Stay legal

A matching decomp contains source code that people wrote by studying a game. The game itself must never be part of the repository. Projects that respect this line are the ones that last.

This is not legal advice. Laws differ between countries; when in doubt, ask someone qualified.

The short version

No game files in the repository. No ROMs, disc images, executables, or extracted files. Each contributor provides their own copy, and .gitignore keeps it out of commits.
No assets. Textures, models, music, text dumps and videos stay out, even “just a few for testing”.
No leaked or proprietary code. Don’t copy from leaked source or SDKs into the project, and don’t build on documentation you have no right to use.
CI secrets stay private. If your CI needs the game, keep it in a private place (a private repository, a private container image, an encrypted secret), never in a public artifact or log.

What this page will cover

Why the original binary is needed to build, and how projects keep it out of git.
Assets: extracting them at build time instead of committing them.
SDK headers and libraries: what projects usually do.
Leaks and clean-room work: why projects refuse leaked material.
Licenses for your own code.