How do you troubleshoot a Claude Sonnet 5 response truncated at max_tokens after migration?

First determine whether the request is running with thinking enabled, which it will be by default unless explicitly disabled, since the most likely cause of new truncation after migration is thinking tokens now consuming part of a max_tokens budget that was previously reserved entirely for the visible response on Sonnet 4.6.

Check the actual thinking-versus-response token split in the response object for this specific request type, rather than assuming a fixed proportion, since how much of the budget thinking consumes depends on both the configured effort level and how much reasoning the adaptive mechanism judged the specific task actually needed.

If confirmed, address it either by raising max_tokens to account for the added thinking budget, lowering effort so the adaptive mechanism reasons less and consumes less of the shared budget, or, for request types where reasoning genuinely isn't needed, explicitly disabling thinking so the full max_tokens allocation goes to the response.

As a broader validation step, since the tokenizer change independently affects how much of max_tokens gets consumed by a given amount of visible content, re-check whether a truncation issue is actually a combination of both factors, the shared thinking budget and the tokenizer's higher token-per-character rate, rather than assuming only one of the two migration changes is responsible.

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