In order to open the developer menu (dev menu), you need to enable it. To do this, right click on Half-Life: Alyx in your steam library and click on "properties".




Next, click on "set launch options".




In the window that opens, type "-dev" without the quotation marks and click "ok".

To use the dev menu, start Half-Life: Alyx and load into a saved game or start a new game. After loading into the game, pause the game. There should be an option at the bottom of the pause menu called "dev menu". Select it to open the dev menu.




This is what the menu looks like open. It will replace your normal pause menu until you revert back to it.




To go back to the default menu, on the dev menu select "game menu" at the bottom.




Then select "proto menu". It will then return to the default menu.