Fast mode in Claude Code: Opus up to 2.5× faster, when is it worth it?

You're debugging under pressure, each Opus reply takes tens of seconds, and you wish it were faster. Fast mode is exactly that: the same Opus model, the same quality, up to about 2.5× faster, at a higher price per token. It isn't a different model but an API configuration that prioritises speed. The question is when the extra cost is worth it.

Turning it on and off

/fast                  # toggle (Space to choose, Enter to confirm)
// ~/.claude/settings.json
{ "fastMode": true }

• When on, a ↯ icon appears next to the prompt. If your current model doesn't support it, Claude Code switches to Opus.
• Supported on Opus 5.5 (generally available), Opus 5 and Opus 4.8 (research preview). Not on Sonnet or Haiku.
• By default it stays on across sessions; organisations can require per-session opt-in with fastModePerSessionOptIn.

Pricing and billing

• On Pro, Max, Ultra, Team and Enterprise plans, fast mode draws only from usage credits, not your plan's included usage, even if you have usage left. Usage credits must be turned on first.
• Team/Enterprise: an Owner must enable fast mode for the organisation.
• Not available on Amazon Bedrock, Google Cloud or Microsoft Foundry.

The cost trap: turning it on mid-session

Fast mode and standard speed keep separate caches. The first time you turn it on in a conversation, you pay the uncached fast-mode price for the entire existing context. The longer the conversation, the pricier that switch. Every later toggle rereads whatever isn't cached. So: turn it on at the start of a session and don't keep toggling.

When to use it

Compared with lowering effort: fast mode keeps quality, lowering only latency and raising price; lowering effort is faster but can reduce quality on hard tasks. You can combine both for simple tasks that need maximum speed.

Risks and caveats

• Separate rate limits: when you hit them, fast mode falls back to standard speed (the icon greys out) and re-enables after the cooldown.
• Out of usage credits: Claude Code retries at standard speed and turns fast mode off for that session.
• Behind an internal gateway: the availability check calls api.anthropic.com directly; if blocked, it may wrongly report fast mode as unavailable.

How to apply it

• Dev: Turn fast mode on at the start of an urgent debugging session; turn it off when moving to long-running work.
• Tech Lead: Enable fastModePerSessionOptIn so nobody accidentally leaves fast mode on all week.
• PM: Track monthly usage-credit spend; fast mode is the line most likely to balloon.