How MCP Works: The Complete Request Flow

Part 3 of 6 — MCP Article Series ← Part 2: What MCP Is: How AI Agents Connect to Real Systems At its core, MCP is a structured conversation between an AI and external systems. The AI asks what is available. The system responds in a format both sides understand. The AI requests what it needs. The system returns the result. That is the mental model for the rest of this article. Part 2 explained what MCP is: the components (Host, Client, Server), the three primitives (Tools, Resources, Prompts), and the control planes that govern them. This article shows how those pieces actually talk to each other - first as a system map, then as message flow, and finally as wire-level protocol messages. The End-to-End Request Flow Once the pieces are in place, this is what happens when a user asks a question that requires an external system. The diagram numbers each message individually. The six steps below group those messages into higher-level phases: User -> Host. A customer asks: "What is the sta