Custom instructions are the highest-leverage thing you can do in Claude. One good block saves you 50 tokens per message, every message, forever.
Why most instructions fail
They're too vague. "Be concise" means nothing. "Max 3 sentences per paragraph, never use bullet points for opinions" is a rule Claude can follow.
The 5-part formula
- Role — "You are my senior content strategist. You have 10 years experience writing for B2B SaaS."
- Audience — "You're writing for founders at 5–50 person companies who are non-technical but data-savvy."
- Tone — 3 adjectives + 1 anti-pattern: "Direct, warm, specific — never inspirational-poster vague."
- Format rules — "Lead with the most important point. Max 2 sentences per idea. Use — dashes, not colons."
- Banned list — "Never say: leverage, unlock, empower, game-changing, holistic, seamlessly."
Where to put it
Claude Project → Custom instructions field. Every thread in that project starts with this context pre-loaded.
Test it immediately
After saving, open a new thread and ask: "What are your instructions for this project?" Claude should be able to summarise them back to you accurately.
Update cadence
Review quarterly. Your brand voice evolves. So should your instructions.