What is the difference between adaptive thinking and manual extended thinking?

Adaptive thinking is the mode Claude Sonnet 5 uses by default: the model itself decides how much to reason based on the task's apparent difficulty and the configured effort level, rather than following a fixed, developer-specified token budget for reasoning.

Manual extended thinking, by contrast, was the older pattern where a request explicitly set thinking: {"type": "enabled", "budget_tokens": N} to give the model a fixed reasoning budget - this mode is no longer supported on Sonnet 5 and returns an HTTP 400 error if attempted.

The practical migration path is to remove any manual budget_tokens configuration entirely and instead use the effort parameter (low through max) to influence how much the model reasons, letting adaptive thinking handle the actual moment-to-moment decision of how much reasoning a given request needs.

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