President Trump and his supporters are hailing his new tariff-averting immigration deal with Mexico as a game-changer and a sign that Trump’s hardball tactics — which even his own party was wary of, in this case — can work.

Critics say the deal represents little more than warmed-over agreements that had already been reached before the tariffs were threatened. They say the measures will have little impact.

The good news is we’re going to find out who’s right.

The Mexico tariff standoff is merely the latest example of Trump’s tendency to conduct foreign policy at the end of a knife. Oftentimes, as the New York Times’s Peter Baker analyzed this weekend, the resulting deal amounts to little more than a face-saving gesture — something to allow Trump to semi-plausibly claim a win without actually changing many of the fundamentals:

These are often dramas of his own making, with him naturally the hero. He stakes out maximalist positions and issues brutal ultimatums to compel action, arguing that extreme problems demand extreme tactics. At times, though, it can seem like little more than smoke and mirrors substituting for serious policymaking, a way of pretending to make progress without actually solving the underlying problem.

The Times has suggested that that’s essentially what this amounts to. In a story Saturday, it reported that the deal struck the day before “consists largely of actions that Mexico had already promised to take in prior discussions with the United States over the past several months.”

What’s more, critics say even those steps are unlikely to be successful. “In general, I don’t think that this agreement stems the flow,” Ali Noorani, executive director of the National Immigration Forum, told The Post this weekend.

That coverage has clearly riled Trump. He has lashed out at the Times for its initial report and now seems to be moving the goal posts, hinting at a previously unknown element of the deal. In tweets, he indicated there was a secret provision that “will be announced at the appropriate time.” He said it required “a vote by Mexico’s Legislative body.”

The prevailing theory is that Trump is pointing to some kind of deal involving asylum rules. The Trump administration has pushed for a so-called safe third country agreement — or “Safe Third” rule — that would require those seeking asylum in the United States to wait in another country for their claims to be processed.

This is easily the most contentious aspect of negotiations that have taken place over the last couple weeks, and it’s difficult to see Mexico’s government agreeing to it. (Indeed, Mexican Foreign Minister Marcelo Ebrard this weekend indicated that the deal averted “drastic measures” such as a “Safe Third” proposal.) It would also be a clear and significant concession from Mexico that would likely have a sizable impact on the influx of Central Americans seeking asylum in the United States.

Whatever the true agreement, though, unlike previous face-saving deals by Trump, this one will be easy to judge according to real, quantifiable results. As The Post’s Philip Bump has documented, the number of border apprehensions and families seeking asylum has skyrocketed in recent months. Border apprehensions are up 1,000 percent since April 2017, to levels not seen since 2006. Apprehensions of family units are up 7,000 percent over the past two-plus years.

If Trump’s deal truly has teeth, we should see reductions in those numbers. Indeed that was the requirement laid out by Trump when he initially made the threat, declaring that the escalating tariffs would remain in place “until such time as illegal migrants coming through Mexico, and into our Country, STOP.” Trump has clearly stopped short of that threat, suspending the tariffs even without concrete reductions. But the monthly numbers moving forward will be telling when it comes to whether he’s actually negotiated anything of substance.

As, too, now, will be whether Mexico actually institutes this mystery provision. A skeptic would suggest Trump is claiming bigger concessions so he has an excuse to drop the hammer if and when the current deal doesn’t produce results. It’s not difficult to see the numbers not adding up to a successful deal, and Trump claiming that Mexico reneged on it or failed the follow through.

But he’s also relaxed his terms here and claimed a big win, and we’re going to find out whether that was wise. Trump’s self-proclaimed reputation as a master dealmaker is truly in the balance.