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Finding Miscompiles for Fun, Not Profit - by Justin Lebar

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Or: You don’t need access to Claude Mythos to spend $10,000 in an afternoon.

Finding Miscompiles for Fun, Not Profit Or: You don’t need access to Claude Mythos to spend $10,000 in an afternoon. Justin Lebar May 28, 2026 ∙ Paid 214 8 Share Update June 1: The day after we published, Anthropic released Opus 4.8 and “ultracode” mode in Claude Code. Our preliminary experiments indicate that together these are significantly better at filtering out low-severity bugs, and that the cost per medium-to-high severity bug found is maybe 1/5 (with very large error bars) that of the workflow described in this article. I’ve worked on compilers for ML for the last decade across Google,

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