Today's concept: CLAUDE.md - teaching Claude your codebase
If you want Claude to stop suggesting bad ideas in your codebase, stop hoping it'll read your mind - write it a CLAUDE.md instead.
CLAUDE.md is a file you create at your repo root that tells Claude Code about your project: conventions you follow, architecture decisions, what state things are in, what to avoid, how you like to collaborate. Think of it like writing a README for the way the code works, not just what it does. When Claude Code opens your repo, it reads CLAUDE.md first - before touching any code.
In long agentic workflows where Claude makes repeated suggestions over many turns, a solid CLAUDE.md up front pays back huge token savings: Claude stops asking "should I refactor this?" or making assumptions about error handling, because it already knows your rules. Without it, you burn tokens steering Claude back on track, turn after turn. This scales poorly - the longer the agent runs, the more you pay for context you should've written once.
Quick check before you scroll: If Claude Code doesn't have a CLAUDE.md in your repo, what might go wrong?
Full breakdown + the answer: frankduah.me/learnings/2026-07-15-claude-md-teaching-claude-your-codebase
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The answer
Claude will make assumptions about your architecture, conventions, and preferences instead of knowing them. It might suggest refactors you don't want, add over-engineered error handling, miss context about why code is structured a certain way, or not know about project constraints. CLAUDE.md prevents this by front-loading the context.