The whole hunk
from line 4, old and new numbered
/
lines
from line 4
44description: Overview of new features and behavior changes in Claude Opus 5.
55---
66
7<Note>
8 Claude Opus 5.5 is the current Opus model. See [What's new in Claude Opus 5.5](https://platform.claude.com/docs/en/models/opus-5-5/whats-new-opus-5-5).
9</Note>
10
711Claude Opus 5 is a step-change improvement over Claude Opus 4.8, with the largest gains in deep reasoning, agentic and long-horizon tasks, and test-time compute scaling. This page summarizes everything new in Claude Opus 5, including mid-conversation tool changes and two breaking changes for code running on Claude Opus 4.8: thinking is on by default, and thinking can be disabled only at effort `high` or below.
812
913## New model
from line 240
236240
237241### Disabling thinking requires effort `high` or below
238242
239On Claude Opus 5, `thinking: {"type": "disabled"}` is accepted only when the effort level is `high` or below. Setting `thinking: {"type": "disabled"}` with effort `xhigh` or `max` returns a 400 error. This rule is enforced on every request to Claude Opus 5 and later models. It is a breaking change from Claude Opus 4.8, where disabling thinking was independent of the effort level. If your Claude Opus 4.8 requests disable thinking at effort `xhigh` or `max`, either keep thinking disabled and set effort to `high` or below, or keep the effort level and remove the `thinking` field.
243On Claude Opus 5, `thinking: {"type": "disabled"}` is accepted only when the effort level is `high` or below. Setting `thinking: {"type": "disabled"}` with effort `xhigh` or `max` returns a 400 error. This rule is enforced on every request to Claude Opus 5. It is a breaking change from Claude Opus 4.8, where disabling thinking was independent of the effort level. If your Claude Opus 4.8 requests disable thinking at effort `xhigh` or `max`, either keep thinking disabled and set effort to `high` or below, or keep the effort level and remove the `thinking` field.
240244
241245With thinking disabled, Claude Opus 5 can occasionally write a tool call into its text output instead of emitting a `tool_use` block, or include internal XML tags in its visible response. Where possible, keep thinking enabled and control token cost with lower effort levels; for integrations that must keep thinking disabled, see [Running with thinking disabled](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-opus-5#running-with-thinking-disabled) for prompting mitigations.
242246