Should you write a letter of recommendation that's lukewarm?

Suppose, hypothetically, one had an assistant who was applying for a graduate program that requires letters of recommendation.

Let’s say the assistant has been totally acceptable. He did what he was told, and made only minimal screwups. But it stopped there. He never seemed very invested and did the bare minimum amount of work expected.

Now he is asking for a letter of recommendation. You are willing to write it, but you know it is not going to be spectacular. Do you agree to write it? Warn the person it is only going to be so-so? Or decline altogether?