PulseAugur
EN
LIVE 03:31:37

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

How we ranked this

Signal score
55 / 100
Composite score across the factors below. Higher = stronger signal that this story matters right now.
Newsworthiness bucket
Significant
New model releases from a frontier lab with specific API changes. [lever_c_demoted from frontier_release: ic=1 ai=1.0]
Source corroboration
Single-source cluster
Only one publisher covered this so far. Single-source stories can still rank when the publisher is high-authority, but they lack cross-source corroboration.
Topics
model release, product
Editorial topic classification. Feeds into how the story surfaces on /topic/<slug> hub pages and into the per-entity coverage mix.
AI-industry relevance
High
Clearly on-topic for AI-industry coverage.
Story freshness
Breaking (< 6h)
Fresh story with cross-source coverage still developing. Ranking may shift as more sources report.

Full methodology in our editorial standards.

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…

RELATED ENTITIES

RELATED TOPICS