Why does max_tokens behave differently on Claude Opus 5 compared to Claude Opus 4.8?

On Claude Opus 4.8, a request that didn't explicitly enable thinking produced no thinking tokens at all, so the full max_tokens budget was effectively available to the visible reply by default.

On Claude Opus 5, thinking runs by default for the same kind of unconfigured request, and thinking tokens draw from that same max_tokens budget before the visible reply gets its share - so the same numeric max_tokens value now has to cover both, rather than just the answer.

The practical effect is that a max_tokens setting that was comfortably generous on Opus 4.8 can become tight on Opus 5 for the exact same request, since part of that budget is now consumed by reasoning the request wasn't previously spending any tokens on.

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