prompting-claude-opus-5-5 changedbuild-with-claude/prompt-engineering/prompting-claude-opus-5-5

Nearest release: v2.1.287, published 4 hours before this site recorded the change. Shown because the two are within 24 hours of each other. Nothing here says the release caused the edit.

Recorded here 1 Oct 2026 21:37 UTC

Lines+2added

Lines−2removed

From line 50 where the diff opens

First seen 23 Sep 2026 this site's first read of the page

Recorded edits2to this page, all time

The whole hunk

from line 50, old and new numbered

lines

from line 50

5050Claude Opus 5 accepts `thinking: {"type": "disabled"}` at `high` effort or below; Claude Opus 5.5 doesn't, and the [migration guide](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide#migrating-from-claude-opus-5) covers the request change. If your Claude Opus 5 integration ran with thinking disabled, four changes go with it:

5151

5252* **Start at `low` effort and measure.** At `low` the model keeps its thinking short. How often it skips thinking altogether depends on your prompts, so measure latency and quality on your own traffic and move to `medium` if quality drops. If time to first token still matters after that, a system prompt line such as "Answer directly without deliberating." can reduce thinking further; measure quality when you add it, because less thinking can lower it.

53* **Remove instructions that stood in for thinking.** If your prompt asked the model to write out its reasoning in the response as a substitute for thinking, remove that instruction and read the reasoning from [summarized thinking](https://platform.claude.com/docs/en/build-with-claude/thinking#summarized-thinking) blocks instead (`display: "summarized"`); a prompt that pushes the model to reproduce its reasoning in the response text can be declined with the `reasoning_extraction` [refusal category](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#refusal-response).

53* **Remove instructions that stood in for thinking.** If your prompt asked the model to write out its reasoning in the response as a substitute for thinking, remove that instruction and read the reasoning from [summarized thinking](https://platform.claude.com/docs/en/build-with-claude/thinking#summarized-thinking) blocks instead (`display: "summarized"`); a prompt that pushes the model to reproduce its reasoning in the response text may be declined with the `reasoning_extraction` [refusal category](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#refusal-response).

5454* **Re-test the thinking-disabled mitigations.** [Running with thinking disabled](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-opus-5#running-with-thinking-disabled) recommends a combined instruction (permission to speak before a tool call, what to do when no tool fits, no internal tags) and removing any rule that tells the model not to think. Both address artifacts that appear on Claude Opus 5 only when thinking is disabled. With thinking always on, check whether you still need the instruction, and remove the no-thinking rule either way.

5555* **Read the response by block type.** Check each block's type instead of assuming the first content block is text: a response may or may not begin with a `thinking` block, whose `thinking` field is empty under the default `display: "omitted"`.

5656

from line 80

8080

8181* **Biology:** The biology safeguards are the same as Claude Fable 5.1's and are new if you're coming from Claude Opus 5. Everyday health and educational questions are unaffected. If the biology classifier gets in the way of your organization's life sciences work, apply to the [Life Sciences Verification Program](https://www.anthropic.com/news/life-sciences-verification-program).

8282* **Cybersecurity:** Finding vulnerabilities in source code is allowed. High-risk dual-use cybersecurity activities are not.

83* **Reasoning extraction:** Requests that push the model to reproduce its internal reasoning in the response text can be declined with the `reasoning_extraction` category, which is new if you're coming from Claude Opus 5. If your prompts ask the model to write out its reasoning in the response, remove those instructions, set `display: "summarized"`, and read the summarized reasoning from the thinking blocks instead; see [Prompts written for thinking disabled](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-opus-5-5#prompts-written-for-thinking-disabled).

83* **Reasoning extraction:** Requests that push the model to reproduce its internal reasoning in the response text may be declined with the `reasoning_extraction` category. If your prompts ask the model to write out its reasoning in the response, remove those instructions, set `display: "summarized"`, and read the summarized reasoning from the thinking blocks instead; see [Prompts written for thinking disabled](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-opus-5-5#prompts-written-for-thinking-disabled). You can still ask for a short explanation of the answer or a summary of the actions taken; see [Keep reasoning in thinking blocks](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#keep-reasoning-in-thinking-blocks).

8484

8585A classifier decline arrives as a normal response with `stop_reason: "refusal"` and a `stop_details` object naming the category. You can have the request retried automatically on a fallback model, except for `reasoning_extraction` declines, which server-side fallback returns to you instead of retrying; see [Refusals and fallback](https://platform.claude.com/docs/en/models/opus-5-5/whats-new-opus-5-5#refusals-and-fallback).

8686