CLAUDE.md context files

CLAUDE.md is a plain-text markdown file that Claude Code reads automatically at the start of every session to maintain persistent project context. You place it in your project directory (or in a few other locations) and fill it with coding standards, architecture decisions, preferred libraries, build/test commands, and hard constraints. Claude loads the file before any user message, so it behaves like a senior engineer who already knows your project's rules without you having to re-explain them every session. The file system is hierarchical: a global file at ~/.claude/CLAUDE.md applies to all your projects, a file at the project root (./CLAUDE.md) applies to that repository, a file at ./.claude/CLAUDE.md scopes to the project, and ./CLAUDE.local.md holds personal notes that are not committed to version control. Nested subdirectory CLAUDE.md files (e.g., packages/frontend/CLAUDE.md) load lazily — only when Claude interacts with files in that subtree — so large monorepos can have per-package rules without paying token costs upfront for every package. CLAUDE.md files can also import other files using @path/to/file syntax, pulling in READMEs, package.json, or any other reference material at launch. Anthropic's prompt caching applies to the file, so after the first load in a session subsequent requests are billed at the lower cache-read rate rather than full input-token price.