Impressive.
Shame Valve still hasn't open-sourced the GoldSource engine yet, though I suppose Nexon and the Sven Coop lead dev have paid licenses that they still want to extract value from.
Everything's open source in the age of LLM-assisted Ghidra...
There is an open Half-Life 1 SDK on Valve's GitHub [1], not sure if it's missing something regarding the engine.
[1] https://github.com/ValveSoftware/halflife