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Sweep 22 Oct 2026 · 04:32Z Build v2.1.308 527 read Stable v2.1.308 Latest v2.1.308 Next v2.1.308 Feeds RSS JSON llms.txt llms-full.txt Unofficial
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migration-guide changedmodels/opus-5-5/migration-guide

Nearest release: v2.1.301, published 5 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.

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First seen 23 Sep 2026 this site's first read of the page
Recorded edits18to this page, all time

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309309* Remove any context-window beta header.
310310* If you rebuild conversation history to update instructions, consider switching to a mid-conversation system message to preserve prompt cache hits.
311311* Verify your stop-reason handling reads `stop_details` on refusals.
312* If you want fast mode, which Claude Opus 4.7 rejects, set `speed: "fast"` with the `fast-mode-2026-02-01` beta header on the Claude API.
312* If you want fast mode, which Claude Opus 4.7 rejects, set `speed: "fast"` on your Claude API requests. Claude Opus 5.5 doesn't need a beta header.
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314314### Claude Opus 4.6 or earlier
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120512054. **Refusal stop details:** When the model declines a request, Claude Opus 5.5 returns a `stop_details` object that names the category of refusal, alongside the `refusal` stop reason. Claude Opus 4.7 returns the same object, so this matters only if your stop-reason handling does not read it yet. No beta header is required, and there is no opt-out. If your stop-reason handling doesn't read it yet, see [Handling stop reasons](https://platform.claude.com/docs/en/build-with-claude/handling-stop-reasons). Claude Opus 5.5 declines in more categories; see [Safety classifiers and fallback](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide#safety-classifiers-and-fallback).
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12075. **Fast mode:** Claude Opus 5.5 supports [fast mode](https://platform.claude.com/docs/en/build-with-claude/fast-mode) (research preview) on the Claude API. Fast mode is not available on Claude Opus 4.7, where requests with `speed: "fast"` return an error. Set `speed: "fast"` with the `fast-mode-2026-02-01` beta header.
12075. **Fast mode:** Claude Opus 5.5 supports [fast mode](https://platform.claude.com/docs/en/build-with-claude/fast-mode) on the Claude API. Fast mode is not available on Claude Opus 4.7, where requests with `speed: "fast"` return an error. Set `speed: "fast"` on your requests. Claude Opus 5.5 doesn't need a beta header.
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120912096. **Computer use toolset and browser use tool:** On the Claude API and Google Cloud, Claude Opus 5.5 supports [computer use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool) as the `computer_toolset_20260801` toolset and the [browser use tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/browser-use-tool) for tasks inside webpages. Claude Opus 4.7 supports neither. On those platforms Claude Opus 5.5 doesn't accept the earlier `computer_20251124` tool; see the [computer use breaking change](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide#computer-use-toolset).
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