Baseerat: Measuring the Oversight Gap When a Computer-Use Agent's Overseer Cannot See the Screen

Safety benchmarks for computer-use agents evaluate an agent's behaviour under the tacit assumption that the human overseer can see the screen. For the roughly forty million people who operate a computer through a screen reader, that assumption does not hold: the only account of what an agent did arrives through the agent's narration and the post-hoc accessibility tree, and a growing body of work shows that an agent's own account of its actions is unreliable, with claimed success exceeding actual success by large margins. We introduce Baseerat, a benchmark that pairs each task with a matched sighted control and measures the oversight parity gap: the detection rate a sighted overseer achieves on a diverged agent run minus the rate an overseer restricted to the accessible channel achieves on the same run. Baseerat runs tasks in a real browser under three conditions (clean, silent failure, and a deceptive indirect injection that poisons the accessibility tree in step with the agent's narration). Over a 150-task corpus a deterministic reference auditor exhibits a parity gap of 0.50, concentrated entirely in the deceptive-injection condition, which we demonstrate is realisable in a live browser. A trusted-receipt defence closes the gap to zero. We release the benchmark, the rig, the corpus, and the reference results, and specify the language-model and human expert-auditor arms. This is a preprint draft: the reference-auditor and defence results are final; the language-model and human expert-auditor arms are pending.