The training data is not automatically in the context scope, and on list tasks LLMs have nearly no way to ensure completeness due to their fundamental characteristics.

To do a task like this with LLMs, you need to use a document for your source lists or bring them directly into context, then a smart model with good prompting might zero-shot it.

But if you want any confidence in the answer, you need to use tools: “here is two lists, write a python script to find the exact matches, and return a new list with only the exact matches. Write a test dataset and verify that there are no errors, omissions, or duplicates.”

LLMs plus tools / code are amazing. LLMs on their own are a professor with an intermittent heroin problem.