The AI you test in the afternoon may not be the AI you test at night, even with the same name

I asked Claude Opus 5 the same 40 questions twice on the same day, a few hours apart. The second time it looked things up about 60% more often, wrote about 50% more, and a score for how well it supports its claims with sources went from 59 to 90. Same model name in every answer. Same questions. Two other models barely changed. And a new version, Opus 5.5, launched in between. The most likely explanation probably wasn't a model swap. It's probably a setting around the model, how hard it is told to think, which tools can adjust quietly in the background. What I took from it: if you compare AI tools, write down every setting, including the ones you never touched. Otherwise you may be measuring the settings, not the AI. Curious whether others have seen the same tool behave differently overnight.