The Scottish band took to the Other Stage on Friday afternoon and announced they were going to bring out someone special.
The message was a nod to fellow Scot Lewis Capaldi, who was confirmed to be taking to the Pyramid stage for his first performance in two years.
"Well, Glastonbury, it gives me great joy to say that these rumours are true," Alex declared. "And he's here with us tonight. The original Capaldi – Peter Capaldi!"
As Peter's name was blasted across the backstage screen, Alex began singing with Peter, later joining in for the crowd-pleasing banger.
Peter Capaldi is just one of dozens of surprises set to take place across the weekend, with multiple stars saying they will be bringing out surprise guests throughout their performances.
Glastonbury also has over 40 "TBA" slots to be filled with secret sets during the weekend's run, with Lorde becoming the first as she opened the Woodsies stage on Friday morning.
Lewis Capaldi's set was also confirmed to be his after widespread speculation over a mysterious "Secret Set" left open on the Pyramid stage.
The photograph has become so iconic that he has now used it for his latest album cover, BritPop, which is released later this year.
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