Librarium is a CLI that does exactly that. One command, many providers in parallel, structured markdown output. Because it's a CLI, any AI agent that can run shell commands can use it. I use it as a skill in Claude Code and it works out of the box.

It also deduplicates citations across providers, so you can see which sources keep showing up independently, which is a useful trust signal when you're researching something you don't already know well.