Anthropic's Claude Opus 5, launched on July 24, 2026, has specific API requirements that can lead to HTTP 400 errors if not met. Specifically, disabling the 'thinking' parameter while requesting 'xhigh' or 'max' effort levels will cause a failure. Developers migrating to Opus 5 must treat 'effort' and 'thinking' as coupled controls, as disabling thinking is only accepted at 'high' effort or below. This new model also features a 1M-token context window and adaptive thinking enabled by default, requiring careful testing of tool use and refusal handling. AI
IMPACT Developers must update their integration strategies for Claude Opus 5, paying close attention to API parameter coupling to avoid errors and ensure proper tool use.
RANK_REASON Frontier-lab model release with system card [lever_c_demoted from frontier_release: ic=1 ai=1.0]
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