A side-by-side comparison of three live-product usability tests shows that a purpose-built synthetic testing platform like Uxia produces fundamentally more reliable and richer results than a general-purpose browser agent (Claude browser extension for this experiment) used as a stand-in usability tester. Reliability. Uxia completed all 3 missions, with every tester finishing end-to-end. Claude fully completed only 1 of 3 missions on the first execution due to abnormal stops: one run was blocked at a domain boundary, another was locked out of the product entirely, and both required reruns with manually adjusted permissions. Behavioral fidelity. All Uxia testers behave within the boundaries of human limits. When blocked, the Claude agent went beyond what a human would do, inspecting the DOM, firing React handlers programmatically, and invoking JavaScript tools, then reported “success” on a flow no human could complete. Insight quality. Uxia insights are grounded in what multiple testers actually experienced, each scored for severity, backed by cross-tester frequency and evidence quotes, and paired with a fix suggestion. Claude’s single-run insights are articulate but unscored, single-perspective, and in the worst case describe flows the agent never actually touched. Coverage. Every Uxia test ran with 5 differentiated synthetic testers (10 by default on standard platform plans); Claude produced one run, one persona, no frequency data. The highest-impact findings, like a resume generator leaking a different person’s identity, were only discoverable because Uxia testers carried realistic personas and files through the full flow. Capabilities. Misclick tracking, heatmaps, journey diagrams or UXIA-Q scoring are available in Uxia and have no equivalent in the Claude extension. Claude’s value. The extension narrates think-aloud friction articulately on pages it can freely access, and it handles login credentials correctly (masked and redacted in logs). Bottom line: a browser agent can describe a page; a synthetic testing platform tells you what real users will actually experience, reliably, repeatably, and at scale. Keywords: usability testing; synthetic users; browser agents; AI agents; UX research; behavioral fidelity