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Anthropic's Claude Opus and Sonnet 5.5 models introduce breaking API changes

By PulseAugur Editorial · [1 sources] ·

Anthropic has released updates to its Claude Opus and Sonnet models, with Opus 5.5 launching on September 22, 2026, and Sonnet 5.5 on September 28. Developers using the Claude API may encounter HTTP 400 errors if they attempt to use specific settings from previous versions, such as `thinking: {"type": "disabled"}` or forced `tool_choice` configurations. The new models require adjustments to how thinking and tool selection are handled, with `thinking: {"type": "between_tools"}` replacing `disabled` in Sonnet 5.5 and `tool_choice: {"type": "auto"}` being the recommended approach for tool selection. Additionally, changes to conversation history editing and tool declarations are necessary to avoid errors. AI

IMPACT Developers must update their integrations to accommodate new API behaviors in Claude Opus and Sonnet 5.5, potentially requiring code changes for tool use and conversation history management.

RANK_REASON New model releases from a frontier lab with specific API changes. [lever_c_demoted from frontier_release: ic=1 ai=1.0]

Read on dev.to — LLM tag →

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

Anthropic's Claude Opus and Sonnet 5.5 models introduce breaking API changes

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29 / 100
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Significant
New model releases from a frontier lab with specific API changes. [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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Breaking (< 6h)
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COVERAGE [1]

  1. dev.to — LLM tag TIER_1 English(EN) · Pesimmoq ·

    Claude Opus 5.5 / Sonnet 5.5: 5 settings that return 400 when you only change the model name

    <p>Claude Opus 5.5 shipped on September 22, 2026 and Claude Sonnet 5.5 on September 28. If you take code that runs on Opus 5 or Sonnet 5 and change only the <code>model</code> string, one of these five settings will return an HTTP 400. All five are in Anthropic's official migrati…

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