BLOG / llms-txt-vs-agents-md-vs-sku-md.mdx
llms.txt vs agents.md vs SKU.md: choose the right responsibility
A practical matrix for Sitemap, Product JSON-LD, llms.txt, agents.md, SKU.md, live catalogs, MCP, UCP, and Checkout.
Do not choose one machine-readable file to do every job. Use each surface for the responsibility its publisher and runtime can actually support, then connect them with explicit links.
SKU.md means merchant-hosted product discovery and stable product knowledge: agents can find products, understand sourced facts, and know where to recheck live commerce. It is not a new checkout protocol.
Responsibility matrix
| Surface | Discovery | Summary or instructions | Product semantics | Stable knowledge | Live state | Transaction authority |
|---|---|---|---|---|---|---|
| Sitemap | URL inventory | No | No | No | No | No |
| Product JSON-LD | Page entity | Product fields | Product/Offer vocabulary | Limited to page data | Observed page values | No |
llms.txt |
Curated readable links | Site overview | Not commerce-specific | Unstructured | No | No |
agents.md |
Repository or agent entry | Operating instructions | Not commerce-specific | Instructions, not product evidence | No | No |
| SKU.md | Product document graph | Sourced product context | ProductGroup, Variant, SKU, GTIN | Yes | Points out where to recheck | No |
| Catalog / MCP / UCP | Capability discovery | Structured responses | Current Product and Variant | Only as the live system provides | Yes | Action-dependent |
| Checkout / order system | Purchase entry | Binding disclosures | Selected line items | No | Revalidated | Yes |
Google’s Product structured-data guide explains how product page markup can qualify for richer search appearances. That is a different contract from a SKU.md knowledge document and still does not guarantee display.
Discovery does not establish authority
A Sitemap can reveal a URL without describing its meaning. llms.txt can curate documentation without proving the linked claims. agents.md can tell a coding agent how to test a repository without becoming a catalog. Product JSON-LD can express an Offer found on the page, but the checkout system can still return a different contextual total.
SKU.md connects stable identity and reviewed knowledge to these surfaces. It should never convert a discovery hint into permission, freshness, or transaction authority.
A selection workflow
- List the question: URL discovery, instructions, product identity, evidence, live conditions, or purchase.
- Choose the narrowest source that owns that question.
- Link stable knowledge to current Catalog or storefront verification.
- Fail closed if Product, Variant, SKU, or GTIN cannot be matched.
- Recheck conditions and obtain user confirmation before Checkout or an order action.
This workflow permits several files to coexist. Duplication is acceptable only when ownership stays explicit and values are checked for consistency.
Common combinations
| Publisher need | Minimal combination |
|---|---|
| Documentation discovery | Sitemap + llms.txt + Markdown pages |
| Coding-agent instructions | Repository README + agents.md |
| Product page semantics | Visible product page + Product JSON-LD |
| Sourced durable product context | SKU.md + source URLs + live verification link |
| Agent-driven purchase | Stable knowledge + negotiated Catalog/MCP/UCP + Checkout |
Shopify’s own 2026 resources illustrate the need for precise attribution: its /agents.md, /llms.txt, and /llms-full.txt behavior is documented for Shopify stores. That evidence must not be generalized into Shopify support for SKU.md.
Further reading
Read What is SKU.md? for the category definition and AI commerce authority handoffs for runtime ownership.