Soul.md surfaced publicly when OpenClaw creator Peter Steinberger posted on X on September 13, 2026 that he'd seen the file after getting a Muse invite code. Its exact schema hasn't been officially documented by Meta, but it applies the established "agent identity in a versionable markdown file" convention to a consumer AI companion's persona rather than a coding agent's project conventions. A reply from Michael Guo claimed Muse ships "almost the exact same 5 Markdown files" as OpenClaw, though that comparison is unverified beyond the tweet itself.