Day 1 used Claude Code to build BookTracker. Day 2 calls the Anthropic API from BookTracker. C5 is the foundation every later Day 2 lab builds on (C6 streaming + tools, C7 RAG, C10 hardening). You start from the full Day 1 solution and finish with a chat endpoint that talks to Claude, remembers the conversation across requests, and caches a large system prompt to save tokens.
