Claude Sonnet 5 Security and Safety: A System Card Analysis for Agent Deployments

Claude Sonnet 5 represents a significant improvement in prompt injection attack resistance compared to its predecessor, Sonnet 4.6, with success rates dropping from about 50% to under 1% in browser use, and effectively 0% when safeguards are enabled. This advancement is critical for those deploying AI agents, emphasizing security over mere capability scores. Although Sonnet 5 is not a frontier model and does not advance the public frontier on offensive cyber capabilities, it demonstrates strengthened defenses, refusing malicious requests more reliably while showing less risky self-initiated behavior. However, this heightened security comes with trade-offs, such as higher refusal rates on legitimate dual-use tasks. The model's robustness is measured with deployment-time safeguards disabled, serving as a lower bound rather than the final security posture, underscoring the importance of maintaining robust system-level defenses. Anthropic's approach highlights the necessity for layered security, ensuring that while model-level robustness is crucial, the overall system's security remains the responsibility of its deployers.

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