Fast Mode & API Breaking Changes

Fast Mode

Fast Mode produces responses 2.5x faster in exchange for a 2x higher cost (verified for Opus 4.8). The underlying model stays Opus 4.8, not a different one. Fast Mode prioritizes resource allocation, without simplifying reasoning.

In CLI: /fast activates the mode for the current session.

When to Use Fast Mode

Fast Mode is worthwhile when response time has direct business value: live demo, intensive pair programming, mechanical code generation in a tight loop. It is not a replacement for Sonnet on simple tasks: Sonnet remains 10x cheaper.

Relevant:

• Repetitive boilerplate generation in an interactive session
• Reformatting or large-scale code conversion
• Demo context where visible latency impacts the experience

Not relevant:

• Background or asynchronous tasks (speed does not matter)
• Simple tasks covered by Sonnet or Haiku
• Tight API budgets

Fast Mode via API (Opus 4.8)

response = client.messages.create(
    model="claude-opus-4-8",
    speed="fast",
    extra_headers={
        "anthropic-beta": "fast-mode-2026-02-01"
    },
    messages=[{"role": "user", "content": "..."}]
)

The beta header is mandatory. Without it, the speed parameter is silently ignored.

Breaking Change: assistant-prefill Removed

Opus 4.6 removed support for assistant prefill: the technique that allowed pre-filling Claude’s response to guide its output format.

# Before (no longer works on Opus 4.6)
messages=[
    {"role": "user", "content": "Respond in JSON"},
    {"role": "assistant", "content": "{"}  # prefill
]

# After — use the system prompt
system="Always respond with valid JSON only."

Impact: any API pipeline using assistant-prefill on Opus 4.6 must migrate to explicit instructions in the system prompt or few-shot examples.

The effort Parameter (API)

The effort parameter replaces budget_tokens on Opus 4.6 for controlling reasoning depth.

output_config={"effort": "medium"}  # low|medium|high|max

budget_tokens remains functional on Opus 4.5 but is deprecated on 4.6. Migrate to effort for new pipelines.

Summary of Opus 4.6 Changes