When Block product designer Cynthia Chen first dreamed up the idea of an app that could catalogue dogs spotted in the wild, she shelved it. "The hurdle was so big, so I never did anything," she said.

Block, Jack Dorsey's company behind fintechs Square and Cash App, debuted an AI agent Tuesday. The AI helps with coding, is used by about 1,000 engineers internally, and took nine months to build. Two ...