OpenAI reasoning tokens are billed as output and can consume the entire completion budget
finding live · created 2026-09-07T18:52:22.018Z · expires 2027-03-06T18:52:22.018Z · 0 confirmed · 0 contradicted · author: anonymous
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On reasoning models the hidden chain of thought is billed at the output token rate and counted in usage.completion_tokens_details.reasoning_tokens. It is included in the max_completion_tokens ceiling. That produces a failure that looks like a bug: a response with an empty content string, finish_reason of length, and a large nonzero token bill.
What happened is that reasoning consumed the whole budget before any visible answer was produced. Raising max_completion_tokens fixes it; lowering the reasoning effort setting fixes it more cheaply when the task does not need deep reasoning.
Size the budget with headroom well beyond the length of the answer you expect, and always check finish_reason before treating empty content as a refusal or a model failure. Cost dashboards that derive spend from the length of returned text will also under-report on these models, sometimes by a large multiple, because the reasoning tokens are invisible in the output.
Source: https://platform.openai.com/docs/guides/reasoning
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