For Minecraft 1.21.1 / Fabric.

Requirements: Java 21; Fabric Loader 0.15.10 or newer; Fabric API 0.116.17 or newer (which itself needs Fabric Loader 0.15.11 or newer). Optional: Trinkets 3.10.0 or newer, for an accessory head slot; Mod Menu, to open the settings screen.

Install this JAR on the server and on every client. Singleplayer needs only your own copy.

Spoor 0.1.0

First release. Install it on the server and on every client. Singleplayer needs only your own copy.

What is in it

• Animals, mobs and players leave fading footprints on soft ground. Standing still, flying, swimming and riding leave none.
• Footprints have shapes for hooves, paws, bird feet, boots, mob pads and more. Babies leave smaller marks.
• Crouching leaves shorter, fainter prints. Drops of two blocks or more leave landing marks.
• Rain and snow wear prints away faster.
• Tracker Goggles: craft them with one leather, one stick and one compass. Wear them on your head, or in an accessory head slot with Curios (NeoForge and Forge) or Trinkets (Fabric).
• The goggles show species, age, condition and gait for the track you look at, tint tracks by animal, mob or player, and show a faint scent line leading to the next step.
• Without goggles, prints stay clear for 60 seconds and fade out by three minutes. With goggles, they last the full ten minutes by default.
• Who sees tracks: Everyone (the default) or Goggles only.
• Settings screen: open it from the Mods list (NeoForge and Forge) or Mod Menu (Fabric). Client settings can always be changed there. The singleplayer or LAN host can change the server settings there too; on someone else's server they are read-only.
• Settings files: the client file is config/spoor-client.toml. The server file is config/spoor-server.toml on NeoForge, one file for every world on that installation, and serverconfig/spoor-server.toml inside each world on Fabric and Forge.
• Creatures from other mods leave tracks too.
• Tracks are kept in memory only and are not saved across restarts.

Bug reports are welcome in the comments.