By Noah Schaffer

Boston’s free festival season is beginning to seem like a musical arms race.

The fest battle is on: Alison Krauss will be performing in Copley Square on July 28th.

Last week, Boston’s Outside the Box Festival added a July 20 City Hall Plaza performance by gospel greats the Blind Boys of Alabama.

Now comes a major lineup addition to the Boston Summer Arts Weekend. Alison Krauss, who has risen from her bluegrass origins to become one of acoustic music’s biggest stars, will be performing at Copley Square Park on July 28. The performance will feature a variety of special collaborators rather than her usual Union Station band, according to WGBH radio host Brian O’Donovan who booked the festival, which is produced by his station and the Boston Globe. O’Donovan notes that Krauss is not otherwise touring this summer.

Earlier that Sunday the fest will pair Galician bagpiper Carlos Nunez with the Boston Landmarks Orchestra.

The festival has also announced that its ticketed, late-night indoor sessions are returning, with promised appearances by Copley headliners including Nunez, Dr. John, Red Baraat, and Angelique Kidjo. Although last year’s late-night shows were a bit light on the hoped-for cross-artist collaborations, they still had the entertaining spirit of an old-time variety show as artists from different genres condensed their acts into 20-minute sets. The late night shows also allow music fans the ability to attend the weekend’s other major free roots music event, the Lowell Folk Festival, without missing the Summer Arts Weekend headliners.