Claude Sonnet 5 vs Sonnet 4.6: A Skeptic's Migration Guide
Claude Sonnet 5 beats Sonnet 4.6 on SWE-bench Verified (85.2% vs 79.6%) and undercuts it on price until August 31 — a combination rare enough to deserve scrutiny. The catches are real: a new tokenizer that inflates token counts on the same text, breaking API changes around sampling parameters and extended thinking, and a discount that ends September 1. A separate Sonnet 5 rate-limit bucket, distinct from the combined Sonnet 4.x pool, makes running both models during a canary rollout unusually practical. For PMs, the move is an eval-first migration before August 31 — capture the discount window only if your regression suite clears.