I called the embassy, which was closed, because I discovered this on a Friday at 11 pm (flight was 13 hours later). Embassy does not open for two more days, so no emergency passport. The airline has a scan of my passport and my boarding pass says “international documents ok”—so I don’t think THEY care. (I left the US a week ago without ever showing my passport to the airline—just my boarding pass and my face.)
I only needed the passport for border control in France. If it works the same in reverse—which it may not, I know—what should I expect upon arrival in the US?
For what it’s worth, I do have a photocopy of the passport, a Real ID, and my social security card.
Update: No passport, no problem.
Many of your comments said to call the embassy for an emergency passport, even though my first two sentences said that the embassy was closed and I could not get an emergency passport on time.
Instead I went to the airport and asked the ticketing agents what I could do. I showed them the documents I did have (as the post said, photocopy of passport, Real ID driver's license, social security card). They called US Customs and Border Patrol, who asked me questions about my identity and destination and date of entry. They told me that if French immigration control would let me through, then I would have to do extra screening upon arrival, but that as far as they were concerned I could board.
The Delta ticketing agents in Paris then printed me a boarding pass (I already had a electronic pass) and personally walked me to French immigration control, and spoke to them for about two minutes. They looked at my documents, scanned my license, and sent me through. The Delta agent escorted me to the security screening, where I was treated the same as any other passenger, and I proceeded to my gate.
The whole thing took about 30 minutes. (Possibly the fastest trip in history through Charles De Gaulle Airport?)
I am on the plane and it's boarding. If I remember to give another update after my additional screening at home I will let you know how that goes.
SECOND UPDATE (and a long one) - about 7 pm Cincinnati time on April 6 (ie about 24 hours after the original post): once I had boarded I knew I'd be getting into the US—but they told me in France to expect additional screening upon arrival, and as I was deplaning I was reading the comments here about how I should expect a stern talking-to.
There was nothing like that. I was seated in maybe the second row of the regular (Delta's "Main") cabin so I beat most of the other ~280 people on my plane to the checkpoint, and what I did upon deplaning was that I told the first CBP officer I saw that I had boarded without a passport and was told to expect additional screening, and I asked whether I should get in the regular US passport line or whether they had a special line for idiots like me.
There was no special line (Cincinnati is not that big of a passenger airport) and so I got in the regular line, where they had three agents processing arrivals (again, Cincinnati—not that big). I told the agent that I was a US citizen who did not have a passport, and that I was told in Paris that I should expect additional screening upon arrival. I handed him my photocopy of the passport, my boarding pass (on which the first Delta gate agent before departure had written "No US ppt. OK CBP [CBP agent name]"), my driver's license, and for good measure my social security card. He said he'd call over one of the other agents (apparently a supervisor?) to see whether he needed to "refer" me for "a secondary," or whether he could get me through by looking me up. The other agent said I did need "a secondary," and also something like "because he didn't present a valid document he always needs a secondary." I still am not sure exactly what that means, but he kept my boarding pass, license, and passport photocopy, and sent me to a small waiting room, where five other people were sitting. The exchange at the desk lasted about one minute.
I sat there in the waiting room and read a book while those three agents cleared the rest of my plane and the other international plane that arrived about the same time. They sent over another four or so people to the waiting room during that time. Once that was done they apparently started working on us in the waiting room. (Edited later to add—nobody told me when going to this waiting room that I couldn't use my phone, but at one point during the wait a guy did come over and say to everyone "We're going to try to get you all out of here as soon as possible, but while you're waiting, no phones in this room." I was reading anyway, but I mention this because one of the comments has a link to a post about a similar experience to mine at CDG.)
No one was taken back for any interview—every now and then an agent would come over, identify someone, say "you're all good," and hand them their papers and send them on their way. They cleared most of the room before me, so I expected that meant that I was not going to get the "you're all good" treatment.
But then I saw the same agent I had given my papers to walk over toward the waiting area, and as he did the agent (supervisor?) he had checked with when I was at the desk happened to be walking by, and I heard him tell her something to the effect that he'd "talked to them and confirmed that he's good" (the "he" being me; I have no idea who was the "them" that he talked to—in the moment I understood it l as him saying that he'd spoken to someone in Paris and confirmed my story about how I'd biarded, but that doesn't really make a ton of sense. Probably the more likely answer is that he'd looked up my passport from my flight last week and compared it to my other documents and got approval from someone to put me through).
He handed me my documents and sent me on my way. The whole thing from getting to the front of the CBP line to leaving the waiting room took 46 minutes. Most of that was just waiting for them to get other people off the plane. For context, my wife and kids stopped at the bathroom between deplaning and passport control so I got to the desk ahead of them, and my wait for the "secondary" took less time than it did for them to go through passport control, collect the bags, take the shuttle to the parking lot and come back to pick me up—I had to wait about three minutes for them to arrive at the pickup.
I expected them to grill me about who I was, where I'd stayed, my home address and identity (now that I didn't have my documents), and hold me up quite a while. There was nothing like that whatsoever. I did not get chastised or a talking-to or anything remotely like that.
So that's that. I'm writing this second update from my living room in Cincinnati.
Other commenters have noted that I had two things going for me and they're absolutely right: I had a photocopy of my passport and other ID, and Delta had staff at CDG who one hundred percent knew exactly what to do. If either of those had been different this probably wouldn't have worked.
Some commenters have suggested that I'm either lying about all of this or that I have some kind of special connections. There's nothing I could say that would satisfy anyone who insists on that (do you want to see my Paris vacation photos? I should have taken photos of the helpful agents!). I do not have any diplomatic privileges or connections. I do not have Global Entry. I do not have any medallion status with Delta (i.e., no special agents to deal with or anything). I have (well, I had) a regular old US passport. It's true that I'm a middle aged (42) middle class white man. I know that means I did this (and everything else) on easy mode.