Reverse engineering for mods
Rule zero: read the real thing, don't guess. Decompiled code, the actual data, a live memory view or a
GPU capture is the spec. Write what you learn into MODLOG.md (names, IDs, offsets, formats) as you go.
Keep all decompiled output and extracted assets outside the mod repo (e.g. ~/<game>-decomp/) and never
publish them. Single-player, offline or servers the user runs. Never attach debuggers or scanners to games
with anti-cheat (see skills/mod-any-game/references/safety.md).
Pick the tool by what the code is (um scan tells you)
Managed .NET (XNA/FNA, Unity Mono, most indie C#)
dotnet tool install -g ilspycmd, thenilspycmd -p -o ~/<game>-decomp <Game>.exe(orAssembly-CSharp.dll). This gives a full C# project you can grep.- dnSpyEx: step through with a debugger, set breakpoints, edit methods.
- MCP: ILSpy-MCP / dnspy-mcp let an agent query types and methods directly.
Unity IL2CPP
- Cpp2IL (or Il2CppDumper) on
GameAssembly.dll+global-metadata.datgives types, fields, method signatures and addresses, plus dummy DLLs for ILSpy. - Method bodies are native: load
GameAssembly.dllin Ghidra/IDA and apply the generated script to name functions. - Live: UnityExplorer, or
il2cpp-frida-mcp.
Java (Minecraft, Slay the Spire...)
Vineflower / CFR / Recaf. For Minecraft, use Loom genSources with Mojang mappings.
Native C/C++ (custom engines, Unreal game code, console recomps)
- Ghidra (free) or IDA, driven through MCP so the agent can decompile, rename, retype and follow
cross-references:
- Ghidra: GhidraMCP (LaurieWired), pyghidra-mcp (headless, with a run-script tool; code-capable tools beat hundreds of tiny ones), ReVa.
- IDA: the official Hex-Rays IDA MCP (IDA 9.4+ Pro/Home; "code mode" runs IDAPython), or ida-pro-mcp.
- Binary Ninja and radare2 have MCP servers too.
- Workflow:
- Strings, then cross-references, then the function.
- Name and type everything you understand. Renames accumulate into a readable program.
- Confirm dynamically (below) before building on a guess. Agents can confidently misidentify things.
- Unreal: dump the reflection data first (UE4SS dumper or Dumper-7); it names most gameplay classes and properties for free.
Dynamic / live
- Cheat Engine: value scans → "find out what writes to this address" → struct → owner. CheatEngine MCP servers exist.
- x64dbg: breakpoints and tracing (x64dbg-mcp; bind it to 127.0.0.1, since some default to 0.0.0.0).
- Frida (frida-mcp, frida-game-hacking-mcp): hook functions from JavaScript, log arguments.
- ReClass.NET rebuilds structs from live memory.
Graphics
- RenderDoc (renderdoc-mcp) captures a frame and shows every draw, the render targets, the constant buffers (view/projection matrices) and the depth buffer.
- That's how you find where to inject geometry or effects (mashup-mods), and which texture holds a sprite atlas.
Data files and asset formats
Use the community tool first:
- Unity: UABEA, AssetRipper
- Unreal: FModel, UAssetGUI
- Bethesda: xEdit, BSArch
- GameMaker: UndertaleModTool
- Genie: genieutils
- Source: VRF, Crowbar
- FromSoft: WitchyBND, Smithbox
- Godot: GDRE Tools
Before reversing a format yourself, search the community archives. Most game formats from the 2000s and 2010s were worked out there:
- XeNTaX (closed in 2023): the forum, wiki and attachments are on archive.org, with a public backup on GitHub;
- ZenHAX: game file research and QuickBMS scripts (posts up to early 2023);
- The definitive guide to exploring file formats (Mr. Mouse, XeNTaX): the classic tutorial on reading an unknown format.
Use them for format knowledge and tools, whatever its origin (betas, leaked SDKs). Don't download game files, leaked code, SDKs or license keys from them.
For an undocumented format:
- Collect several stock files. Compare sizes, and hex-dump the headers (
xxd | head). Look for magic numbers, counts, offsets and tables of fixed-size records. - Form a hypothesis for the header, then the frame/record layout, then the compression. Check it by parsing every stock file without errors.
- Write a reader that decodes to something viewable (PNGs, JSON) and look at it.
- Write the writer, and prove it with a round trip: decode → encode → decode, compared against the
original. The AoE2 SLD sprite writer was accepted only when it round-tripped the stock knight at
0.9/255 mean error (
examples/aoe2-de-civ/sld.py). - Only then write new files. Test them in game with one asset before batch-converting.
Make it an oracle
- Engine logic you port (for a simulator, a trainer, a reimplementation): record real traces from the
game (positions, velocities per tick) and replay them against your port. In the Terraria Eye of Cthulhu
work, the Eye's velocity matched 99.9% once the port used the action applied on frame t+1. Float32
constants mattered too (
0.2f≠0.2). A leftover mismatch was traced to a hidden buff (Happy!, x1.21 move speed). Replays find what reading the code misses. - For long RE runs:
- a journal file;
- small verified steps;
- cap attempts per problem (about 3 identical failures, then change approach);
- commit every confirmed fact.
