Claude Opus 5.5 Fast

• ⚡ Same Opus 5.5 model served with a faster inference configuration
• 📏 Full 1M-token context window, up to 128K output tokens
• 🧠 Adaptive thinking that cannot be fully disabled on 5.5
• 👁️ Vision plus text inputs, text output
• 🔧 Tool calling and structured outputs supported
• 🎯 Built for long-running agentic coding and knowledge work
• 🏢 Anthropic's Opus line; Fast is an opt-in serving mode
• 🆕 Successor to the Opus 5 Fast serving tier in the same family

Anthropic

Anthropic PBC is an American artificial intelligence company headquartered in San Francisco. Structured as a public benefit corporation, the lab develops large language models under the Claude name, with a research emphasis on building reliable, steerable,…

Claude Opus 5.5 Fast is a speed-optimized serving tier for Anthropic's Opus 5.5 model rather than a separate model. Anthropic's platform documentation describes fast mode as the same model with a faster inference configuration, with no change to intelligence or capabilities; on the Claude API it is enabled through a speed setting together with a fast-mode beta header.

It exposes the same specification as Claude Opus 5.5: a 1M-token context window served by default with no beta header, up to 128K output tokens (300K via a Batch API beta), vision, tool calling, and adaptive thinking that cannot be turned off.

Against its direct predecessor, Claude Opus 5 Fast, any capability change is inherited from the base generation rather than from the serving tier itself, since both tiers simply run the corresponding Opus model on a faster path. Anthropic's migration guide is aimed at users moving from Claude Opus 5 and notes behavioural differences to account for, and it also documents that Claude Opus 4.7 rejects fast-mode requests entirely.

Practically, the Fast tier suits latency-sensitive interactive coding and agent loops where wall-clock time matters more than per-token efficiency; for batch or throughput-driven workloads, the standard Opus 5.5 endpoint is the more natural default, as are lighter siblings such as Claude Sonnet 5 or Claude Fable 5.1 where full Opus-class reasoning is not required.