Larry B. Kupers got a nasty surprise when he read a client’s plea deal. The federal prosecutor had inserted a clause that would bar his client from withdrawing from his plea even if exculpatory or impeachment evidence was later found.

Now, two years later, the clause is standard issue in all plea deals in San Francisco, as well as in San Diego and New York. The use of “Brady waivers,” named for the landmark U.S. Supreme Court case Brady v. Maryland, has slowly been working its way across the country, and defense lawyers such as Mr. Kupers, an assistant federal public defender, are worried the trend will erode the strength of Brady. The 1963 decision and its progeny require prosecutors to turn over exculpatory and witness impeachment evidence to the defense.