Anthropic Reports Claude Wrote 80% of Code in May, Long-Task Autonomy Extends to 16 Hours

According to Anthropic's report released on June 5, titled "When AI Builds Itself," Claude has written over 80% of the code merged into the company's main repository as of May 2026, up from single-digit percentages before Claude Code launched in February 2025. The report also revealed that engineer productivity surged eightfold compared to 2024, with Claude now handling code writing and execution while engineers focus on planning and review.

Claude's capacity for sustained autonomous task execution has doubled approximately every four months. The latest Claude Mythos preview version can maintain uninterrupted work for over 16 hours, compared to 12 hours for Claude 4.6 Opus in March 2026 and just four minutes for Claude 3 Opus in March 2024. However, Anthropic clarified in the report that fully autonomous design and development of successor models through recursive self-improvement has not yet been achieved.