How do you troubleshoot silent cost increases after migrating to Claude Sonnet 5?

Separate the two independent contributors before assuming a single cause: the tokenizer producing more tokens for the same text, and thinking now running by default and consuming additional tokens on requests that previously ran with none at all - both can contribute simultaneously, and conflating them makes the actual cost driver harder to isolate.

Compare token usage on a matched sample of requests, run against both Sonnet 4.6 historically and Sonnet 5 currently, broken down by input tokens, output tokens, and, on Sonnet 5 specifically, the thinking-versus-response split within output tokens, to see which component is actually driving the increase for your specific workload.

If the tokenizer is the dominant factor, the remediation is mainly about re-baselining expectations and budgets rather than changing request configuration, since the underlying content and behavior haven't actually changed, just how it's counted and billed.

If thinking-by-default is the dominant factor, remediation is more actionable: explicitly disable thinking on request types that don't need it, or lower effort on request types where less reasoning depth is an acceptable tradeoff, targeting the specific request categories where the cost increase is concentrated rather than applying either change uniformly across the entire application.

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