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MCP Architecture

Tool Design & MCP Integration

Definition

MCP's client-server design: a host application coordinates one MCP client per server, communicating across a data layer (JSON-RPC 2.0) and a transport layer (STDIO / Streamable HTTP with SSE).

Key points

  • MCP follows a client-server architecture where an MCP host — an AI application like Claude Code or Claude Desktop — establishes connections to one or more MCP servers. The host creates one MCP client for each MCP server; each MCP client maintains a dedicated connection with its corresponding MCP server.
  • Roles (verbatim):
    • MCP Host: "The AI application that coordinates and manages one or multiple MCP clients."
    • MCP Client: "A component that maintains a connection to an MCP server and obtains context from an MCP server for the MCP host to use."
    • MCP Server: "A program that provides context to MCP clients" — regardless of where it runs (local or remote).
  • Two layers:
    • Data layer: the JSON-RPC 2.0 based protocol for client-server communication, including lifecycle management and core primitives (tools, resources, prompts, notifications). The inner layer.
    • Transport layer: communication mechanisms/channels — connection establishment, message framing, and authorization. The outer layer.
  • Transports:
    • Stdio transport: standard input/output streams for direct process communication between local processes on the same machine — optimal performance, no network overhead. Local servers typically use STDIO and serve a single client.
    • Streamable HTTP transport: HTTP POST for client-to-server messages with optional Server-Sent Events (SSE) for streaming. Enables remote communication; supports standard HTTP auth (bearer tokens, API keys, custom headers); "MCP recommends using OAuth." Remote servers typically use Streamable HTTP and serve many clients.
    • SSE appears as the streaming option within Streamable HTTP, not as a separate standalone transport.
  • Lifecycle: MCP is a stateful protocol. Initialization is a capability negotiation handshake — client sends initialize (with protocolVersion, e.g. 2025-06-18, and capabilities), server responds, then client sends notifications/initialized.

Why it matters for the exam

  • Tests the host/client/server roles and the one client per server / dedicated connection rule.
  • Tests the data-layer vs transport-layer split and which transport is local (STDIO) vs remote (Streamable HTTP).

Common gotchas

  • It is one client per server, not one client for all servers — the host coordinates multiple clients.
  • SSE is streaming within Streamable HTTP, not a third standalone transport.
  • The API MCP connector is remote-HTTP only — local STDIO is not supported there.

See also

Sources

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