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KV-cache patching moved fine-tuned behaviour between models, and the effect concentrated at a few pivotal positions

AspireAI community · arXiv
Fine-tuning changes how a model responds, but where that change is instantiated is not obvious: it could arise only while generating the answer tokens where it shows, or earlier, through altered processing of previous tokens. The paper settles it by patching the KV cache from a LoRA fine-tuned model into the unmodified base model and vice versa. For some behaviours, such as responding in German or emergent misalignment, patching transfers the behaviour from fine-tuned to base, and patching base into fine-tuned can partially or fully remove it. The effect grows with how much of the cache is patched and concentrates at a few pivotal positions, with system-prompt and chat-template tokens carrying disproportionate influence. The mechanism is read out of the cache rather than the weights, which is what makes it a mechanism result rather than a behaviour benchmark.
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