RRSI is a Google Research framework that automatically evolves LLM agent harnesses through a regularized search loop: proposing edits, screening candidates with a critic for benchmark-specific logic, evaluating them, and selecting winners while preventing overfitting to the training tasks. It ships with three domain instances (terminal coding, document work, and engineering design) built on benchmarks like Terminal-Bench 2.1 and Harvey LAB.
It is most valuable when an LLM agent's harness (prompts, tools, control flow, memory) needs systematic optimization and manual iteration has plateaued or is at risk of overfitting to evaluation tasks. It is also useful when auditable evidence of which edits produced measured gains is required, since every edit records its hypothesis, score, cost change, and verdict in git worktrees and JSONL history.