Caching is borked for GPT 5 models

[vb]

I’ll add my input by linking two more recent independent reports on the same issue. Hope this helps pinpoint the root cause!

Caching rate drop after switching to Responses API

Recently switched app backend to Responses API and migrated from gpt-5 to gpt-5.1. Seeing cache rate (% cached input tokens) dropping from 40% to 0% after this change. What am I doing wrong? Does caching work differently for Responses API vs Chat Completions API?

Possible Cache Issue on GPT-5-mini and GPT-5-nano

Hello everyone, I wanna report something I been testing these last days, because maybe is helpful for other devs too. I made a big batch of tests (around 30 runs) using different models: gpt-5.1, gpt-5-nano, and gpt-5-mini. And right now, only GPT-5.1 is showing consistent caching behavior. When I tested GPT-5-mini and GPT-5-nano, the cache almost never hits. For example: I tried 20 prompts with exact same system input, and only 2 requests got cached successfully. The rest came with cache…