physics_npc_solver

This entity is not in the stock FGD files by default. It should not be placed (in Hammer) because it can't be configured with keyvalues.

Code classes:

• CPhysicsNPCSolver
• CLogicalEntity
• CServerOnlyEntity
• CBaseEntity (all ents)

physics_npc_solver is a point entity present in all Source games. It's an entity used to keep physics objects and NPC's from looking weird when they touch. This is what causes physics objects to fall off an NPC's head.