Parthenon Law: A Self-Evolving Legal-Agent Framework
For legal professionals and AI researchers, this work addresses the need for domain-adapted, self-improving agent architectures in high-stakes legal settings.
The paper identifies three obstacles to deploying LLM agents in legal domains and introduces Parthenon, a self-evolving legal-agent framework that improves performance on legal-matter tasks through an anti-leakage learning loop. A large-scale study on Harvey LAB with 12,510 agent trajectories shows that frontier agents struggle with matter completion, and Parthenon substantially improves SOTA model and harness performance.
As agents grow more capable, legal-domain LLM agents promise to turn document-heavy matters into reviewable work products -- yet reliable deployment faces three obstacles: no large-scale evidence on how today's strongest model-and-harness combinations behave on end-to-end legal matters; no agent architecture adapted to the legal vertical, only general-purpose harnesses; and, in a setting that keeps shifting with new facts, authorities, and deadlines, no mechanism for systems to learn from their own outcomes. We address each. A large-scale empirical study on Harvey LAB -- $12{,}510$ agent trajectories -- shows that even frontier agents remain far from completing matters in a single pass: per-criterion accuracy climbs with stronger models while strict matter completion stalls. We then introduce \textsc{Parthenon}, a self-evolving legal-agent framework that factors Model, Harness, Agent roles, legal Knowledge, deterministic Tools, and procedural Skills into auditable surfaces for source traceability, date and number grounding, deliverable compliance, and issue closure. Finally, an anti-leakage learning loop converts scored failures into task-agnostic edits to skills, tools, and knowledge, letting the system improve with experience -- as a firm refines its checklists and playbooks after each matter -- without touching model weights. Across our large-scale empirical analysis, \textsc{Parthenon} substantially improves the performance of state-of-the-art models and harnesses on legal-matter tasks.