Model Context Protocol: What It Is and Why It Matters
Anthropic's open standard for connecting LLMs to external tools and data. What MCP solves that function calling doesn't, the host/client/server architecture, and why every major IDE and AI assistant is adopting it.
In November 2024, Anthropic open-sourced the Model Context Protocol — a standard for connecting LLMs to external tools and data sources. Within six months, it was adopted by VS Code, Cursor, Zed, Replit, Sourcegraph, Block, and dozens more. Here's why it matters.
The problem MCP solves
Every AI assistant that integrates with external tools reinvents the same integration layer: authentication, serialization, error handling, discovery. Without a standard, every LLM application builds N×M integrations — N models × M tools. MCP turns this into N + M: each tool builds one MCP server, each model builds one MCP client.
The architecture: host, client, server
- Host: the application the user interacts with (Claude Desktop, VS Code, Cursor). Manages connections and permissions.
- Client: runs inside the host, maintains a 1:1 connection with one MCP server.
- Server: an external process that exposes tools, resources, or prompts over the MCP protocol.
The four MCP primitives
| Primitive | What it is | Example |
|---|---|---|
| Tools | Functions the LLM can call (model-controlled) | search_web(), run_query(), send_email() |
| Resources | Data the host can read (app-controlled) | File contents, database rows, API responses |
| Prompts | Reusable prompt templates | code-review, summarize-doc, explain-error |
| Sampling | Server asks the LLM to generate text | Agentic servers requesting model inference |
Transport: stdio vs. SSE
Local MCP servers communicate over stdio (stdin/stdout) — simple and zero-configuration. Remote servers use Server-Sent Events (SSE) over HTTP for network deployment. The protocol itself is JSON-RPC 2.0 in both cases.
MCP is the USB-C of AI tool integration. Once a tool has an MCP server, it works with any MCP-compatible host — without custom per-application integration code.
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