[klippy]

HLSDK is just a base for coding your HL mod (it's Half Life Software Development Kit), it's not directly found in the game and mods (like CS) use it as a base and expand upon it. That's what developers use to develop the "mod library" (those binaries in $moddir/dlls and $moddir/cl_dlls).

Anyway, to code something like Metamod you only need interfaces between the engine and mod library which can be found in HLSDK, not the binary itself.