"s&box ships a headless dedicated server that can run an unpublished local project"

You want to run a dedicated server for your s&box project during development but assume you need to publish the package first, or you're searching for Garry's Mod/Source-1 style server setup (sv_tickrate, workshop.txt, dm_crossfire) that doesn't apply to s&box.

s&box has its own first-class headless dedicated server — a separate Steam app (1892930, distinct from the client 590830). It installs via SteamCMD with anonymous login and runs headless (no GPU/render window). Pointing +game at a local .sbproj path makes connecting clients stream and hotload the project's code and assets straight from the server — the sanctioned dev/iteration workflow, and the only supported way to run server-only code (published sbox.game packages have server code stripped).

Discovery is via Steam lobbies / the multiplayer UI, not a paste-an-IP server browser. The dedicated server surfaces to clients when Networking.QueryLobbies(filters, includeServers:true, ct) is called with includeServers = true (a bool most lobby code leaves false), then Connect(id) joins it.

Install and launch

# SteamCMD (anonymous login works)
steamcmd +login anonymous +app_update 1892930 validate +quit

# Launch headless
sbox-server.exe +game "C:/path/to/project.sbproj" +hostname "My Dev Server"

Optional flags: +net_game_server_token <tok>, +port <n>, +net_query_port <n>.

Traps to know

1. Refresh SteamCMD metadata first: +app_info_update 1 +app_info_print 1892930. Without the refresh, the install may return Missing configuration. After the refresh, it installs normally. Prefer the branch matching your installed client build.

2. +game <path> loads the project but does NOT activate networking. The game code must call Networking.CreateLobby itself, or it silently runs a headless single-player world. A dedicated process cannot call Networking.QueryLobbies (unable to query lobbies on a dedicated server), so its host path must skip lobby-search flows and create/advertise directly.

3. Steam drops custom Networking.SetData when aggregate game tags exceed 128 characters — the log reads Steam game tags exceed 128 char limit ... Data dropped. Use tiny discovery metadata (e.g. g=x,p=1,d=1).

4. Compare .version to clients. An older public build may not recognise +game — always verify the server and client builds match. Check for updates before reporting issues.

5. net_game_server_token (persistent server Steam ID) is documented as available "once the game is publicly released" — whether a local/unpublished project gets a usable token and shows up in global discovery is unverified. Test before relying on it.

The dedicated server app is a headless instance of the same engine, running the same game assembly. Pointing it at a local .sbproj means the server has the full unstripped project (including server-only code that sbox.game publish strips), and connecting clients receive the code and assets over the network — no publish/package step needed during development.