Very large codebase modernization/conversion to newer libraries for interop and compliance reasons. Fan out only works if you have enough to do without the agents stepping on each others work. It is amazing when you can do it. Conversions make it even easier because you have a baseline metric of tested functionality to meet. We did pull off mass fanout on an ambitious greenfield internal tool that went very well. It feels like my entire job now is figuring out how to parallelize as much work as possible for the agents.