An agreement about how two independently written programs talk—message shapes, sequences, promises—so that strangers' code cooperates without bespoke glue per pairing. Agent practice uses two kinds: tool protocols connect an agent to capabilities and data (the Model Context Protocol is the prominent labeled example), and agent protocols connect it to other agents that reason, plan, and answer in their own time (Agent2Agent is the labeled example). Either way the payoff is arithmetic: adapters stop multiplying as connectors × agents (Chapter 6).