YC Startup School: Claude Code Creator Boris Cherny

Anthropic deleted over 80% of Claude Code's system prompt for Opus 5, and the model became more intelligent, according to Claude Code creator Boris Cherny at YC Startup School 2026. Cherny shared a playbook for building AI products, including cutting system prompts, using higher-level prompting, and spawning thousands of agents, noting that Anthropic took Arc AGI to 30%, up from low single digits or low teens.

YC Startup School: Claude Code Creator Boris Cherny Anthropic just deleted over 80% of Claude Code’s system prompt for Opus 5, and the model got MORE intelligent, not less. Fresh off the Opus 5 launch, Claude Code creator Boris Cherny sat down with YC at Startup School 2026 and laid out the exact playbook for building products when the model keeps outrunning them. We went through the full 36 minutes and pulled out every stealable move the ablation method, SKIP - not a real violation, higher-level prompting, dynamic workflows that spawn thousands of agents so you can rebuild your AI product for Opus 5 without watching a second of it. In this guide you’ll find: Why Opus 5 Runs for Weeks With No Scaffolding and Ignores Prompt Injection The Delete Playbook: Cut 80% of Your System Prompt and Rebuild It Line by Line Product Overhang: The Model Capabilities Startups Aren’t Shipping Give Claude Harder Tasks Than You Think It Can Do: Bun’s Zig-to-Rust Rewrite in 11 Days Verification Is the Skill Nobody Gets Right: The Two-Week Prompt That Live-Blogged Itself Spawn Thousands of Agents From One Prompt: Dynamic Workflows and Self-Maintaining Codebases 👋 Just a quick note before you continue If you are serious about building or investing in AI, you can subscribe here: Here’s a good place to start: ↳ Alex Karp’s Playbook for Which AI Companies Survive ↳ 19 great new details about Cursor’s rise Try premium free for 7 days. Or get 50% off this week only. Cherny frames the launch with one data point: Anthropic took Arc AGI to 30%, “before the best score was in the low single digits or low teens .” Everything below is his account of how to build products for a model moving that fast.