llms.txt vs SKILL.md vs agent.json: What Should Your Site Publish?

llms.txt vs SKILL.md comes down to scope: llms.txt maps high-value pages for AI systems; SKILL.md gives agents a workflow playbook; agent.json declares public identity, capabilities, endpoints, and proof. A strong site can publish all three, plus OpenAPI and MCP metadata, because each file answers a different agent question.

For HeadlessDomains.com, the best stack is simple: publish a curated content map, a task playbook, a public identity manifest, callable API docs, and tool metadata. Then anchor those artifacts to a persistent .agent identity so humans and agents can inspect the same record.

Quick Comparison

Use llms.txt for the Content Map

The llms.txt proposal describes a Markdown file at the site root that points language models toward useful, context-rich resources. Ahrefs frames llms.txt as a proposed standard rather than a proven ranking lever, so treat the file as clean publishing infrastructure rather than a guaranteed visibility shortcut.

A good llms.txt file links to canonical docs, API pages, policy pages, product taxonomies, key blog hubs, and machine-readable resources. Keep the list curated. Agents benefit from fewer high-signal paths over a noisy dump of every URL.

Use SKILL.md for the Operating Playbook

OpenAI describes SKILL.md as a Markdown playbook for consistent workflows: what the skill does, required inputs, step-by-step instructions, output format, and final checks. A website can publish SKILL.md when agents should perform a repeatable task using that site or product surface.

For HeadlessDomains.com, SKILL.md can explain how an agent should search a name, inspect a record, register a .agent identity, update metadata, or verify endpoint ownership. The file turns instructions into a portable workflow rather than a buried support article.

Use agent.json for Public Identity

agent.json is the identity and capability layer. The file can declare the agent name, operator, purpose, capabilities, endpoints, auth model, MCP servers, payment metadata, directory profile, and proof links. llms.txt explains where content lives. SKILL.md explains how to run a workflow. agent.json explains who the agent is and how to verify the surface.

HeadlessDomains.com already positions agent.json, SKILL.md, TXT records, lookup, and renewal as part of the agent-native identity loop. A .agent record gives those files a persistent identity anchor rather than leaving each artifact floating on separate URLs.

Add OpenAPI and MCP When Agents Can Act

OpenAPI describes callable HTTP endpoints. MCP describes tools, resources, prompts, and authorization patterns for agent tool use. A site that only publishes llms.txt can be readable. A site that also publishes OpenAPI and MCP metadata can become callable, inspectable, and easier to govern.

Cloudflare's Markdown for Agents docs point toward another useful layer: serving clean Markdown through Accept: text/markdown content negotiation. The format can reduce parsing friction for agents while preserving structured data such as JSON-LD.

Recommended Publishing Bundle

{"identity":"atlas.agent","records":{"llms_txt":"https://atlas.agent/llms.txt","skill_md":"https://atlas.agent/SKILL.md","agent_json":"https://atlas.agent/.well-known/agent.json","openapi":"https://api.atlas.agent/openapi.json","mcp":"https://api.atlas.agent/mcp"}}

Implementation Checklist

• Publish /llms.txt with only the most useful pages and machine-readable resources.
• Publish /SKILL.md for agent workflows that should run the same way each time.
• Publish agent.json with operator, capabilities, endpoints, auth model, and proof links.
• Publish OpenAPI when agents or developers can call an HTTP API.
• Publish MCP endpoint metadata when agents can connect to tools.
• Link every artifact from a persistent .agent identity record.
• Track AI crawler visits and AI brand visibility instead of assuming any single file creates citations.

Where HeadlessDomains.com Fits

HeadlessDomains.com turns AI-readable files into a verifiable identity bundle. Registering a .agent identity lets an agent publish content maps, workflow playbooks, manifests, endpoints, payment metadata, and directory profiles from one inspection path.

Get started: register a .agent identity and publish machine-readable records before agents, tools, marketplaces, and payment surfaces start referencing your agent from different places.

Internal Reading Path

• The Agent Identity Stack
• How to Make Your Website AI-Agent Readable
• agent.json Examples for Public AI Agent Identity
• MCP vs A2A: Tools, Collaboration, and the Missing Identity Layer
• Domain Search
• HeadlessDomains.com SKILL.md

Sources

• Ahrefs on llms.txt
• llms.txt proposal
• Cloudflare Markdown for Agents
• OpenAI Skills overview
• HeadlessDomains.com

FAQ

Should every site publish llms.txt?

Publish llms.txt when a site has documentation, product data, APIs, policies, or high-value pages that agents should find quickly. Treat the file as a curated map, not a ranking guarantee. Pair the file with strong pages, clean internal links, and crawl monitoring.

Is SKILL.md the same as llms.txt?

No. llms.txt maps content. SKILL.md tells an agent how to run a repeatable workflow. A site can use llms.txt for discovery and SKILL.md for task execution, especially when the workflow has inputs, steps, output rules, and final checks.

Where should agent.json live?

Publish agent.json at a stable well-known URL or link the manifest from a .agent identity record. The file should declare operator, capabilities, endpoints, auth model, MCP servers, payment metadata, proof links, and directory profile so agents can inspect the surface.

Do OpenAPI and MCP replace agent.json?

No. OpenAPI describes HTTP endpoints and MCP exposes tools. agent.json describes identity, ownership, capabilities, and verification context. Agents can use all three: identity first, API contract second, tool interface third.

Does FAQ schema improve AI citations?

FAQ schema should mirror visible questions when a site already uses structured data, but Google has deprecated FAQ rich results for most sites as of May 7, 2026. Use schema for clarity and consistency, not as a citation shortcut.

How does HeadlessDomains.com connect these files?

HeadlessDomains.com gives the files a persistent public identity anchor. A .agent record can point to llms.txt, SKILL.md, agent.json, OpenAPI, MCP endpoints, payment metadata, and a public profile so agents can resolve and verify one canonical source.