Why does Claude Sonnet 5's tokenizer change matter for migration?

Sonnet 5 uses a new tokenizer that maps the same input text to roughly 30% more tokens than Sonnet 4.6's tokenizer did, meaning token counts, cost estimates, and context-window capacity calculations calibrated against Sonnet 4.6 don't transfer directly.

Because per-token pricing itself is separate from this change, the practical effect is that an equivalent request costs more in absolute terms even without any pricing change, and a max_tokens value that was generous on Sonnet 4.6 can become tight on Sonnet 5 purely from this token-count shift, independent of the separate thinking-by-default change.

The recommended fix is mechanical but essential: re-run token counting against Sonnet 5 specifically, using the token counting API on representative prompts, rather than reusing figures measured against Sonnet 4.6 or assuming the tokenizer change is negligible.

Invest now in Acorns!!! 🚀
Join Acorns and get your $5 bonus!

Acorns is a micro-investing app that automatically invests your "spare change" from daily purchases into diversified, expert-built portfolios of ETFs. It is designed for beginners, allowing you to start investing with as little as $5. The service automates saving and investing. Disclosure: I may receive a referral bonus.

Invest now!!! Get Free equity stock (US, UK only)!

Use Robinhood app to invest in stocks. It is safe and secure. Use the Referral link to claim your free stock when you sign up!.

The Robinhood app makes it easy to trade stocks, crypto and more.

Webull! Receive free stock by signing up using the link: Webull signup.