SAN FRANCISCO — Defendants in the Silicon Valley “no-poach” case have made a rare and risky move to avoid either going to trial or shelling out an extra $55 million to settle antitrust claims over their hiring practices.

Lawyers for Google Inc., Apple Inc., Adobe Systems Inc. and Intel Corp. filed a petition for a writ of mandamus Thursday in the U.S. Court of Appeals for the Ninth Circuit challenging U.S. District Judge Lucy Koh’s rejection of a proposed $324.5 million settlement. In it, they ask the appeals court to intervene and approve the deal, arguing Koh “committed clear legal error” that could make it harder to settle class actions.