Getting British Spelling Instead of American Spelling From AI

You put “use British English spelling” in the system prompt. The first three paragraphs are fine. By paragraph nine there is a color , and by the end there is an organization . The instruction was not ignored; it was outvoted. The symptom The characteristic pattern is not uniform failure. It is a document that starts correct and degrades — and the degradation is usually inconsistent within the document, so you get colour in one paragraph and color two paragraphs later, sometimes in the same sentence as behaviour . Long outputs are worse than short ones, and a long conversation is worse than a single call. A second symptom is domain-specific: the spelling holds in ordinary prose and fails in technical contexts. Code comments, API field names, CSS properties and library names are American by convention ( color is a CSS property; serialize is what the method is called), and text near them pulls the surrounding prose across. Both patterns point at the same cause, and it is not that the model did not read the instruction. Why it drifts back Each token is sampled from a distribution conditioned on everything in the context. The system prompt is part of that context, but so are the two thousand tokens the model has generated since, and so is the enormous prior from training data in which American spelling outnumbers British by a wide margin in almost every technical domain. At the start of a response the instruction is close by and there is little else in the context, so it dominates. As the response grows, the local statistics of the text being generated carry more weight relative to a single instruction several thousand tokens back. And the drift is self-reinforcing in exactly the way described in mid-answer code-switching : once one American spelling is in the context, the conditional probability of the next one rises. The key insight for fixing it is that spelling is not a mode the model is in. There is no British-English state that gets set and then holds. Each word i