What is the difference between migrating from Sonnet 4.6 versus from Sonnet 4.5 or earlier?

Migrating specifically from Sonnet 4.6 only requires handling the changes introduced with Sonnet 5 itself: adaptive thinking on by default, manual extended thinking rejected, non-default sampling parameters rejected, and the new tokenizer.

Migrating from Sonnet 4.5 or an earlier version requires applying those same Sonnet-5-specific changes plus an additional set of older breaking changes that were already introduced between Sonnet 4.5 and Sonnet 4.6 - specifically, assistant message prefilling being rejected and differences in how tool parameters are JSON-escaped.

This distinction matters because code still running on a pre-4.6 Sonnet version has more total ground to cover during migration than code already running on 4.6, even though both end up on the same Sonnet 5 model - the older codebase simply has a longer list of accumulated changes to work through.

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