Fast mode (research preview)

Fast mode provides significantly faster output token generation for Claude Opus 4.6. By setting speed: "fast" in your API request, you get up to 2.5x higher output tokens per second from the same model at premium pricing.

Supported models

Fast mode is supported on the following models:

• Claude Opus 4.6 (claude-opus-4-6)

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)
• Same model weights and behavior (not a different model)

Basic usage

curl https://api.anthropic.com/v1/messages \
    --header "x-api-key: $ANTHROPIC_API_KEY" \
    --header "anthropic-version: 2023-06-01" \
    --header "anthropic-beta: fast-mode-2026-02-01" \
    --header "content-type: application/json" \
    --data '{
        "model": "claude-opus-4-6",
        "max_tokens": 4096,
        "speed": "fast",
        "messages": [{
            "role": "user",
            "content": "Refactor this module to use dependency injection"
        }]
    }'

Pricing

Fast mode is priced at 6x standard Opus rates for prompts <= 200K tokens, and 12x standard Opus rates for prompts > 200K tokens. The following table shows pricing for Claude Opus 4.6 with fast mode:

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.

Rate limits

Fast mode has a dedicated rate limit that is separate from standard Opus rate limits. Unlike standard speed, which has separate limits for ≤200K and >200K input tokens, fast mode uses a single rate limit that covers the full context range. When your fast mode rate limit is exceeded, the API returns a 429 error with a retry-after header indicating when capacity will be available.

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

For tier-specific rate limits, see the rate limits page.

Checking which speed was used

The response usage object includes a speed field that indicates which speed was used, either "fast" or "standard":

curl https://api.anthropic.com/v1/messages \
    --header "x-api-key: $ANTHROPIC_API_KEY" \
    --header "anthropic-version: 2023-06-01" \
    --header "anthropic-beta: fast-mode-2026-02-01" \
    --header "content-type: application/json" \
    --data '{
        "model": "claude-opus-4-6",
        "max_tokens": 1024,
        "speed": "fast",
        "messages": [{"role": "user", "content": "Hello"}]
    }'

{
  "id": "msg_01XFDUDYJgAACzvnptvVoYEL",
  "type": "message",
  "role": "assistant",
  ...
  "usage": {
    "input_tokens": 523,
    "output_tokens": 1842,
    "speed": "fast"
  }
}

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

Retries and fallback

Automatic retries

When fast mode rate limits are exceeded, the API returns a 429 error with a retry-after header. The Anthropic SDKs automatically retry these requests up to 2 times by default (configurable via max_retries), waiting for the server-specified delay before each retry. Since 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". Set max_retries to 0 on the initial fast request to skip automatic retries and fail immediately on rate limit errors.

Since setting max_retries to 0 also disables retries for other transient errors (overloaded, internal server errors), the examples below re-issue the original request with default retries for those cases.

import anthropic

client = anthropic.Anthropic()

def create_message_with_fast_fallback(max_retries=None, max_attempts=3, **params):
    try:
        return client.beta.messages.create(**params, max_retries=max_retries)
    except anthropic.RateLimitError:
        if params.get("speed") == "fast":
            del params["speed"]
            return create_message_with_fast_fallback(**params)
        raise
    except (anthropic.InternalServerError, anthropic.OverloadedError, anthropic.APIConnectionError):
        if max_attempts > 1:
            return create_message_with_fast_fallback(max_attempts=max_attempts - 1, **params)
        raise

message = create_message_with_fast_fallback(
    model="claude-opus-4-6",
    max_tokens=1024,
    messages=[{"role": "user", "content": "Hello"}],
    betas=["fast-mode-2026-02-01"],
    speed="fast",
    max_retries=0,
)

Considerations

• Prompt caching: Switching between fast and standard speed invalidates the prompt cache. Requests at different speeds do not share cached prefixes.
• Supported models: Fast mode is currently supported on Opus 4.6 only. Sending speed: "fast" with an unsupported model returns an error.
• TTFT: Fast mode's benefits are focused on output tokens per second (OTPS), not time to first token (TTFT).
• Batch API: Fast mode is not available with the Batch API.
• Priority Tier: Fast mode is not available with Priority Tier.

Next steps