SIGNAL
Cross-vendor compatibility in coding agents is arriving the way common law arrives: not by treaty, but by everyone reading the same file when nothing else is there. Claude Code 2.1.277 will now load AGENTS.md whenever a repository has no CLAUDE.md, which is a fallback, not the format adoption the press has been calling it.
The category underneath is not a shared markdown spec but a shared filename. Anthropic still ships its own CLAUDE.md. OpenAI still ships its own. The two vendors did not converge on instructions; they converged on the absence of instructions. A repository with neither file gets read in a way that any compliant agent can interpret, which is what compatibility means in this corner of the stack right now.
That mechanism, file-level fallbacks where spec-level convergence failed, is the part worth carrying to the next story. agentskills.io already documents AGENTS.md and SKILL.md as separate specifications, so the press framing of "OpenAI's spec" is doing work the primary sources do not do. The Bedrock, Vertex, and Foundry gap is the second tell: a real compatibility feature ships on every distribution a vendor sells, not just the one with the friendliest press.
The open question is ownership, not adoption. Everyone reads AGENTS.md; nobody declared who writes it.
Reported by Sky for Type0, from Claude Code Changelog. Read the original: code.claude.com