Sonnet 5's sticker price is unchanged, but it now costs more tokens to do the same work

Sonnet 5 shipped with a new tokenizer, though that change wasn't called out in the announcement. Simon Willison tested it directly and found the same English text now produces about 1.4x more tokens than it did on Sonnet 4.6, with Spanish text coming in around 1.33x and Python code around 1.27x. The per-token price is unchanged ($3 per million input tokens, $15 per million output tokens), but the number of tokens needed for the same task has gone up.

Right now this is masked by introductory pricing ($2/$10 through August 31), which makes the migration look free or even like a discount. Once standard rates return on September 1, the same higher token counts, now 20 to 40% above pre-migration levels, get billed at full price, producing a meaningfully bigger invoice even though the price card never changed. Most cost dashboards track price per token, not token volume, so they won't flag this as an anomaly.

The likely explanation isn't that Anthropic is being sneaky, it's that a bigger context window and better tool use probably required a denser tokenizer, and the introductory pricing is a genuinely good deal while it lasts. The real issue is timing: anyone migrating this month who doesn't recheck their numbers on September 1 will open an invoice that looks wrong.