Protocol Architecture in Model Context Protocol

Lessons in this section

1. Protocol Architecture

   The three layers of MCP (application, message, transport), the three kinds of messages, progress updates, cancellation on each transport, and how server requests became Multi Round-Trip Requests.

2. JSON-RPC Messages

   Every field of MCP requests, responses and notifications, id matching, the core methods, _meta and resultType, error codes, tool errors versus protocol errors, and why servers are now stateless.

3. Transports

   How MCP messages travel: stdio with process lifecycle, line framing and sandboxing; Streamable HTTP with one POST per request, SSE streams and MCP headers; legacy HTTP+SSE; and local versus remote trade-offs.

4. Discovery and Versioning

   server/discover, per-request versions in _meta, the -32022 unsupported version error, server and client capabilities, list changes through subscriptions/listen, the legacy initialize handshake, and clean shutdown.