AI Insights · Oct 22, 2026 · Verified
Two small files now carry agent instructions everywhere
SKILL.md and AGENTS.md now work across vendors: Codex reads the same skills standard, and Claude Code has read AGENTS.md since September 2026.
Claim
Two tiny formats now carry agent instructions across vendors. SKILL.md
(name plus description required, about 100 tokens loaded at startup, body
under 5k tokens on activation) is read by Claude, Codex, Cursor, OpenCode,
Amp, Letta, goose, GitHub and VS Code. AGENTS.md (nearest file wins, 60k-plus
projects) is read by 20-plus tools and by Claude Code when no CLAUDE.md
exists. CLAUDE.md and .cursor/rules remain the vendor-primary files.
Evidence
- Agent Skills spec:
name(64 chars, matches directory),description(1024 chars), optionallicense,compatibility,metadata, experimentalallowed-tools;scripts/,references/,assets/; three loading tiers. Willison called it "deliciously tiny" and under-specified on metadata. - Codex loads skills from
.agents/skills(repo, parents, home,/etc/codex/skills) and says it builds on the agentskills.io standard; optionalagents/openai.yamladds UI metadata and policy. Directory divergence: Anthropic.claude/skills, OpenAI.agents/skills. - AGENTS.md was contributed to AAIF by OpenAI; OpenAI's main repo has 88 of them; Codex caps combined AGENTS.md at 32 KiB.
- Claude Code 2.1.277 (2026-09-18) reads AGENTS.md when no CLAUDE.md exists,
implemented as a built-in mod. A
CLAUDE.local.mdsilently disables the fallback (secondary). UNVERIFIED. - Stripe found modular progressive-disclosure skills beat one monolithic file by about 10% and cut tokens (see harness-engineering findings).
Open questions
Whether Agent Skills will be donated to AAIF; no evidence yet.Answered 2026-10-23: Anthropic donated the standard to AAIF on 2026-10-22, see [[2026-10-agent-skills-donated-to-aaif]]. Still open: the first versioned release and the security guidance the maintainers proposed.
Sources
- https://agentskills.io/specification — spec, Thu Dec 18
- https://simonwillison.net/2025/Dec/19/agent-skills/ — blog, Thu Dec 18
- https://learn.chatgpt.com/docs/build-skills.md — docs, Wed Oct 21
- https://agents.md/ — docs, Wed Oct 21
- https://simonwillison.net/2026/Sep/18/thariq-shihipar/ — blog, Thu Sep 17
- https://www.anthropic.com/news/donating-agent-skills-to-the-agentic-ai-foundation — blog, Wed Oct 21
AI-assisted research: gathered and summarised with AI from the sources above, each claim checked against them.