agentskills.io specification (the open skills standard)

The open, cross-vendor specification that formalizes agent-skills as a portable format. Per adk-agents-with-skills, skills authored to this spec are universal — the same skill runs across “Gemini CLI, Claude Code, Cursor, and 40+ other products.” This is the agent-skills form factor maturing from each tool’s native skill directory into a single shared standard.

A skill’s shape

Each skill bundles: metadata (name, description), a detailed instruction body, and optional reference resources.

Progressive disclosure (the core mechanism)

A three-level loading architecture so the agent pays context cost only for what it actually uses:

L1 — metadata (~100 tokens): names + descriptions, loaded at startup so the agent knows what exists.
L2 — instructions (<5,000 tokens): the full skill body, loaded only when the skill is activated.
L3 — resources: external references, loaded on demand.

The claim: this cuts baseline context usage by ~90% vs. a monolithic system prompt adk-agents-with-skills. (Vendor-stated figure, not independently benchmarked here.)

Why it matters to the thesis

It sharpens the agent-skills insight — capability as portable markdown an agent loads — into an interoperable one: the same markdown procedure now moves across vendors. It also operationalizes the “load knowledge on demand” idea that distinguishes the llm-wiki‘s index-then-drill pattern; progressive disclosure is the procedural analog of the wiki’s index → page drill-down. Related substrate: model-context-protocol.

Distribution (skills as packages)

As the format standardizes, skills get published/installed like packages — e.g. node-js-functional-patterns-skill via an MCP/skills marketplace, and Hermes’ “Skills Hub” (hermes-agent). A nascent skill-marketplace layer riding on this spec.

Canonical first-party home

Anthropic publishes the spec and a reference set of skills in its official anthropic-skills repo — the spec (./spec/), a template, example skills, and a Claude Code plugin marketplace in one place. So the standard isn’t only a third-party agreement; a major vendor authors and ships it directly, including the source-available document skills that power Claude’s production file handling.

The widest install matrix so far (2026-07-28)

perplexity-api-platform-skills is a second major vendor authoring against the spec, and it stretches the portability claim further than anything here. One repo is simultaneously a Claude Code plugin marketplace, a Codex plugin marketplace, and a plain skills/ directory — no symlinks, no separate plugins/ tree. npx skills add perplexityai/api-platform-developers reportedly “fans out to ~70 agents”, gh skill add supports version pinning, and there’s a zero-install fallback that makes the point sharply: “Any agent that can fetch a URL can use a skill directly.”

That last path is the standard reduced to its minimum: the artifact is a markdown file at a URL, and any agent that can read it can use it, no installer in the loop. Set against ponytail‘s 15-host roster wired per-host and renwei-writing‘s Cola-specific directory, the delivery layer is visibly consolidating even where it’s still packaged per-client.

The same repo bundles a docs MCP server in its root .mcp.json alongside the skills, so procedure (skills) and current facts (MCP) install together — the skills×MCP pairing as one artifact rather than two decisions.

agent-skills · adk · adk-agents-with-skills · model-context-protocol · llm-wiki · node-js-functional-patterns-skill · anthropic-skills · perplexity-api-platform-skills