I Thought the Model Drifted. My Cache Key Was Serving Tuesday.

Have you ever watched an LLM endpoint return a clean answer that belonged to a different prompt entirely? I spent forty-eight hours blaming sampling noise, temperature, and a free model that would not sit still. The request logs looked honest enough, and the health check on the box stayed green the whole time. The bug was quieter than that: a cache key that hashed the user message and ignored everything else that actually changes a completion. I was trying to keep a small eval loop cheap, which is a very ordinary instinct. Free-model access is useful when you want overnight volume without treating every call as precious. I parked a thin HTTP wrapper on a free server, hashed each prompt, and stored the JSON body on disk so retries would not hammer the model. Does that sound reasonable? It did, until two different system prompts started colliding on the same key and I spent a day chasing "nondeterminism" that was just a hash. I ran that wrapper against MonkeyCode's free model access on the free server option because I wanted a boring place to reproduce the cache bug, not a production SLA. Disclosure: This article was prepared as part of MonkeyCode's product outreach. Nothing below depends on a named model, a quota, or a hardware claim. The lesson is the key function, and it still applies if you delete the product name from the stack. What I walked into The wrapper looked like every weekend cache I have written under time pressure. Incoming POST bodies were reduced to user_message , run through hashlib.sha256 , and written under ./cache/<hex>.json . A hit returned the file. A miss called the model, then wrote the file. I even logged X-Cache: HIT so future-me would feel scientific. That design has one attractive property and one fatal one. The attractive property is that identical user text becomes free after the first call. The fatal property is that user text is not the request. System prompt, temperature, stop sequences, tool schemas, and even a date injected into th