Claude Sonnet 5: Adaptive Thinking and Effort in Python

Anthropic shipped Claude Sonnet 5 on June 30, 2026, and made it the default model on the free and Pro tiers the same day. The headline is that it closes most of the agentic gap with Opus at Sonnet prices. The part that will quietly break your code is smaller and easier to miss: Sonnet 5 thinks by default. If you port a Sonnet 4.6 script by swapping the model ID, the same request now spends extra output tokens on reasoning, and an output limit that used to be plenty can suddenly truncate your answer.