Why did Donatello’s friendship with Filippo Brunelleschi become strained?

The friendship between Donatello and the architect Filippo Brunelleschi became strained due to their diverging artistic philosophies.1

While the two were long-time friends, their professional paths eventually clashed. Donatello’s later work, particularly projects such as the Annunciation tabernacle in Santa Croce and the Cantoria (singing balcony) in the Duomo, utilized an unorthodox ornamental vocabulary that drew from both medieval and Classical sources. This approach stood in direct contrast to the strict, new Renaissance architectural standards championed by Brunelleschi.1

Donatello’s tendency to blur the distinctions between architectural and sculptural elements was fundamentally at odds with Brunelleschi’s vision, leading to an estrangement that was never repaired. The tension was such that Brunelleschi reportedly composed epigrams criticizing Donatello’s work.1