MCP Architecture
Host, client and server as roles; tools, resources and prompts by who decides to use each; one tool call in real 2026-07-28 JSON; the two ways a call fails; and the change that defines the current spec: no handshake, so any instance can answer.
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After this section you can
- Trace one create_support_ticket call from tool_use to tool_result across host, client and server
- Choose tool, resource or prompt by who decides to use it, and send a failure down the right error channel
- Explain what replaced the initialize handshake and why a request no longer needs a particular server instance
MCP Architecture: One Tool Call, End to End
One tool call in real 2026-07-28 JSON, the two ways it fails, and the change that defines the spec: no handshake.
An AI application (the host) runs one MCP client per server, and the model only ever talks to the host. A tool call is one JSON-RPC request from that client to its server. In 2026-07-28, the current spec, every request carries its own context, so there is no handshake and no request depends on which instance saw the last one.
Numbers are step 3’s messages: blue a request, green a result, amber what the model wrote. The figure pans sideways on a phone.