The agent that builds its own plugins still opens a skill file first.
Even an agent that writes its own plugins starts from a written skill. Self extension does not end harness work; someone still owns the code.
DeepSeek just put Harness v0.2 into public preview with a desktop app. The pitch is everything is a plugin: tools, skills, even the interface. Creator mode lets the agent write a new plugin through chat, install it, and check it works.
Watch their own demo of a Pomodoro timer plugin. Before it writes package.json and the client code, it loads a plugin-development SKILL.md. That file is the procedure.
I keep watching people treat self-extension like the end of harness work. It is not. A plugin the agent wrote for itself is still code. Someone has to own it. Someone has to write how it is supposed to behave. Someone has to decide when it gets retired.
That is the same lesson as AGENTS.md, just one layer deeper. Instructions as an artifact. Documentation instead of hoping the model remembers. If your fleet can invent tools on the fly, your review gate and your retirement rule matter more, not less.
Who owns the last plugin your agents shipped without you noticing?