Anthropic launched Claude Opus 5.5 on September 22, 2026, a model designed for long-running agentic coding with a default 1M token context window, 128k max output tokens, and always-on adaptive thinking, priced at $4/$20 USD per million tokens. The model is available across multiple platforms including Claude API, Amazon Bedrock, Google Cloud, and Microsoft Foundry, with new features including fast mode and mid-conversation tool definitions.