HTTP Fundamentals

What You're Building

In this lesson, you'll build a simple HTTP server using Python's built-in http.server module. This server will handle JSON requests and responses — the foundation for MCP's HTTP/SSE transport.

HTTP in 60 Seconds

HTTP (HyperText Transfer Protocol) is how clients and servers communicate on the web. For MCP, you need to understand just a few key concepts:

Request Structure

Every HTTP request has:

• Method — What action to take (GET, POST, etc.)
• Path — The URL path (/mcp, /api/tools)
• Headers — Metadata (Content-Type, Authorization)
• Body — The actual data (for POST/PUT requests)

POST /mcp HTTP/1.1
Host: localhost:8080
Content-Type: application/json
Content-Length: 45

{"jsonrpc": "2.0", "method": "tools/list"}

Response Structure

Every HTTP response has:

• Status Code — Success (200), error (400, 500), etc.
• Headers — Metadata about the response
• Body — The actual response data

HTTP/1.1 200 OK
Content-Type: application/json
Content-Length: 52

{"jsonrpc": "2.0", "id": 1, "result": {"tools": []}}

HTTP Methods for MCP

MCP's HTTP transport uses just a few methods:

Important Headers

These headers matter for MCP:

# Request headers
Content-Type: application/json       # We're sending JSON
Accept: application/json, text/event-stream  # We accept JSON or SSE
MCP-Protocol-Version: 2025-11-25    # MCP version

# Response headers
Content-Type: application/json       # For single responses
Content-Type: text/event-stream     # For SSE streaming

Status Codes to Know