#editor

A -joinlocal second instance connects to whichever process holds the loopback port 127.0.0.1:55333, so the precondition to check is socket ownership, not which editor is in play mode.

A -joinlocal second instance never runs the in-game Join UI: it connects straight to the editor's loopback dev-host socket, so it presents an empty invite code. Any host that verifies the code on the wire will correctly reject it, breaking the local two-peer test even though every other hop is healthy. It's a harness artifact, not a product bug.

When the editor console (or any text field) holds keyboard focus, every Input.* read in game code returns nothing, so a feature toggled from the console looks dead while the exact same feature works from its keybind. A debug camera in that state is pixel-identical to a broken one. Game code cannot take focus back; the fix is to make the silence loud.

A project resource type that claims a file extension the engine already owns can lose a registration race, with no error. The losing type's ResourceLibrary.GetAll returns empty and files deserialize as the engine type instead. Which side wins is decided per editor launch. Audit the extension with asset_types before you commit to it.

The editor can report a clean compile for both the game and editor compilers while the running assembly is still stale. A green status is necessary but not sufficient. Confirm the load with a symbol that is new this session.

Running a second s&box process under the same Steam account (an editor or dev instance alongside the published client) correlates with P2P joins to a live remote host failing rendezvous inside the engine's fixed ~3.5s connect budget. Close every second same-account instance before drawing any conclusion from a P2P connectivity failure.

A fresh git worktree of an s&box project cannot dotnet build, because the editor generates Code/<project>.csproj and gitignores it, and the csproj references the Steam engine DLLs by relative paths tied to the main tree's directory depth. A naive copy fails with MSB3245 when the worktree nests one level deeper. The durable fix rewrites the csproj with absolute engine paths and redirects OutputPath into an isolated folder. That second half matters everywhere: the default OutputPath is the shared .vs/output/ folder with no project segment, so any CLI build on the machine writes into the same directory. A sibling build there empties a running editor's TypeLibrary, and every game component reads as MissingComponent while compile_status still reports Success. Build headless with the editor closed and output redirected, or pass -p:OutputPath to keep the build out of the shared directory.

GameObject.Destroy() in edit mode is deferred; a query fired right after returns the previous build's objects.

The editor compiles and verifies its ProjectRoot working tree. A branch that lives only in a git worktree is invisible to it, and compile_status results silently reflect the wrong code.

The engine ships its built-in [McpTool]s as full C# source, not compiled DLLs: check addons/tools/Code/Mcp/*.cs for canonical patterns before inventing your own.

A runtime-generated world root torn down with GameObject.Destroy() keeps rendering in edit mode because the deferred queue isn't flushed: use DestroyImmediate and sweep all matching roots.

The editor discovers a library under Libraries/ only if the folder contains exactly one .sbproj file. A hand-vendored library without one is silently ignored, producing a wall of CS0246 errors on every type it defines.

An Input.config action bound to an editor/host-reserved key (F1-F3, F7, F8, Escape) silently never fires in Play. The capture map also differs between editor and published client -- bind game hotkeys to plain letters and verify in both.

If sbox-launcher.exe stays open during a Steam update, files vanish mid-install. Validate via steam://validate/590830.

A C# static list/registry survives editor Play stop/start and code hotloads, holding references to destroyed GameObjects: gate iteration on IsValid(), don't rely on clearing alone. It can also mask a new scene's state when stale high-priority entries outrank it. Hand registrations back in OnDestroy.

A structural .razor change can throw a hotload exception that deregisters the project's entire toolset while compile_status still reports Success. Restart the editor to recover.

The editor MCP port is one engine-global preference, not a per-project setting. When the configured port is already bound by another open editor, the second editor logs a bind failure and starts no MCP server at all; it does not auto-increment to a free port. Bump the port by hand before (or after) launching the second editor, and have agents discover the live port by scanning and matching the Project field instead of pinning a number.