Put your standing instructions in CLAUDE.md at the project root. Claude Code reads it automatically on every session start, no prompting required.

Useful things to put there:

• Tech stack and conventions (“We use Zod for validation, never raw JSON.parse”)
• What commands to run for tests/build/lint
• Things Claude should always or never do in this repo
• Context that would take 2 minutes to re-explain each session

The file is just markdown. Keep it short and opinionated. It’s a briefing, not documentation.