Someone asked about making all the zombies headcrabless, to better fit the RE2 theme.

Researching this, I found that there really isn't a simple way of going about this. Most of the advice is editing the SDK itself for those making a sourcemod. Little explained how to do anything of the sort in vanilla Half Life 2.

I was able to sort-of do this using nothing but the console, but it's a pretty janky solution.

I can't guarantee this won't cause issues, especially if you can't run the game at a smooth framerate, as it involves a timer firing a couple times a second.

Use at your own risk.

1. Open steamapps/common/half-life 2/hl2/cfg
   
   
2. Create a new cfg file here, name it whatever. For example, "headcrabless.cfg". If you need to, make a blank txt file and change the extension to cfg. It will still open in Notepad.
   
   
3. In the file, paste the following code:
   
   
   ent_create logic_timer refiretime 0.5 startdisabled 0 ontimer"npc_zombie*,SetBodygroup,0" ontimer"npc_headcrab*,Kill" ontimer"npc_fastzombie*,SetBodygroup,0"
   
   
4. Save the file.
   
   
5. Open HL2 and go into a game. If you play RE2:S, you'll spawn next to quite a few zombies, making for a perfect test.
   
   
6. Open the console (the game should pause) and type in "exec NAME" where NAME is the name of your cfg file, without the extension. (For my example, I'd type "exec headcrabless")
   
   
7. Close the console, resume the game, and observe your handiwork. If you did it right, the zombies around you should change to be headcrabless.
   
   
8. The entity only lasts for that map, so when you transition to a new map, you'll need to execute the cfg file again.

It creates an invisible logic_timer entity. If you do any mapping then you probably already know the logic entities and what they do, but if you don't, fret not. It isn't terribly important for an end user. Suffice to say, this one is set to fire every half second. If it makes your game lag, you can try making it fire every second, every 2 seconds, whatever. Of course, the longer the refire, the more a headcrab can exist before being thanos-snapped.

Anyway. Every time the timer fires, it sets any npc_zombie or npc_fastzombie to have its bodygroup changed to hide the headcrab. Then it looks for any npc_headcrab (which should include fast headcrabs, but also maybe poison headcrabs which might not be intended, but oh well...) and delete it in an instant. So if you kill a zombie and it spawns a headcrab, it will vanish before it can do anything to you.

But yeah, again, this is VERY jank and if it causes problems, well, you were warned.

Edited out the script bit about turning cheats on and off because, apparently, you don't need cheats on to create entities. Neat!
I'm too used to multiplayer games like No More Room In Hell or Left 4 Dead 2 where doing such things would be locked behind cheats for obvious reasons. Didn't occur to me that in this singleplayer game they wouldn't have bothered.