Why is thinking on by default a breaking change when migrating to Claude Opus 5?

Because max_tokens caps thinking and the visible reply together as one shared budget, a request that previously produced no thinking at all on Opus 4.8 now consumes part of that same budget on thinking before Opus 5 even starts on the visible answer.

If an application's max_tokens value was tuned assuming the full budget went to the visible reply, as it did by default on Opus 4.8, that same value on Opus 5 can leave less room than expected for the answer - the request still returns successfully with HTTP 200, but with stop_reason: max_tokens and the answer cut off mid-sentence, rather than throwing an obvious error.

This makes it a quiet breaking change: nothing in the API contract technically fails, so it's easy to miss during migration testing unless truncated responses are specifically checked for on requests that previously fit comfortably within budget.

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