MCP Overview — Model Context Protocol

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What is MCP?

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Architecture

• Host — An AI application (ChatGPT, Claude) that initiates connections to servers. Each host runs one or more clients.
• Client — A protocol client within the host that maintains a 1:1 connection to a server. Handles capability negotiation, request routing, and session management.
• Server — A program that exposes tools, resources, and other capabilities over MCP. Servers are stateless between sessions.

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Transports

• stdio — Communication over standard input/output. Used for local servers running as subprocesses.
• Streamable HTTP — Communication over HTTP with server-sent events. Used for remote servers accessible over the network.

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Connection Lifecycle

1. The client sends initialize with its capabilities and preferred protocol version.
2. The server responds with its own capabilities and the negotiated protocol version.
3. The client sends notifications/initialized to confirm the session is ready.
4. Requests flow in both directions for the duration of the session.

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What Can Servers Expose?

• Tools — Functions the model can call. Used for actions like fetching data, running computations, or modifying state.
• Resources — Data the model can read. Identified by URIs with MIME types. Used for structured data, files, and (in MCP Apps) interactive HTML UIs.
• Prompts — Reusable prompt templates. Not currently used by MCP Apps.

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