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

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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 Smallest Request That Turns On Fast Mode

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."
    }
  ]
}

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:

No waitlist: Fast mode is out of research preview for Claude Opus 5.5.
No beta header required: Requests to Opus 5.5 no longer require the fast-mode-2026-02-01 beta header (though requests that still send it will keep working). Note: If you are still calling claude-opus-5 or claude-opus-4-8, keep sending the beta header.
Default rate limits: Opus 5.5 now has a default fast mode rate limit across the Start, Build, and Scale usage tiers.
Azure support: Available on the first-party Claude API and on Claude in Microsoft Foundry for deployments hosted on Azure (not yet available on AWS/Bedrock or Batch API).

Pricing & When to Use It

Opus 5.5 Fast Mode: $8 USD / MTok (Input) · $40 USD / MTok (Output)
Fast mode pricing applies across the full context window (including requests over 200k input tokens), and prompt caching or data residency multipliers apply on top.
Rule of thumb: Since fast mode is not available with the Batch API, save "speed": "fast" for interactive paths where a user is actively watching tokens stream in, and route offline jobs to standard/batch.

Common Gotchas When Migrating Old Requests

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

The thinking parameter returns a 400 error: Both thinking: {"type": "disabled"} and thinking: {"type": "enabled", ...} will fail. Omit thinking entirely and use the effort parameter (low, medium, high) to control reasoning depth.
tool_choice types any and tool return a 400 error: Use auto with strict tool use instead.
Computer use toolset version: On the Claude API and Google Cloud, computer use now requires computer_toolset_20260801 (the older computer_20251124 returns a 400).

Read the full breakdown

The complete side-by-side curl/SDK examples, the model comparison table and the rate limit checklist are in the original article on CoreMesh: Claude Opus 5.5 Fast Mode.

This article was drafted by an AI system. Facts were checked against Anthropic's official documentation, which the original article lists as sources, and a human reviewed it before publishing.

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