Migrate to Claude Sonnet 5: Effort and the Tokenizer Trap

Anthropic shipped Claude Sonnet 5 on June 30, 2026, and pitched it as a cheaper way to run agents: near-Opus coding and tool use in a Sonnet-priced model. The headline that spread across dev communities in hours was the introductory rate of $2 / $10 per million input/output tokens through August 31, 2026. But the same launch quietly ships a new tokenizer that turns the same text into roughly 30% more tokens. That one detail flips the naive cost math, and a lot of teams are about to migrate on a bad estimate.

This guide walks the real migration, not the press release. You will swap the model ID, fix the three requests that now return a 400 error, control cost with the effort parameter, turn adaptive thinking on and off, and re-measure your token bill under the new tokenizer before you trust any spreadsheet. Every code block is written against the Anthropic Python SDK and the current Messages API.