" I Know What You Did Last Summer " was written backstage spontaneously on one of Taylor Swift 's The 1989 World Tour stops , in which Mendes was an opening act and Cabello was the surprise guest as part of the group Fifth Harmony . " We were just backstage . There were like 100 people in my dressing room at the time , and it was madness back there , but I had this guitar and I was just jamming out , and we basically wrote a pre-chorus and a chorus in like 30 minutes , " Mendes said in a video released about the making of this song . " And we planned to go into the studio , and it was like an 11-hour studio session where we finished writing the song . " After a live performance on '' Live with Kelly and Michael '' , Cabello said , " We were just kind of jamming out . We were n’t really just writing a song . We kind of started spitting out melodies and put lyrics to it ; I do n't think we knew we had a song . " The artists co-wrote " I Know What You Did Last Summer " with its producers Ido Zmishlany and Noel Zancanella , with Bill Withers receiving songwriting credits for the sampling of his 1971 single " Ai n't No Sunshine " and Ryan Tedder also receiving songwriting credits . Cabello described " I Know What You Did Last Summer " as a " conversation between two people in a relationship where it 's dying , but nobody wants to admit that it 's dying . " The song is sung in A minor , with a tempo of 114 bpm and a time signature of
The 1989 World Tour
The 1989 World Tour
I Know What You Did Last Summer (song)