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We report a preregistered randomised experiment on whether an autonomous computer-using agent can complete professional assignments to an acceptable standard. A controller launches independent Claude Code sessions (\texttt{claude-opus-5-5}) to perform ten validated scenarios in ten occupations inside a synthetic economy with hidden oracles, planted errors and prompt-injection traps. Each scenario is run under five levels of computer access, from file access without a shell to full computer use with browser, office tools and an enterprise portal, plus a self-repairing variant. Acceptance requires deterministic completion, no critical violation and a blind quality score of at least 70. Over 150 runs, acceptance is 0.00, 0.87, 0.90, 0.90 and 0.83. The shell premium is $+0.87$ [0.67, 1.00]; the preregistered primary contrast, full computer use minus shell, is $+0.03$ [0.00, 0.10], a precise null because every arm with a shell is at a ceiling. Accepted deliverables cost about one dollar of API charges; a cheaper model reaches 0.85 acceptance at 0.40 dollars. In an exploratory experiment, bounded self-repair converts interrupted sessions into accepted deliverables (0.85 against 0.05). The supervision margin, not the budget, separates the Autonomous Labor Frontier from the acceptance ceiling. Human arms were not executed; no displacement estimate is implied.