When House Republicans on Monday night passed the rules that would govern the chamber for the next two years under Speaker Kevin McCarthy, McCarthy hailed the supposed win for transparency.

Even as he was doing so, though, McCarthy (R-Calif.) was being rather opaque about the agreement that got him to that point: namely, the parameters of a still-mysterious deal he cut with the hard-right Freedom Caucus to secure their votes in the speaker election.

Punchbowl News on Monday first reported the existence of a three-page document outlining the terms, which it referred to as an “addendum.” The whole thing remains shrouded in mystery: McCarthy is now reportedly denying the existence of an official “addendum.” But some members have confirmed they’ve seen such a document, and top McCarthy allies have either talked around its existence or flatly declined to discuss the matter.

The chairman of the House Rules Committee, key McCarthy ally Rep. Tom Cole (R-Okla.), spoke about it as if it was a mystery to him. “I’m sure it exists, because I read about it from you guys [in the press] all the time,” he told Axios Monday. “It has to be out there.”

Added Rep. Dusty Johnson (R-S.D.): “I’m not at liberty to discuss whether I’ve seen it or not.”

What we do know is that there is apparently some kind of document out there — though it’s unclear that it’s “official” or technically an “addendum” to the House rules — and that many Republicans are in the dark about it. (Punchbowl reported that members aren’t allowed to keep a copy, apparently for fear of it leaking.)

We know the broad strokes of the deal McCarthy struck, but not its full scope. And that raises the obvious question: Why? And why the apparent lack of curiosity from House Republicans about forcing the issue?

At least one member is pushing for answers. Rep. Nancy Mace (R-S.C.) indicated over the weekend that she might vote against the rules package due to the lack of transparency. She ultimately relented — just as McCarthy allies voted for him for speaker despite grumbling about being in the dark — but said she will continue to push for its release.

“If it’s not okay for the far left to cut deals in secret, then why is it okay for a few on the far right to cut deals in secret?” Mace said at Tuesday’s meeting of the House GOP conference.

What we know about the concessions McCarthy made is that they apparently contain some of the more controversial deals — especially as compared to the rules package itself, which contained relatively few and passed with little fuss. Among them, as The Post’s Marianna Sotomayor and Leigh Ann Caldwell report:

The deal also apparently included concessions on committees. But there too, precisely what form those concessions take isn’t clear.

We know that McCarthy has agreed to things like putting a certain number of hard-right Republicans on the influential Rules Committee, but McCarthy has said he didn’t promise anyone chairmanships. Ally Rep. Richard Hudson (R-N.C.), who said he has seen the document, told Axios that it included, “No names, just representation.”

At the same time, Rep. Byron Donalds (R-Fla.) claimed over the weekend that he had secured a slot on the House Republican Steering Committee, which decides committee chairmanships and assignments, in exchange for flipping his speaker vote back to McCarthy:

This doesn’t necessarily suggest that Donalds’s name appears on whatever document might exist. But his statement highlights how little we know about the extent or the particulars of the committee promises.

As for why, the obvious answer is that it’s just not terribly helpful to put yourself on the record agreeing to these things. Being forced to agree to such extensive concessions reinforces that McCarthy is a diminished speaker at the mercy of a small number of holdouts. Sharing specifics would mean McCarthy would have to account for them and his ability to live up to the deal publicly. Some — such as cutting defense spending — open the GOP up to discord and criticism (and those are just the terms for which we know the basic outline).

While that would explain keeping this information from the public, though, it doesn’t explain why even House members appear to be in the dark. Those members have declined to truly press the issue, voting for McCarthy and then the rules package when they could have used both for leverage. McCarthy got what he wanted, ultimately winning both votes without being forced to show the hand that he has left himself with.

As Mace notes, for a party that has hailed its new rules as ushering in an era of transparency and empowering the rank-and-file members, it’s an inauspicious start.