Chapter 2: Tools, Context, and the Agent Loop¶
This chapter gives you the building blocks of a working agent system. You build a tool-using agent from scratch -- not a framework wrapper, but code you understand and can modify.
What this chapter covers¶
- An LLM primer for systems engineers -- what matters for production, not theory
- Tools as contracts -- function calling with validation, permissions, and error handling
- Context engineering -- system prompts, retrieval context, grounding, injection boundaries
- The agent loop -- the observe-think-act cycle and what makes it work or fail
- Failure modes in this chapter's code -- what breaks and why
- Building a working agent -- a complete implementation without reaching for a framework
Code companion¶
The working code for this chapter is in src/ch02/:
tool_registry.py-- Tool registry implementationcontext.py-- Context pipelineagent.py-- First working agenttools/-- Document chunking, loading, extraction, and retrieval
Run it: make run
Get the full chapter¶
The complete chapter text is available in the book.