Todd R. Howe, the star prosecution witness in a sweeping corruption trial centered on Albany, was arrested on Thursday night, just hours after he admitted in court that he had violated the terms of his cooperation deal.

The arrest of Mr. Howe, a former lobbyist and loyalist to Gov. Andrew M. Cuomo and his father, was a stunning development in the case against Joseph Percoco, a former top aide to the governor, and three other defendants.

One provision of Mr. Howe’s cooperation deal was that he “commit no further crimes whatsoever.” But on Thursday, Mr. Howe, under a grueling cross-examination, acknowledged that he tried to defraud a credit card company after his cooperation agreement had been finalized.

Prosecutors often rely on witnesses whose admissions of past crimes could weaken their credibility. Salvatore Gravano helped the government bring down nearly 40 gangsters — most notably John J. Gotti — even though Mr. Gravano had admitted his participation in 19 murders.