Democrats demanded an immediate briefing and even Republicans conceded they had no information on an agreement the administration has declined to release.
Senator John Thune of South Dakota, the majority leader, said that he had not yet seen the deal but said he expected the administration to convene with lawmakers as the process continues.
With support from Markwayne Mullin and Robert F. Kennedy Jr., the kratom industry is pursuing a potentially lucrative policy. Mr. Mullin owns equity in a company that could benefit.