Claude Opus 5.5 from Anthropic is now available on AI Gateway as anthropic/claude-opus-5.5 .

Opus 5.5 is Anthropic’s strongest Opus model for agentic coding, long-running agent tasks, and professional knowledge work. It communicates progress, findings, and next steps clearly, making extended work easier to supervise.

It has a 1M-token context window and supports up to 128K output tokens. Compared with Opus 5, Anthropic has stated that Opus 5.5 performs at a similar level to Fable 5.1, but is ~30% faster and ~40% cheaper per task than Opus 5.

The new model includes two API changes that can turn previously valid requests into HTTP 400 errors:

Thinking is always adaptive. Requests that disable thinking or set a fixed thinking budget are rejected. Use an effort level and prompting to control reasoning depth.

Forced tool choice is unsupported. Requests cannot require a tool call or force a specific tool. Use automatic tool selection and prompt the model when it should call a tool.…