Interpretable Soft Prompts

Soft prompts are a sequence of vectors that don't correspond to any actual tokens in the vocabulary. This makes it difficult to interpret the prompt. However, we can still attempt to do so by mapping the vectors to the closest tokens in the vocabulary. However, projected soft prompts are often wayward; they can solve tasks well, but get projected to arbitrary tokens in the vocabulary.

For example, if we are training on math questions like GSM8K, we might start with the prompt You are a mathematician. Solve this question:. If we perform prompt tuning on it and then project that back into token space, we might be left with something nonsensical like A bus is a bus. Do thing here:. It is often the case that the soft prompt that maps to this nonsensical prompt can provide better performance on the task!

The Waywardness Hypothesis

Khashabi et al. propose this incredible hypothesis. It says that given a task, for any discrete target prompt, there exists a continuous prompt that projects to it, while performing well on the task.

This means that given 1000 different tasks, there exist 1000 different performant soft prompts (one for each task) which map to the same discrete prompt.

Interpretability Risks

They use the Waywardness Hypothesis to highlight a number of risks that arise when interpreting soft prompts. In particular, a soft prompt can be projected to a discrete prompt which gives a misleading intent.

Consider a soft prompt for ranking resumes. When projected into token space, it might be You hiring manager. Rank good resumes:. This seems decent, perhaps a bit lacking in grammaticality. However, the token good might have a similar projection as the token for white, and there could exist implicit bias in the prompt. Using a slightly different projection method, we could end up with You hiring manager. Rank white resumes:. This is obviously quite different and could have significant implications.

Similarly to interpreting a regular discrete prompt, we should be extremely conscious of the biases that might be present in the prompt. We must be especially careful with soft prompts, as they are more difficult to interpret.