Define thinking (reasoning) in the context of Claude Opus 5?

Thinking, also called reasoning, is Claude working through a problem step by step before producing its final answer, and on Claude Opus 5 it runs by default on every request rather than needing to be explicitly turned on.

This is a first for the Opus model family - every Opus version before Opus 5 had thinking off by default unless a request explicitly set it, such as with thinking: {"type": "adaptive"} - though Claude Sonnet 5 had already introduced this on-by-default behavior before Opus 5 did.

The amount of thinking Claude Opus 5 actually performs on a given turn is adaptive and controlled by the effort parameter, so higher effort levels generally allow deeper reasoning, at a correspondingly higher token cost.

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