Fix GMod "App ID Specified by Server Is Invalid"

The message "Unable to connect to server, app id specified by server is invalid" comes from Steam itself, in a small popup, and it reads as though your server is at fault. In reports on Valve's own issue tracker and on the Steam client beta forum, for other Source and Steam games, it appears after a steam://connect link. This guide gets your players in, then rules out the one server-side setting worth checking: your Steam game server token.

Where this message comes from

Two Valve-side reports describe the same popup:

• A bug report on the Steam Client Beta group from August 2023, where steam://connect/IP:PORT links stopped working. Players there named ARK, Conan Exiles, 7 Days to Die and Team Fortress 2.
• An issue on Valve's Source-1-Games tracker, where a steam://connect link to an SDR-enabled Team Fortress 2 server gave the same message.

Neither report mentions Garry's Mod or a token. We can't say these reports explain every case, but they show the popup can follow a steam://connect link, so joining another way is the quickest test.

Join from inside Garry's Mod instead

Joining with the game's own console does not go through a Steam link:

1. In your server's panel, copy the Server address from the left rail, under the server's name. Take the whole thing, including the number after the colon.
2. Open Garry's Mod and open the Developer Console. GMod server: connect, Workshop, gamemodes shows how to switch it on.
3. Type connect <your address> and press Enter, for example connect 27.124.105.40:27015.

If that gets you in, hand the same address to everyone who gets the popup.

Rule out the Steam token

Garry's Mod servers should run with a Steam Game Server Login Token (GSLT), and the token has rules of its own. If you made one yourself, check these against Facepunch's wiki:

1. App ID. The token must be created for App ID 4000, which is Garry's Mod. Facepunch says the "App ID of the base game" field should always be 4000. The number 4020 is what the wiki uses for downloading the dedicated server, so it is the wrong number for a token.
2. One token per server. Facepunch says each server must have a unique token. Using one token on two or more servers is the likely cause of Connection to Steam servers lost. (Steam error code 6) and players being disconnected.
3. Still valid. Changing your Steam password invalidates all of your tokens. An invalid or expired token prevents the server from starting, so a token problem shows in the Console on boot.

To check or replace yours, open the Startup tab and find the Rank in the server browser card. It shows your saved token (only its last four characters), with Change and Remove buttons, or walks you through creating one. After saving, restart the server.

If the Console prints No account token specified; logging into anonymous game server account on boot, no token is set. Facepunch says servers without one suffer a severe penalty in the server list ranking.