settings   # force a localization (dev builds have no bundle ID)

Notes

• The remaps are active exactly while Keystone runs: quitting (or toggling it off) clears the mapping and hands every key back to stock macOS. Launch at login keeps it seamless across restarts.
• A force-quit or crash can't clean up after itself; the next launch, toggle, or reboot sweeps the mapping away.

License

MIT, see LICENSE. Bundled third-party software and its licenses are listed in THIRD-PARTY-NOTICES.md.