Model Context Protocol: Servers, Clients, Tools, Resources, Prompts

Desk needs to look up an order. In one design, a Desk developer writes the lookup function inside Desk's harness, together with its description and its input schema. In the other design, the team that owns the order system runs a small separate program. Desk asks that program which tools it offers, and then calls the lookup through it.

The model sees the same tool in both designs. What differs is who wrote the description, how many other products can reuse the work, and how much Desk must trust a program that its own developers did not write.