Everything in GUO belongs to one of five layers. The layer decides what a change must prove before it is merged, whether it is on by default, and whether it ships in the game you download. The owner approved the layers on 2026-10-13; the binding text is the layer table in AGENTS.md and CONTRIBUTING.md.

• L0 is held to parity: it must behave exactly as ClassicUO does. Any deliberate difference carries a PORT DEVIATION (GUO) marker, and docs/upstream/GUO_DIVERGENCES.md lists them. See Parity and Drift.
• L1 options are off by default and need a profile field, an Options entry, proof that with the option off the game draws exactly as before, a scripted scenario and a section in docs/data_formats.md. Anything that changes how the world is drawn or how input works also needs an ADR. An option may not start another program, run Python or call a network service. Enhanced HD and Regional audio and recorded NPC voices are L1 options.
• L2 packs are typed content only: no code, and nothing runs when one is installed. See Content Packs.
• L3 editor code is compiled only into the editor (#if TOOLS), and every AI feature also sits behind the editor's AI switch (Editor Settings > GUO > AI > Enabled). With it off, AI menus are hidden and no AI work runs; manual authoring always stays. See Editor.
• L4 Labs live in godot/GUO/src/Labs/<feature>/ and are compiled only into a Debug build made with -p:GuoLabs=true. Release and export builds leave them out even when the flag is set. Each Lab needs a README, a doctor launcher that says what to install, a scenario and a smoke line. The registry is empty today: no Lab ships yet.

1. A new chat command, profile field, Options entry, shader or edit to ported code starts with a short proposal or ADR that the owner decides. The first question: would ClassicUO ship it? If not, it belongs in a pack, the editor or Labs.
2. One feature per pull request, about 1,500 lines or 20 files at most unless agreed first.
3. Generating content belongs in the editor or Labs; the game only reads packs.
4. Anything that needs an outside program has a doctor launcher, and its failure text names that launcher.
5. Every L1 and L4 feature has a scenario under tools/scenarios/ and a line in the smoke.
6. Content that can be a pack goes to the Store.
7. Contributors and agents fill in the same review template: layer, toggle, parity proof, scenario, doctor and docs section.

dotnet build godot\GUO\GUO.csproj -c Debug -p:GuoLabs=true

godot/GUO/src/Labs/README.md explains how a Lab registers its chat commands and Options rows.

• Content Packs: what an L2 pack may carry
• Contributing: how to send a change
• Architecture: how the port is put together