BEIJING — China vows to stem the supply of the powerful opioid fentanyl flowing into the United States. It pledges to target exports of fentanyl-related substances bound for the United States that are prohibited there, while sharing information with American law-enforcement authorities.

Such promises, echoed in the recent meeting between the countries’ presidents, ring familiar.

They first emerged in September 2016, when the Obama administration said China and the United States had agreed on “enhanced measures” meant to keep fentanyl from coming into the United States. But in its official statements or state media reports made at the time, the Chinese government never specified the steps it intended to take, and its follow-up has been patchy at best.

So when the Trump administration said on Saturday that President Xi Jinping of China had agreed to designate fentanyl as a controlled substance in “a wonderful humanitarian gesture,” analysts said there was little to cheer about.

“It’s in many ways all theater from the White House and very little serious substance,” said John Collins, executive director of the International Drug Policy Unit at the London School of Economics. “It seems to me the same story again.”