What they say

MCP is a “universal, open protocol” that enables AI models to securely connect to external data sources and tools.¹ It’s been called “USB-C for AI” — a universal plug that lets any model talk to any tool.

What it actually is

MCP is JSON-RPC 2.0² transported over stdio (or SSE for remote servers). That’s it.

A “tool” is a JSON schema describing function parameters. The LLM decides to call it, the host serializes the call as a JSON-RPC request, sends it to a subprocess over stdin, and reads the response from stdout.

The protocol in pseudocode

// Client → Server (over stdin)
{"jsonrpc": "2.0", "method": "tools/call", "params": {"name": "read_file", "arguments": {"path": "/foo"}}, "id": 1}

// Server → Client (over stdout)
{"jsonrpc": "2.0", "result": {"content": [{"type": "text", "text": "file contents..."}]}, "id": 1}

The “extra steps”

1. Capability negotiation — client and server exchange supported features at startup (standard handshake pattern)
2. Resource discovery — server advertises available tools via a tools/list method (service registry pattern)
3. Schema validation — tool parameters are defined as JSON Schema (input validation)
4. Lifecycle management — initialize/shutdown semantics (standard session management)

What you already know

If you’ve called a REST API and read the JSON response, you understand 90% of MCP. The difference is that instead of HTTP, the messages go over stdin/stdout to a subprocess — and instead of you deciding what to call, the LLM decides.

# REST API: you decide to call it
curl https://api.example.com/weather?city=NYC

# MCP server: LLM decides to call it, message goes over stdin
echo '{"jsonrpc":"2.0","method":"tools/call","params":{"name":"get_weather","arguments":{"city":"NYC"}},"id":1}' | node mcp-server.js

Same data, different pipe.³

Footnotes

1. Model Context Protocol announcement — Anthropic, November 2024. The blog post framing; read the spec first. ↩

2. JSON-RPC — Wikipedia — the underlying wire protocol. The method, params, and id fields in every MCP message come directly from this standard. ↩

3. Standard streams — Wikipedia — stdin/stdout as an IPC mechanism predates the web. MCP’s local transport is just two processes communicating over pipes, the same way Unix tools have always talked to each other. ↩