Users are reporting issues with the prompt caching feature in OpenAI's GPT-5.6 models, specifically the sol, terra, and luna variants. Despite paying the new 1.25x cache-write fee and observing cache writes, the `cached_tokens` field in the API response consistently shows zero. This behavior is reproducible with simple API calls and differs from GPT-5.5, where caching functions as expected with significant cost savings. The unreliability of the caching mechanism raises concerns about billing accuracy and the validity of cost-saving calculations for users relying on this feature. AI
IMPACT Potential for increased operational costs and unreliable performance for users relying on GPT-5.6's caching feature.
RANK_REASON User-reported bug in a specific model variant's feature.
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