On Omarchy Quattro 4.0, DHH said AI agents wrote 100% of the shipped code for the August 14 release, while he mainly reviewed architecture and sensitive model-layer changes.
In the same Lex Fridman podcast conversation, he said that approach has proved far trickier on Basecamp and HEY, where mature, revenue-generating codebases carry years of embedded decisions and legacy logic.
At Rails World on September 23, DHH still declared 37signals "pencils down," with AI-agent-written code now the default and hand-written code the exception across the company.
The gap suggests agents perform far better on greenfield projects built under their own direction than on inherited systems, where understanding why old modules were designed a certain way remains the harder task.