Claude Opus 5.5: The System Card
Zvi Mowshowitz @TheZvi · substack · 2026-09-23 · ★★★ · archived
Detailed critique of the Opus 5.5 system card, arguing it is effectively a Tier 2 cyber model.
Summary
Zvi reads Opus 5.5 as the strongest cyber-capable Claude released, matching or beating Mythos 5.1 on internal evals, and argues it is in practice a Tier 2 cyber model even though Anthropic places it lower. He notes Anthropic deployed Tier 2-style safeguards anyway (activation probes, lightweight classifiers, and a dedicated LLM check) and that red-teaming found decomposable exploits but no universal jailbreak. Follow-ups: 'Claude Opus 5.5 Should Raise Your Ambitions' (Sep 26). Verified by WebFetch and the Substack archive API; the entry already links the wordpress mirror.
Archived text
Page title: Claude Opus 5.5: The System Card
Page description: Introducing the world’s most powerful model, at least by some measures like Artificial Analysis or any standard benchmark list, which is now Claude Opus 5.5.
Metadata archived 2026-09-29; see Summary for content.
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