Prompt Cache Diagnostics GA: Raise Hit Rate Without Rewriting Everything

Coffee Summary

• FACT (changelog Sep 10, 2026): Prompt Cache Diagnostics is generally available in the Responses API for GPT-5.6 and later supported models.
• Compare cache reuse against a previous response, see miss reasons, and follow troubleshooting guidance.
• Goal: improve reuse (latency + cost) without inventing a brand-new prompt architecture on day one.
• Works alongside earlier Prompt Caching dashboard metrics.
• Still model- and API-surface constrained — verify your model is supported.

What happened

OpenAI’s September 10, 2026 changelog marks Prompt Cache Diagnostics as GA on the Responses API for GPT-5.6+ supported models. The feature is aimed at a concrete debugging loop: compare how much of a prompt was reused versus a previous response, identify why a cache miss happened, and apply troubleshooting steps to improve reuse.