Claude Sonnet 5 Launches in Harvey: Legal AI Advancements

Anthropic’s Claude Sonnet 5, the latest iteration of its mid-tier large language model, is now available in Harvey, the AI-driven legal assistant platform. Officially launched on June 30, 2026, Sonnet 5 aims to deliver significant improvements in legal accuracy and multi-step task execution, targeting both litigation and transactional workflows.

According to Harvey, Sonnet 5 achieved a 5.8% all-pass score on the platform’s Legal Agent Benchmark (LAB), a stringent evaluation designed to simulate complex legal tasks at law firms. The model excelled in drafting tasks, particularly in energy and natural resources, real estate, and capital markets. On the BigLaw Bench, a separate benchmark focused on large law firm use cases, Sonnet 5 posted an impressive 91.3% score—surpassing all previous Sonnet and Opus models. Notable strengths included risk assessment, compliance, and case management.

“Sonnet 5 brings a meaningful jump in legal quality over Sonnet 4.6,” commented Niko Grupen, Head of Applied Research at Harvey. “In early testing, it was both more accurate and more precise than its predecessor, delivering stronger answers in fewer words.” However, Grupen also noted that the model still faces challenges with dense, specialized analytical tasks, such as tax and structured finance, and remains imperfect in completing every step of complex multi-step tasks without error.

Claude Sonnet 5 represents Anthropic’s effort to make advanced ‘agentic’ AI capabilities—autonomous task execution, tool usage, and enterprise workflow integration—more accessible and affordable. Positioned between the costlier Mythos- and Fable-class models and the older Sonnet 4.5, it balances performance with practicality for mainstream enterprise adoption. The model also adheres to Anthropic’s Cyber Verification Program, ensuring enhanced reliability and safety for real-world deployments.

Harvey plans to roll out Sonnet 5 to eligible U.S. customers in the coming days, with availability in the EU and Australia to follow shortly. As Anthropic continues expanding its AI offerings across platforms like AWS, Microsoft Foundry, and soon Google Vertex AI, Sonnet 5 could play a pivotal role in bridging the gap between frontier AI capabilities and everyday enterprise AI deployment.

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