The Pink Elephant in Your Prompt

Cross-posted. This post was first published on the Bonsai blog. I’ve been playing with this idea in my head that we can get better results if we blind our agent to certain things. You know the drill. You already have something and you want a fresh take on it, so you paste the old one in and write “don’t base it on this”. But to disregard the existing result, the agent needs to know about the existing result, which defeats the purpose of the ask. It’s the old pink elephant problem: tell someone not to think of a pink elephant, and now that’s all they can think about. I wanted proof, at least for agents, so I ran 720 trials against Claude Haiku 5.5 and Claude Sonnet 5.5. The experiment I asked for the same thing every time: a Python RateLimiter class with an allow(key) method. There are about five standard ways to build one, so it’s easy to count what the model picked. With no existing code in the prompt, both models go for the same one, a sliding window log, 88% of the time. That’s its…