Build a Prompt-Injection Eval for Claude Sonnet 5 Agents

On June 30, 2026 Anthropic shipped Claude Sonnet 5 and made it the default model on Free and Pro. The headline is agentic muscle at a lower price: it plans, drives browsers and terminals, and runs multi-step tasks that used to need Opus-class models. Anthropic also claims it is better at refusing malicious requests and resisting hijack attempts in prompt-injection attacks than Sonnet 4.6.