Fast mode

Fast mode delivers up to 2.5x higher output tokens per second at premium pricing. Set speed: "fast" on your request to opt in. Fast mode is available on Claude Fable 5.2 and Claude Opus 5.5. Fast mode for Claude Opus 5 and Claude Opus 4.8 is in research preview. Claude Mythos 5.2 supports fast mode too.

How fast mode works

Fast mode runs the same model with a faster inference configuration. There is no change to intelligence or capabilities.

• Up to 2.5x higher output tokens per second compared to standard speed
• Speed benefits are focused on output tokens per second (OTPS), not time to first token (TTFT)
• Compatible with streaming, where the OTPS gain is most visible

Basic usage

Fast mode works only on supported models. See Availability for details, including which models need a beta header.

On Claude in Microsoft Foundry, fast mode is a separate model, such as claude-opus-5-5-fast. For Claude Fable 5.2, it is claude-fable-5-2-fast. For Claude Mythos 5.2, it is claude-mythos-5-2-fast. Send fast mode requests to a deployment of the fast model, and leave out speed. A request that sets speed on a deployment of the model without -fast returns an error.

client = anthropic.Anthropic()

response = client.messages.create(
    model="claude-opus-5-5",
    max_tokens=4096,
    speed="fast",
    messages=[
        {"role": "user", "content": "Refactor this module to use dependency injection"}
    ],
)

for block in response.content:
    if block.type == "text":
        print(block.text)

Rate limits

Fast mode has a dedicated rate limit that is separate from standard rate limits. When you exceed your fast mode rate limit, the API returns a 429 error with a retry-after header that says when capacity is available. On Claude Models on Google Cloud (Google managed), Google Cloud manages rate limits and quotas.

Claude Opus 5.5 has a default fast mode rate limit in the Start, Build, and Scale usage tiers, so you don’t need to request access to use fast mode with it. Organizations in the Evaluation tier can use fast mode with Claude Opus 5.5 once they move to the Start, Build, or Scale tier. The same applies to Claude Fable 5.2. It also applies to Claude Mythos 5.2.

The response includes headers that indicate your fast mode rate limit status:

To see your fast mode rate limits, go to the Rate limits page in the Claude Console.

Checking which speed was used

The response usage object includes a speed field that indicates which speed was used, either "fast" or "standard". Requesting speed: "fast" on a model that doesn’t support fast mode returns an error, and so does exceeding fast mode’s rate limits or capacity (a 429 or 529). When a request with speed: "fast" succeeds, usage.speed is "fast". Claude Opus 4.6 is an exception: instead of returning an error, a request with speed: "fast" and the fast-mode-2026-02-01 beta header succeeds, but runs at standard speed. In this situation, usage.speed shows "standard". On Claude Models on Google Cloud (Anthropic managed), a Claude Opus 4.6 request with speed: "fast" returns a 400 error even with the header.

client = anthropic.Anthropic()

response = client.messages.create(
    model="claude-opus-5-5",
    max_tokens=1024,
    speed="fast",
    messages=[{"role": "user", "content": "Hello"}],
)

print(response.usage.speed)  # "fast" or "standard"

{
  "id": "msg_01XFDUDYJgAACzvnptvVoYEL",
  "type": "message",
  "role": "assistant",
// ...
  "usage": {
    "input_tokens": 8,
    "output_tokens": 12,
    "speed": "fast"
  }
}

To track fast mode usage and costs across your organization, see the Usage and Cost Analytics API.

Retries and fallbacks

Automatic retries

The SDK automatically retries fast mode 429 errors up to two times by default (configurable with max_retries), waiting for the delay in the retry-after header before each retry. Because fast mode uses continuous token replenishment, the retry-after delay is typically short and requests succeed once capacity is available.

Falling back to standard speed

If you’d prefer to fall back to standard speed rather than wait for fast mode capacity, catch the rate limit error and retry without speed: "fast". On Claude in Microsoft Foundry, the deployment sets the speed, so retry on a deployment of the model without -fast instead. Set max_retries to 0 on the initial fast request to skip automatic retries and fail immediately on rate limit errors.

Because setting max_retries to 0 also disables retries for other transient errors (overloaded, internal server errors), the following examples resend the request after those errors.

client = anthropic.Anthropic()


def create_message_with_fast_fallback(max_retries=0, max_attempts=3, **params):
    try:
        return client.with_options(max_retries=max_retries).messages.create(**params)
    except anthropic.RateLimitError:
        if params.get("speed") == "fast":
            del params["speed"]
            return create_message_with_fast_fallback(max_retries=max_retries, **params)
        raise
    except (
        anthropic.APIStatusError,
        anthropic.APIConnectionError,
    ) as error:
        if isinstance(error, anthropic.APIStatusError) and error.status_code < 500:
            raise
        if max_attempts > 1:
            return create_message_with_fast_fallback(
                max_retries=max_retries, max_attempts=max_attempts - 1, **params
            )
        raise


message = create_message_with_fast_fallback(
    model="claude-opus-5-5",
    max_tokens=1024,
    messages=[{"role": "user", "content": "Hello"}],
    speed="fast",
    max_retries=0,
)

Considerations

• Batch API: Fast mode is not available with the Batch API.
• Priority Tier: Fast mode is not available with a Priority Tier commitment.

Pricing

Fast mode is priced at a multiplier on standard rates across the full context window, including requests over 200k input tokens. On Claude Models on Google Cloud (Google managed), Google Cloud sets fast mode prices. See Cloud platform pricing.

Fast mode pricing stacks with other pricing modifiers:

• Prompt caching multipliers apply on top of fast mode pricing.
• Data residency multipliers apply on top of fast mode pricing.

For complete pricing details, see the Pricing page.

Availability

Fast mode is available only on the following models. If you were using fast mode on an earlier model, migrate to a model that supports fast mode.

To access fast mode on Claude Opus 5, contact your account manager or join the waitlist.

Next steps