Claude Opus 5.5

Anthropic's flagship for agentic coding.

Knowledge cutoff Jun 2026. Thinking is adaptive and always on; extended thinking (budget_tokens) and forced tool use are not accepted. The Batches API supports 300K output with the output-300k-2026-03-24 beta header.

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{
  "model": "claude-opus-5-5",
  "messages": [
    {
      "role": "user",
      "content": "Hello, world."
    }
  ]
}