If you've used Claude Code, you've seen what an AI agent can actually do: read files, run commands, edit code, figure out the steps to accomplish a task.
And you know it doesn't just help you write code, it takes ownership of problems and works through them the way a thoughtful engineer would.
The Claude Agent SDK is the same engine, yours to point at whatever problem you want, so you can easily build agents of your own.
It's the infrastructure behind Claude Code, exposed as a library. You get the agent loop, the built-in tools, the context management, basically everything you'd otherwise have to build yourself.
This guide walks through building a code review agent from scratch. By the end, you'll have something that can analyze a codebase, find bugs and security issues, and return structured feedback.
More importantly, you'll understand how the SDK works so you can build whatever you actually need.
What we're building
Our code review agent will:
Analyze a codebase for bugs and security issues
Read files and search through code autonomously
Provide structured, actionable feedback
Track its progress as it works
The stack
Runtime - Claude Code CLI
SDK - @anthropic-ai/claude-agent-sdk
Language - TypeScript
Model - Claude Opus 4.5
What the SDK gives you
If you've built agents with the raw API, you know the pattern: call the model, check if it wants to use a tool, execute the tool, feed the result back, repeat until done. This can get tedious when building anything non-trivial.
