NOTE

[[mcp-moc|MCP]] Architecture

authorclaude-sonnet-4-6 aliasesmcp-model, mcp-layers, mcp-lifecycle, mcp-standard-model title[[mcp-moc|MCP]] Architecture statusactive date2026-04-25 typepermanent

MCP Architecture

The Model Context Protocol (MCP) follows a layered, client-server architecture designed to bridge AI applications (Hosts) with external data and tools.

The Layered Model

MCP separates the "what" (data) from the "how" (delivery) through two distinct layers.

1. Data Layer (Inner)

The data layer implements a JSON-RPC 2.0 based exchange protocol that defines message semantics and lifecycle management.

  • Lifecycle Management: Handles connection initialization, capability negotiation, and termination.
  • Feature Primitives: Defines the structure for Tools, Resources, and Prompts.
  • Notifications: Supports real-time, one-way updates (e.g., notifications/tools/list_changed) to keep the Host synchronized with Server state.

2. Transport Layer (Outer)

The transport layer manages communication channels and message framing. The Data Layer is transport-agnostic, allowing the same JSON-RPC format to work across different media.

  • Stdio Transport: Best for local process communication. Messages are sent via stdin/stdout and must be carefully framed to avoid corruption from standard logging.
  • Streamable HTTP Transport: Uses HTTP POST for client-to-server messages and Server-Sent Events (SSE) for server-to-client streaming. This enables remote servers and standard web authentication (OAuth).

The Initialization Handshake

MCP is a stateful protocol that begins with a mandatory capability negotiation sequence:

  1. Initialize Request: The Client sends its protocolVersion and supported capabilities (e.g., roots, sampling).
  2. Initialize Response: The Server responds with its own version, serverInfo, and supported capabilities (e.g., tools, resources).
  3. Initialized Notification: The Client sends a notifications/initialized message to confirm it is ready to process requests.

Participants

  • MCP Host: The main AI application (e.g., Claude Desktop, VS Code) that coordinates multiple Clients.
  • MCP Client: A component within the Host that maintains a 1:1, dedicated connection to a single Server.
  • MCP Server: A standalone program providing context or tools. Each Server operates in its own isolated scope.

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