Simon Willison in conversation with Cat Wu & Thariq Shihipar, Anthropic

This is a rare inside look at how Anthropic actually uses its own coding agents: their internal agent 'Claude Tag' now lands 65% of product PRs, and they explain the engineering decisions behind that number rather than just stating it. The discussion of shrinking system prompts by 80% for frontier models — because over-constraining hurts model judgment — is directly applicable if you maintain agent prompts today.

• Claude Tag lands 65% of Anthropic's product PRs; the team treats it as a coworker embedded in public Slack channels, which requires a public-by-default communication culture.
• System prompts were cut by 80% for newer models — detailed examples and hard rules that helped older models actively constrain frontier models' judgment.
• Auto mode runs a Sonnet classifier over every tool call and bash command in context, enabling dynamic permissions and defending against prompt injection.
• The PRD-to-implementation cycle has compressed from 6–12 months toward a week, shifting engineering value from execution to product taste.

Part of Issue Nº 002: How Anthropic ships with its own agents, evals in the trenches, and the new physics of small teams