We built our own and aren't done open sourcing it but before that I got to a really good place with opencode plus some custom agents, pi family is good too although I haven't used it as much. We made an agent to design a spec, one to implement by dispatching subagents, one to validate against the plan, things like that. All of this helps claude/gpt too IME. For open models it has helped them stay out of loops (e.g. Kimi's but WAIT) and for frontier it helps them stay on task and not invent bloated patterns