Thoughtworks: a week of agent-only development shipped a running app and a build that was often red

Laycock describes an internal experiment run by Giles Alexander, Thoughtworks' Regional CTO for Europe. Senior engineers, infrastructure consultants and product people spent a week building an airline application with no code written by hand. She says they got about 80% done. The team's own estimate was about two thirds of their time at the whiteboard and one third on the harness, and the agents kept producing code after the working day ended.

The application ran, "but the build was frequently red." Laycock lists candidate causes: too many agents working in parallel and stepping on each other's changes, a system not modular enough for separate work, or changes arriving faster than the checks could finish. She says the team has not settled which. The experiment ran twice with different teams. She also cites DORA's 2025 research as linking AI adoption to higher delivery throughput and lower delivery stability.

Why it matters: This is one firm's account of two runs, with no published data, so treat the time split as an estimate. The useful part is where it broke. A red build only works as a signal if it stays rare, and her own question about batch size, with teams moving from stories to epics, is the one to test before you add more parallel agents.