Why does Claude Sonnet 5 run with thinking on by default?

On Sonnet 4.6, a request that didn't include a thinking field ran without any reasoning step at all; on Sonnet 5, that same unconfigured request now runs with adaptive thinking automatically, reflecting Anthropic's broader shift toward reasoning-by-default across its current model generation.

This change is framed as raising the baseline quality of unconfigured requests - rather than requiring every integration to explicitly opt in to reasoning to get its benefit, Sonnet 5 assumes reasoning is generally worth the extra tokens unless a request explicitly opts out.

The tradeoff is that requests which previously ran cheaply and quickly with no reasoning at all now consume additional tokens and time by default, which is why Anthropic's guidance specifically calls out revisiting max_tokens budgets and explicitly disabling thinking, via thinking: {"type": "disabled"}, for latency-sensitive workloads like streaming chat or simple classification where reasoning adds little value.