Point taken. It may very well be a case of "witch eating crackers".
I just really didn't know how to answer her. Traditionally whoever is hosting lets everyone know what the main dish will be and everyone replies with what they are bringing, filling in what's needed. So far, in addition to the main dish, we have an appetizer, a salad and a dessert. I honestly didn't want to tell her what to bring. People don't usually ask, they just tell the hostess.
If she had just said, "I'll bring wine", I admit I would have still rolled my eyes (privately) but at least that's definitive. I honestly don't know if she's looking to cook something or pick something up at the grocery store premade or pick something up to go from a restaurant, all of which people have done in the past and all of which are perfectly fine. But it's up to each person to decide that's what they want to do as opposed to me telling her, Cook xyz dish please, or Go to ABC restaurant and pick up an order of their xyz dish.
I suppose I could have just said, "please bring a side" but again, is that really guidance? It's vague enough that if she hadn't yet figured out what to bring, I don't think that would have helped.
To answer Lynn's question, all the conversation about what everyone was bringing took place immediately after I sent the email saying what I was making, in the midst of which Amy sent the email saying she was available after all. I actually debated whether to include her in that email chain because she wasn't coming, but I did it as a courtesy figuring that if her trip got canceled, she'd still want to come.
I ran into Amy in the restroom yesterday (we work in the same office) and she asked me what she should bring. I just said whatever sounded good to her, anything was fine. She kept pushing and I said I really didn't know what to suggest. She said she wondered if I needed help with the main dish. (It's a casserole that I put together last night and will throw in the oven tonight so I'm not sure what help she could offer upon arrival.) So I finally just told her to bring a specific appetizer that she could either pick up from the grocery store or from a restaurant or might even have in her pantry already. I didn't feel comfortable suggesting anything that might take much effort because it's not my place to do that.
What would I have told anyone else who asked? Probably the same thing I initially said to Amy: whatever sounded good, anything was fine... but the thing is, no one ever does ask. We all just step up and volunteer what we're bringing.
Anyway, like I said, it probably is a case of a witch eating crackers... which is why it's nice to have this forum to hash this all out in.