Boris Cherny: We Cut 80% of Claude Code’s Prompt
Jul 27, 2026 · 35:52
Boris Cherny, creator of Claude Code, joins Diana Hu at Y Combinator's Startup School 2026 to discuss how Opus 5 eliminates prompt injection, why Claude Code deleted 80% of its system prompt for the new model, and how builders can 'unhobble' AI by giving Claude harder problems. He reveals that Opus 5 is not prompt-injectable thanks to three safety layers, and that deleting most instructions actually makes the model smarter. Cherny explains the concept of product overhang—the gap between model capability and existing products—and shows how a single prompt using dynamic workflows rewrote Bun's entire codebase from Zig to Rust in 11 days. He also describes a two-week prompt that spawned thousands of agents to rewrite Claude's Electron app in Swift, and shares how Claude now maintains its own codebases with daily routines removing dead code and unifying abstractions. Cherny argues coding is solved for most tasks but advises CS students to learn practical application and build things people want.
