Terraria is where Universal Modder began. Its first worked example, Fal Arsenal, adds a homing missile launcher, a tactical nuke that craters the world, a chain-lightning rifle, a black-hole gun, three enemies and a two-phase Drone Mothership boss, with every sprite generated by fal.
Updated 2026-09-28
Field notes2
Working / released2
Engine.NET / XNA / FNA
RouteMod-loader API
How Universal Modder approaches Terraria
Terraria runs on .NET (XNA/FNA), so the route is tModLoader: free on Steam for Terraria owners. The agent decompiles with ILSpy to read how vanilla items, projectiles and NPCs work, writes C# mod classes, and verifies in a throwaway world. Sprites come from fal, cut out and fitted to Terraria's exact sizes with um sprite.
Loader / tool
What it's for
tModLoader
The official-community loader, free on Steam for Terraria owners.
Versions that worked:Terraria 1.4.5.8 (decompiled), tModLoader 1.4.4.9 base for the in-game agent | Terraria 1.4.4.9 via tModLoader 2026.07 (GitHub build), Steam; vanilla 1.4.5.8 decompiled for reference
The Eye of Cthulhu fight was ported from Terraria's decompiled source into a vectorized simulator: boss AI, servants, player physics, gun and damage rules. A PufferLib PPO agent trained in it (99.7% win rate in the sim), and the policy was exported to C# and runs inside the real game through a tModLoader mod. Real traces were replayed against the sim…
Versions
Terraria 1.4.5.8 (decompiled), tModLoader 1.4.4.9 base for the in-game agent
Anti-cheat
none (single player)
Built by
Claude Code (Opus 5.5)
Date
2026-09-28
6 gotchas recorded
Trace replay disagreed on every dash.
The numba sim slowly diverged from the numpy one.
The player moved faster in the real game than in the sim.
Five weapons (homing missile launcher, a tactical nuke that craters the world, a chain-lightning rifle, a black-hole gun, an orbital strike), three new enemies and a two-phase Drone Mothership boss. All sprites were generated with fal. It's built as a tModLoader mod, tested in game, and recorded for an X video that got ~400k views. Code:…
Versions
Terraria 1.4.4.9 via tModLoader 2026.07 (GitHub build), Steam; vanilla 1.4.5.8 decompiled for reference
Anti-cheat
none (single player)
Built by
Claude Code (Opus 5.5)
Date
2026-09-28
7 gotchas recorded
Recording from inside the game leaked 17 GB of memory in one take.
SDL_AUDIODRIVER=disk recorded nothing.
The recording's tail was lost.
Takes stopped matching.
tModLoader refuses to start.
The agent killed its own shell.
Sprites came out with grey fringes or a floor shadow.
Is Terraria a good first game for Universal Modder?
Yes. tModLoader is mature, the code is readable .NET, and there's a full worked example in the repo to learn from.
Can Universal Modder mod Terraria?
Yes. The Universal Modder knowledge base has 2 field notes for Terraria, and 2 of 2 are marked working or released. Routes used: Mod-loader API.
Which AI agents have modded Terraria?
Claude Code. Universal Modder works the same way in Claude Code, Codex, Cursor, Gemini CLI, Copilot and OpenCode.
Is it safe to mod Terraria with an AI agent?
Keep to single-player, offline modes or servers you host, back up saves first (the agent runs um backup), and never ship game files. The anti-cheat notes recorded for this game: none (single player).