Could it be to make openspec specific files AGENTS.md files upgradable by openspec (when installing a new version of openspec) without taking the risk of affecting the generated project.md file ?

I suppose AGENTS.md, like the various rules / workflows / commands that come from openspec could change overtime with new versions of openspec, so they are part of the openspec domain and apply to any project, while project.md and openspec/changes are related to your project only and so will differ from one project to the other.

By keeping a nice separation between the two it allows safer openspec updates without taking the risk of affecting your own project files.

I think we can think of this as a separation of concern principle.

If that would be the reason, then it would make no sense to have this Block auto prefixed on existing root/AGENTS.md files:

<!-- OPENSPEC:START -->
# OpenSpec Instructions

These instructions are for AI assistants working in this project.

Always open `@/openspec/AGENTS.md` when the request:
- Mentions planning or proposals (words like proposal, spec, change, plan)
- Introduces new capabilities, breaking changes, architecture shifts, or big performance/security work
- Sounds ambiguous and you need the authoritative spec before coding

Use `@/openspec/AGENTS.md` to learn:
- How to create and apply change proposals
- Spec format and conventions
- Project structure and guidelines

Keep this managed block so 'openspec update' can refresh the instructions.

<!-- OPENSPEC:END -->

I have experimented on this for weeks now. At least for me it works, when:

• openspec/AGENTS.md only containts the Openspec Flow
• root/AGENTS.md containts all Agent relevant information, coding principles and rules
• openspec/project.md is pretty useless and only contains for me te sections "Purpose" and "Tech Stack". Everything else has been removed.

This works on private as well as corporate projects for me.