system prompt — The standing instructions a model receives before any user input — defining its role, rules, tools, and tone for the whole conversation.
Why it matters
First-hand account of how the Claude Code team decides what belongs in a system promptThe standing instructions a model receives before any user input — defining its role, rules, tools, and tone for the whole conversation.Full definition → and what to delete, from the person who built it.
Transcript
Fresh off the launch of Opus 5, Claude Code creator Boris Cherny joins Diana Hu at Startup School 2026 to talk about what the newest models can do, how Claude Code came to be, and what it means to build products when the underlying capabilities keep accelerating.
Transcript: https://www.ycrootaccess.com/p/boris-cherny-building-claude-code
Apply to Y Combinator: https://www.ycombinator.com/apply
Work at a startup: https://www.ycombinator.com/jobs
Chapters:
00:00 — Intro
00:07 — What Makes Opus 5 Different
02:06 — Solving Prompt Injection
03:21 — Why Claude Code Deleted 80% of Its System Prompt
06:37 — Press Delete on Your AI Product
07:20 — How to Rebuild Your System Prompt
10:30 — Product Overhang and “Unhobbling” AI
14:26 — Give Claude Harder Problems
19:32 — Prompt Engineering Is Changing
21:57 — The Two-Week Claude Code Prompt
24:42 — Running Thousands of AI Agents
30:15 — Coding Is (Almost) Solved
32:20 — What Every CS Student Should Still Learn
