Why AI Models Drift From British to American Spelling in Long Outputs

AI language models often start following British spelling instructions correctly but gradually revert to American spelling as a response grows longer. This happens because each word is generated based on the full context, and American spelling dominates most technical training data, slowly outweighing the original instruction. The drift is self-reinforcing: once an American spelling appears, the probability of further American spellings increases. Technical content such as code, CSS properties, and API names is almost exclusively American by convention, pulling surrounding prose along with it. Reliable fixes require deterministic post-processing rules targeting specific spelling pattern classes, rather than simply repeating or emphasising the original instruction.