Just because something is first, doesn't mean it's the standard. The standard is put in place by how many harnesses use a specific pattern, and that has been AGENTS.md for a long while now. Long enough that Claude Code not supporting it is really just an act of forcing people into a proprietary workflow, because of course it's in their interest to make moving away as painful as possible given how little moat the LLM models themselves have. In fact, I think Claude Code has been pretty much the only one not supporting AGENTS.md. Check out https://agents.md if you want more information on it.