DarwinX: Evolving Agent Harnesses Through Natural Selection

Treats agent self-improvement as population-level selection over harnesses (prompts, tools, skills, control flow) with the model frozen, using a preserve-and-extend contract to prevent regression and a verifier-based fitness signal. Reaches 84.7% on Terminal-Bench 2.1, 68.3% on TerminalWorld held-out, and 93.0% pass@1 on WebArena-Infinity; a Terminal-Bench harness transfers unchanged to SWE-bench Verified.