Ouroboros: A Self-Developing Coding Harness with Reviewed Core Evolution

Long-horizon agents are model–harness systems, yet most harnesses remain fixed after design. We present Ouroboros — a self-developing agent harness whose tools, context assembly, prompts and core implementation improve through reviewed commits that become the runtime for later work. Core evolution proceeds in two modes. In recursive free evolution, improvement is itself a task and completion can schedule the next evolution cycle. In experience-driven core evolution, ordinary work and social interaction expose bugs, rough edges, and inefficient context construction leading to reviewed structural changes. On Terminal-Bench 2.1, an Opus 5 run scores 86.97% (86.74% after trajectory audit), the best result reported on this benchmark. An Opus 5 run on OSWorld-Verified reaches 90.69%, above the best previously reported score, and a five-rollout CL-Bench campaign sets a new state of the art at 0.2301 (all three comparisons as of August 2026). Hope is the longest-running publicly documented Ouroboros deployment: a 161-day living-agent experiment in free evolution under governed human communication across seven surfaces, where people surface faults and proposals but the agent decides which changes to pursue. Because a self-developing agent may rewrite its own code and select new model APIs, operational safety is a primary design problem: guardrails must remain authoritative under evolutionary pressure. Benchmark campaigns use frozen seeds, while Hope continues live evolution on a separate lineage.