What is the difference between disabling thinking on Opus 5 vs Fable 5?

On Claude Opus 5, thinking can be explicitly disabled with thinking: {"type": "disabled"}, but only at effort level high or below - attempting this at xhigh or max effort returns an HTTP 400 error, since those higher effort levels require thinking to stay on.

On Claude Fable 5 and Claude Mythos 5, adaptive thinking is always on with no disabled mode available at any effort level at all - it's simply not a configuration option the way it is, with restrictions, on Opus 5.

This reflects a broader pattern across Anthropic's current lineup: Opus 5 offers a genuine, if now restricted, choice about whether thinking runs, while the Mythos-tier models treat always-on adaptive thinking as a fixed characteristic rather than a togglable setting.

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