Install

Mods load through MelonLoader. Install it into Cairn once, then each mod is one .dll in the Mods\ folder.

Close Cairn before adding or updating mods — it holds each mod file open while running.

1 · Locate the game folder

Steam → right-click Cairn → Manage → Browse local files. The folder with Cairn.exe:

C:\Program Files (x86)\Steam\steamapps\common\Cairn\

2 · Install MelonLoader

  1. Get MelonLoader.Installer.exe from MelonLoader releases.
  2. Run it; point Select at Cairn.exe.
  3. Latest stable → Install. Patches in place.

3 · Launch once

Run Cairn from Steam. MelonLoader generates its folders on first boot — then quit. Layout:

Cairn\
├─ Cairn.exe
├─ version.dll      (MelonLoader hook)
├─ MelonLoader\     (loader + logs)
├─ Mods\            (drop .dll files here)
└─ UserLibs\        (shared libs, if a mod needs one)

4 · Add a mod

  1. Download a mod's .dll from its page (e.g. CairnRoutes).
  2. Make sure Cairn is closed.
  3. Copy the .dll into Mods\.
  4. If the mod lists a Requires library (e.g. CairnAPI), drop that into Mods\ too.
  5. Launch. It loads at startup — check the mod's keys (F8 routes, F6 beats, F9 tools, F4 co-op).

Verify

MelonLoader\Latest.log lists each mod by name and version as it loads.

Trouble

No mods load
The .dll must be directly in Mods\, and version.dll must sit next to Cairn.exe. Re-run the installer if not.
Missing dependency
Add the library named in the mod's Requires box to Mods\.
Copy fails — file in use
Cairn is still running. Quit fully (both windows in co-op), then copy.
Remove a mod
Close Cairn, delete its .dll.
Uninstall everything
Installer → Un-Install, or Steam → Verify integrity of game files.