Claude Opus 5.5 Fast Mode: Enable It With Request Examples (No Beta Header Needed)

Posted on Oct 17 • Originally published at coremesh.online

Fast mode is officially out of research preview for Claude Opus 5.5. You no longer need to join a waitlist or pass the old beta header to get faster token streaming.

Here is the minimal setup, what changed in the October 2026 release notes, pricing details, and the breaking parameter gotchas to watch out for when migrating older Opus requests.

💡 * Originally published on CoreMesh * — Practical AI and coding guides, tested fixes, and runnable prompts.

The request body is a standard Messages API payload with one extra field, "speed": "fast":

{ "model": "claude-opus-5-5", "max_tokens": 1024, "speed": "fast", "messages": [ { "role": "user", "content": "Summarize this stack trace in two sentences." } ] } Enter fullscreen mode Exit fullscreen mode Three things to check in that body: model is claude-opus-5-5, the official Claude API ID. speed is "fast". Leave the field out and you get standard speed. There is no thinking field. Passing older thinking parameters on Opus 5.5 will trigger a 400 error (see the gotchas section below). What Changed Recently The latest release notes bundle several updates for Opus 5.5 fast mode:

Opus 5.5 introduces behavior changes unrelated to fast mode that often break copied-over requests:

I put together the full side-by-side curl/SDK examples, model comparison table, and rate limit checklist in the complete note on CoreMesh:

👉 Read the full guide: Claude Opus 5.5 Fast Mode on CoreMesh

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