Their entire company owes its history to mods.

HL's engine GoldDrc was originally a mod for Quake. Team Fortress Classic was based on a quake mod. Counterstrike was a HL mod they bought out. Portal was a student game they bought. Dota 2 was based on a WC3 map. Left 4 Dead was a mod made by Turtle Rock while working on CS:CZ (so, yet again a mod, although a mod based on their own engine this time and build in house). Underlords was based on a Dota 2 mod.

Deadlock is original, but based on characters and lore from the game they made from the WC3 map.

Deadlock and L4D are arguably the only true original creations.

Valve knows their bread is buttered by outside creation using tools and platforms they can provide and then fold in if it catches their attention.

> HL's engine GoldDrc was originally a mod for Quake.

GoldSrc is based on Quake 1 code with valves own modifications and a little Quake 2 added in, if I remember correctly. I wouldn’t call that a “mod”, they bought a commercial license for the engine and made a game with it.

You’re trying to use this to say that valve are unoriginal? I really don’t think that’s a criticism you can lob at the half life series.

I think we'll see some more creativity with S&box soon as well!

You are confusing an engine and an idea.

GoldSrc is a continuation of Q1 engine but it's development is of separate lineage even from Q2 and it was a fully licensed agreement. Setting and ideas are all original for HL.

TFC is a re-imaging of TF from Q1 but it's codebase is separate from Q1 TF.

TF2 is a sequel developed in-house.

HL2 is a series of sequels developed in-house.

EDIT: Portal has the same core developers and the same game mechanics, but both the setting and script are Valve original.

Sure, Steam pivoted their path of a game developer studio to a game publishing house but that's doesn't mean they never did anything themselves.