Setting the launch options to -vulkan or SteamDeck=1 %command% is required to start the game from the beginning, otherwise the game will crash upon loading the level. If the player loads farther into the game, the game will be playable, however there will be noticable graphical artifacts such as nonessential geometry disappearing and reappearing.

Note that setting the launch options to SteamDeck=1 %command% will remove your ability to delete save files, due to the feature being missing in the Steam Deck's variant of the UI used in Portal. However, SteamDeck=1 %command% -nogamepadui will cause your game to act the same as if there were no launch options at all. This still might be useful since the game still won't act irregularly in the main menu, but instability will still occur in-game.