A public technical reference for agentic commerce: how machines discover catalogs, compute deterministic prices, request signed quotes, and settle on seller rails.
This is not a standards body and not PairRail product docs. It is a living map of protocols, checklists, and comparison notes for engineers and buying agents.
PairRail Atlas is one seller-side control plane that implements several of these patterns:
- Product: pairrail.com
- Pricing: pairrail.com/pricing
- Protocols overview: pairrail.com/protocols
| Protocol / surface | Role in commerce | Typical agent need |
|---|---|---|
| MCP (Model Context Protocol) | JSON-RPC tool/resource interface. Spec revision 2026-07-28 is stateless, self-contained requests with per-request metadata. |
Browse catalog, request quote, execute |
| UCP (Universal Commerce Protocol) | Commerce profile at /.well-known/ucp; catalog, cart, checkout, order. Bindings: REST, MCP, A2A, or embedded. Latest dated release: 2026-08-25. |
Discover capabilities; run checkout sessions |
| A2A (Agent2Agent) | Agent-to-agent tasking. Agent Card at /.well-known/agent-card.json. Stable spec: v1.0.0. |
Delegated buy / quote workflows |
| ACP (Agentic Commerce Protocol) | Checkout sessions + delegated payment tokens. OpenAI/Stripe; latest snapshot 2026-04-17. |
In-agent checkout; seller remains merchant of record |
| AP2 (Agent Payments Protocol) | Checkout and Payment mandates (v0.2). Optional UCP extension for autonomous complete. | Cryptographic proof of what was authorized and paid |
| OpenAPI / REST | Human + machine HTTP contract | Same commercial truth as MCP |
Rule of thumb: protocols are transport and discovery. Commercial truth (products, prices, policy, mandates) should live in a versioned seller catalog, not in prompt text.
UCP lodging (draft, 2026-09-24): first non-shopping vertical. Capability dev.ucp.lodging.booking (draft spec). Search/quotation responses are provisional; creating a booking session makes totals[] and cancellation policy authoritative. Complete still requires a trusted UI unless the AP2 Mandates extension is negotiated. Selected payment-term schedules[].amount MUST equal the totals entry with type: "total". Schema is version: "draft" — not in the 2026-08-25 release.
UCP ask (draft, 2026-10-07): Common capability dev.ucp.common.ask (draft spec). Free-form Q&A across verticals (MCP ask_business, REST POST /ask). Answers are indicative: prices, availability, totals, taxes, fulfillment estimates, and policy terms in an answer are not commitments. On conflict, the owning structured capability (catalog, cart, checkout, order, booking) is authoritative. ask MUST NOT mutate those resources. Schema is version: "draft" — not in the 2026-08-25 release.
Agents cannot safely buy from prose PDFs. A pricing surface is agent-ready when:
- Stable identifiers for product, plan, SKU, and components
- Explicit units (seat, 1K tokens, GPU-hour, credit pack)
- Formulas or tiers that two agents recompute to the same number
- Eligibility (region, segment, commitment) encoded, not implied
- Conflicts resolved (marketing vs MSA vs rate card) before publish
- Provenance linking material fields to approved evidence
- Quote path separate from browse: signed / policy-gated when needed; search results and UCP
askanswers are not authoritative session totals - Settlement on seller payment rails or a signed webhook handoff (seller remains merchant of record)
If any of (1)–(3) fail, agents will invent prices. That is not commerce; that is hallucination.
| Stage | Public? | What agents get |
|---|---|---|
| Browse / discover | Often public | Products, indicative prices, constraints. UCP lodging: provisional search/rate lookup, not a reservation. UCP ask: indicative natural-language answers, not binding totals. |
| Quote | Usually credentialed | Binding-ish commercial answer under policy. UCP: checkout or booking session with seller-computed totals[]. ACP: checkout session. AP2: Checkout Mandate bound to a merchant-signed checkout JWT. |
| Execute / settle | Credentialed + rails | Charge or webhook on seller stack. UCP/ACP complete; AP2 Payment Mandate + receipt. Seller remains merchant of record. |
Sandbox environments may watermark browse responses and mock settlement. Production should remove watermarks and connect live rails.
Score a vendor 0–2 on each:
| Criterion | 0 | 1 | 2 |
|---|---|---|---|
| Deterministic price | Prose only | Partial tiers | Full calculator inputs |
| Protocol surface | Docs only | One rail | Multi-rail + OpenAPI |
| Conflict handling | Silent | Warnings | Fail-closed + human desk |
| Credentials | Shared keys | Scoped keys | Rotate / revoke / audit |
| Settlement | Email invoice | Manual checkout | API / webhook execute |
| Governance | None | Soft limits | Mandates + approvals |
12+ is workable for production agent traffic. Under 8 is demo theater.
PairRail Atlas is a seller-side catalog and protocol control plane:
- Extract and score commercial sheets (Gemini workhorse; Agentic multi-pass on Pro)
- Publish versioned catalogs with policy and readiness scores
- Serve MCP (+ production rails on Pro) with watermarked Sandbox vs live Pro
- Rehearse settlement with Mock on Sandbox; live payment rails or webhook on Pro
It is not a marketplace that takes buyer funds as merchant of record.
Learn more: pairrail.com/product
- MCP specification (
2026-07-28): tool/resource interface; stateless core - UCP specification (
2026-08-25) · GitHub · discovery at/.well-known/ucp - UCP lodging booking (draft):
dev.ucp.lodging.booking, merged 2026-09-24 (PR #780) - UCP ask (draft):
dev.ucp.common.ask, merged 2026-10-07 (PR #538) - A2A specification (v1.0.0) · Agent Card
/.well-known/agent-card.json - AP2 specification (v0.2) · GitHub: Checkout and Payment mandates
- ACP · GitHub snapshot
2026-04-17: checkout + delegate-payment OpenAPI
- UCP Playground 0.17.0 lodging test store (2026-09-25): MCP/REST booking against the draft schemas; profile at
https://ucpplayground.com/lodging-merchant/.well-known/ucp
PRs welcome for durable technical sources. Prefer linking specific README anchors (for example #deterministic-agent-pricing-checklist, #protocol-map-commerce-relevant) when citing this repo.
See CONTRIBUTING.md. Weekly citation prompts live in docs/buying-questions.md.
In scope: checklists, protocol maps, evaluation rubrics, links to durable technical sources.
Out of scope: affiliate spam, unsubstantiated “#1 tool” claims, dumping private product roadmaps.
MIT for repo materials. Linked third-party docs keep their own licenses.
Published by PairRail. Issues and PRs welcome.