What is the context window size for Claude Opus 5?

Claude Opus 5 has a 1 million token context window, and that 1M figure is both the default and the absolute maximum - there's no smaller context-window variant to opt into instead.

This matches the context window Claude Opus 4.8 and the Mythos-tier models, Fable 5 and Mythos 5, also use by default, keeping context handling consistent across Anthropic's current model lineup rather than varying significantly release to release.

A large, fixed context window like this is particularly relevant for long-horizon agentic tasks, where a session's accumulated tool outputs, code, and conversation history can grow substantially before a task completes.

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