Two years ago I was checking in for a CDG-IAD flight. For some reason, the AF agent couldn't understand that I was passing through the U.S. enroute to a third country and I put Transit to XXX via IAD on the APIS information. Had a big argument with the agent because even though I need a visa to enter the U.S. the agent claimed that I needed an ESTA as well, saying that "Any passenger who is not a US Citizen/Resident needs an ESTA regardless of their nationality".

Long story short, she gave up talking to me and called a "Marshall", who turned out to be a CBP agent, and took around 20 minutes to arrive. He checked my passport, asked me about my trip to the US, and told the agent to check me in as there were no issues.

So yes, there are CBP agents at CDG.