# Anthropic Claude Opus 5.5 rejects disabled thinking, causing 400 errors

Digest AI · Generative AI & Models · published 2026-10-23T17:13:00Z

Canonical: https://digestai.news/story/anthropic-claude-opus-5-5-rejects-disabled-thinking-causing-400-errors

## Summary

Anthropic has released Claude Opus 5.5, Fable 5.1, and Sonnet 5.5, introducing breaking changes to the API that may disrupt existing automation. The most significant change is that Claude Opus 5.5 no longer accepts requests to disable or explicitly enable thinking; including `thinking: {"type": "disabled"}` or `thinking: {"type": "enabled"}` now returns a 400 error. Developers must omit the `thinking` field entirely and instead use the `effort` parameter to control the depth of reasoning.

Additionally, the new model rejects forced tool specifications using `tool_choice` set to `any` or specific tool names. Users are required to switch to `auto` mode and utilize strict tool use configurations. The release also discontinues the fast mode for the previous Opus 4.7 model. While these changes do not affect the Claude Code tool directly, they impact any custom automation or application that calls the Claude API directly, such as news gathering or article generation systems.

Developers are advised to review their code for instances of the deprecated `thinking` and `tool_choice` parameters. The article notes that similar code written for Sonnet 5 may break in five places when migrated to Sonnet 5.5. These updates are part of a broader ecosystem shift, coinciding with OpenAI deprecating older model snapshots like gpt-4o-2024-05-13 and o1 on the same date.

## Key points

- Claude Opus 5.5 returns a 400 error if thinking is disabled or explicitly enabled.
- Developers must omit the thinking field and use the effort parameter for control.
- Forced tool_choice specifications are rejected; users must switch to auto mode.

## Why it matters

Developers using the Claude API for automation must update their code immediately to avoid service interruptions. The removal of explicit thinking controls and forced tool choices changes how agents are built, requiring a shift to effort-based reasoning and auto tool selection.

## Sources

1. [Claude Opus 5.5, disabling thinking results in 400 error](https://note.com/bhrtaym/n/n25ba72cd1f10?hl=en) (note.com, 2026-10-23)

## Cite

Digest AI, "Anthropic Claude Opus 5.5 rejects disabled thinking, causing 400 errors", 23 October 2026, https://digestai.news/story/anthropic-claude-opus-5-5-rejects-disabled-thinking-causing-400-errors

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