Half-Life Cheats

Load game with the command line: hl.exe -dev -toconsole. Then, press ~ if the orange text on the screen does not disappear. During gameplay, press ~ to display the text prompt and type in sv_cheats 1. Press ~ and enter one of the following console commands

Single player

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Multiplayer/Deathmatch

boot_camp
bounce
datacore
lambda_bunker
snark_pit
stalkyard
subtransit
undertow

Hazard course

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item_airtank
item_antidote
item_battery
item_healthkit
item_longjump
item_security
item_sodacan
item_suit
ammo_357
ammo_9mmAR
ammo_9mmbox
ammo_9mmclip
ammo_ARgrenades
ammo_buckshot
ammo_crossbow
ammo_egonclip
ammo_gaussclip
ammo_glockclip
ammo_mp5clip
ammo_mp5grenades
ammo_rpgclip
weapon_357
weapon_9mmAR
weapon_9mmhandgun
weapon_crossbow
weapon_crowbar
weapon_egon
weapon_gauss
weapon_glock
weapon_handgrenade
weapon_hornetgun
weapon_mp5
weapon_python
weapon_rpg
weapon_satchel
weapon_shotgun
weapon_snark
weapon_tripmine
weapon_quantumdestabilizer

Start the game with one of the following command line parameters

You must be within the loading area of the desired monster for this cheat to work. For example, you cannot load up Barney while fighting the Nihilanth. Enter give <monster name> and the corresponding monster will appear. Note: Activate the /noclip first, or you will be stuck inside the monster.
Zelter

People

monster_human_grunt
monster_barney
monster_scientist
monster_generic
monster_gman

Monsters

monster_sentery
monster_headcrab
monster_alien_slave
monster_alien_grunt
monster_bullchicken
monster_zombie
monster_apache
monster_ichthyosaur
monster_bigmomma
monster_barnacle
monster_tentacle
monster_gargantuan
monster_nihilanth
monster_houndeye
monster_rat
monster_cockroach
xen_tree
xen_spore_small
xen_spore_medium
xen_spore_large

Corpses

monster_barney_dead
monster_scientist_dead
monster_hevsuit_dead

You can enter bind "p" "say Hello" or bind "p" "teamsay i need backup" and press [Enter] at the console window to bind those and other chat commands.

To activate codes easier, bind them to keys. For example, type /bind v "impulse 101" at the console window to bind the "HEV suit, weapons, and ammo " code to the "V" key.

In the first level, enable the /impulse 101 code and go to either one of the two security officers (Barneys) guarding an area. Use the /impulse 76 code to spawn a Grunt near them. Do not spawn the Grunt in a wall or Barney or the game will end with an error message. Kill the Grunt quickly and leave it on the floor for a few seconds. Barney should say "Ew, what's that stench?" or something similar.

Normally when you want to spawn Barney at the Nihilanth episode, you will get an error message. To solve this problem, go to Lamba Core teleport to Xen. Enable the /noclip code. After the scientist finishes his teleport to Xen, go through and dark. Then, type give monster_barney repeatedly until you arrive. You will go to a strange island and not have a Barney, but you must type it. After that, you can play and fight with Barney at the alien world. You also can do this at Nihilanth teleports. The v1.1.1.0 patch no longer allows this to happen.

When you enter the room where the lasers are shooting out the walls, run through. There should be a Barney (security officer) on the floor. Watch carefully and you can see the laser go across and literally cut him in half.

When you begin a game and get your suit, look at the lockers. One will be marked "Freeman" (your character's last name). Walk straight to it and you will see a photo of his little boy or girl.

On the level where you are picking up your HEV suit before the accident, enable the /impulse 101 code and select the crowbar. Walk up to a scientist and wait for him to do a scripted event such as scratching his chin while looking at computer readings. Keep hitting him with the crowbar. When the scripted event ends, depending on how many times you have hit him, there will be an explosion of meaty chunks.

Enable cheat mode, then use the /impulse 76 code to activate the Grunt-O-Matic. A message to start a new game in order for it to work will appear. If you want to continue where you currently are, just quicksave, then quickload the game. Once you have quickloaded, type in the code again to spawn a Grunt. Note: You must have the /nocliping code on before you spawn the grunt because he will be spawned close to you. If this is not done, a message stating that the monster spawned is stuck in a wall may appear and the game will end. Do not spawn the Grunt in a closed position or the same message will appear. The Grunt you spawn will see you as a hostile. To have more fun, use the /notarget code and spawn a number of grunts to do your alien hunting.

In multi-player on LAN games, create a game and choose a level with the Zapper in it. This is the gun that you hold the Right Mouse Button to charge and shoot. Aim it at the ground and you will jump high. Note: Be careful where you aim when you fall because you will lose health. This does not work in Story mode.

In the "Apprehension" level, you will reach a part where you go through two red doors reading "CAUTION" that lead into an area with a large sunken in area, two military officers, and a turret gun. Kill the two officers (you can use the turret gun if desired). Then, turn and face the direction opposite the wall with the door and the large window. Enable the "No Clipping" code and walk forward through the natural wall. You will see a large yellow shaft. When you get closer, you will see a huge rocket inside.

Do not waste precious pistol ammo on headcrabs. Use your crowbar, as it is more useful.

Enable the console window and go to the Hazard course. When you reach the bridge to get the hazard suit, jump of it into the sludge. You should be teleported to Nihilanth automatically.

As soon as you enter the "We've Got Hostiles" level, turn to your right and use the /noclip code to float through the glass window with the security guard being eaten by the zombie. When you are in there, press the switch that opens the Silo and the Silo door will open. Float back out of the room, disable the noclip code, then go through the Silo door to start the next level.

Get the Crossbow in single player mode and do a quick save with the weapon that you want to zoom in with. Change to the Crossbow and zoom in, then quick load and your other weapon will be zoomed in. You can now change weapons and they will all be zoomed in. To come out of zoom, change back to the Crossbow and press the second fire button until it zooms out again.

During the mission 'On A Rail' (the part of the game where you have to stop the rockets from firing and your in the sewers or something on a trolley). Turn on 'noclip' and float towards the top of the rocket. once you get to the top you can see a pic of a developer of the game. its his face tiled about 50 times on a square. there is a ladder at the top that brings you up another level with the same pic.

P.S - this Easter Egg is only reachable using 'noclip'. there is no other way to get up that high.

A technique used in a lot of Ego-Shooters is the Rocket-Jump where you point a Rocket Launcher at the ground and fire off a rocket while jumping to be able to jump way higher than usually. The downside is that you lose health by doing this and since a rocket in Half-Life takes away lots of health it wouldn't really be of good use.

Instead you can exploit another possibility in Half-Life: Using the secondary fire mode of the Gauss Cannon instead of the rocket launcher will also launch you high into the air but will not directly deal any damage (only eventual falling damage).

Put Trip Mines onto a wall in form of a small staircase. You can actually jump on them and even use them to reach places you're not supposed to reach just yet or even places that aren't supposed to ever be reached like roofs that can be used as shortcuts.

Use a Snark (a bug) when facing a wall and jump onto it. Directly throw another one and repeat until you've reached the height you wanted to reach. This way you are able to reach places you're not supposed to reach just yet or even places that aren't supposed to ever be reached like roofs that can be used as shortcuts.