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Anthropic's Claude Opus 5 API errors linked to disabled thinking and effort settings

By PulseAugur Editorial · [1 sources] ·

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]

Read on dev.to — Claude Code tag →

AI-generated summary · Google Gemini · from 1 sources. How we write summaries →

Anthropic's Claude Opus 5 API errors linked to disabled thinking and effort settings

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Significant
Frontier-lab model release with system card [lever_c_demoted from frontier_release: ic=1 ai=1.0]
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model release, product
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High
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59 days old
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COVERAGE [1]

  1. dev.to — Claude Code tag TIER_1 English(EN) · Creeta ·

    Disabling thinking can 400 your Opus 5 calls

    <p>A migration that looks like a one-line model change can fail before your agent writes a token. The sharp edge is the interaction between Opus 5 effort levels and disabled thinking.</p> <h2> Which Opus 5 API setup triggers the HTTP 400? </h2> <p>The Opus 5 API setup that trigge…

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