The Fourth Engine
AEO, GEO and SEO are read engines — AI agents crawl and cite a brand's content. MCP is the call engine: agents don't read the brand, they invoke it, directly, via a Model Context Protocol server. It is the fourth engine of brand discovery. A brand with a live endpoint is MCP-enabled: its switchboard is on. This is the ACI 55 measured on it.
Already on the call engine
initialize at mcp.onepeloton.com/mcp — serverInfo peloton-mcpmcp.paypal.com/mcpmcp.stripe.comThe full 55
| Brand | Category | MCP | Status |
|---|---|---|---|
| Peloton | Sporting Equipment (indoor & outdoor) | 60 | Live endpoint |
| PayPal | Fintech & Consumer Finance | 40 | In registry |
| Stripe | Fintech & Consumer Finance | 40 | In registry |
| Alterra Mountain Co. | Ski & Mountain | 0 | Not yet callable |
| Amazon | Retail & E-commerce | 0 | Not yet callable |
| Apple | Technology / Consumer Electronics | 0 | Not yet callable |
| Arc'teryx | Sporting Equipment (indoor & outdoor) | 0 | Not yet callable |
| Best Buy | Retail & E-commerce | 0 | Not yet callable |
| Burton | Ski & Mountain | 0 | Not yet callable |
| CeraVe | Beauty & Personal Care | 0 | Not yet callable |
| Chase | Fintech & Consumer Finance | 0 | Not yet callable |
| Chipotle | Restaurants & QSR | 0 | Not yet callable |
| Columbia | Fashion & Apparel | 0 | Not yet callable |
| Dell | Technology / Consumer Electronics | 0 | Not yet callable |
| Disney+ | Media & Streaming | 0 | Not yet callable |
| Domino's | Restaurants & QSR | 0 | Not yet callable |
| e.l.f. Beauty | Beauty & Personal Care | 0 | Not yet callable |
| eBay | Retail & E-commerce | 0 | Not yet callable |
| Electronic Arts | Online Gaming & Platforms | 0 | Not yet callable |
| Everlane | Fashion & Apparel | 0 | Not yet callable |
| Ford | Autos & Trucks | 0 | Not yet callable |
| Gap | Fashion & Apparel | 0 | Not yet callable |
| Glossier | Beauty & Personal Care | 0 | Not yet callable |
| Technology / Consumer Electronics | 0 | Not yet callable | |
| Hyundai | Autos & Trucks | 0 | Not yet callable |
| IKEA | Retail & E-commerce | 0 | Not yet callable |
| Intuit | Fintech & Consumer Finance | 0 | Not yet callable |
| L'Oréal | Beauty & Personal Care | 0 | Not yet callable |
| McDonald's | Restaurants & QSR | 0 | Not yet callable |
| Mercedes-Benz | Autos & Trucks | 0 | Not yet callable |
| Michael Kors | Fashion & Apparel | 0 | Not yet callable |
| Microsoft | Technology / Consumer Electronics | 0 | Not yet callable |
| Netflix | Media & Streaming | 0 | Not yet callable |
| Nike | Sporting Equipment (indoor & outdoor) | 0 | Not yet callable |
| Nintendo | Online Gaming & Platforms | 0 | Not yet callable |
| PlayStation | Online Gaming & Platforms | 0 | Not yet callable |
| Puma | Sporting Equipment (indoor & outdoor) | 0 | Not yet callable |
| Reformation | Fashion & Apparel | 0 | Not yet callable |
| Rivian | Autos & Trucks | 0 | Not yet callable |
| Robinhood | Fintech & Consumer Finance | 0 | Not yet callable |
| Roblox | Online Gaming & Platforms | 0 | Not yet callable |
| Rossignol | Ski & Mountain | 0 | Not yet callable |
| Samsung | Technology / Consumer Electronics | 0 | Not yet callable |
| Spotify | Media & Streaming | 0 | Not yet callable |
| Spruce Peak | Ski & Mountain | 0 | Not yet callable |
| Starbucks | Restaurants & QSR | 0 | Not yet callable |
| Stöckli | Ski & Mountain | 0 | Not yet callable |
| Subaru | Autos & Trucks | 0 | Not yet callable |
| Target | Retail & E-commerce | 0 | Not yet callable |
| Toyota | Autos & Trucks | 0 | Not yet callable |
| Ulta Beauty | Beauty & Personal Care | 0 | Not yet callable |
| Under Armour | Sporting Equipment (indoor & outdoor) | 0 | Not yet callable |
| Vail Resorts | Ski & Mountain | 0 | Not yet callable |
| Walmart | Retail & E-commerce | 0 | Not yet callable |
| Xbox | Online Gaming & Platforms | 0 | Not yet callable |
How it's measured
The MCP Engine score is deterministic — no model in the path. For each brand we do two checks: a lookup in the official MCP registry (does a server list a remote endpoint on the brand's own domain?), and a live handshake probe (does mcp.<domain>/mcp or <domain>/mcp answer a JSON-RPC initialize with a real server identity?).
Live callable endpoint = 60 · listed in the official registry = 40 · reported alongside the ACS, never inside it.
Almost no brand is callable yet — and that gap is the whole point. AEO, GEO and SEO are the read engines a brand already competes on. MCP is the one it can still get to first.
The measurer is on the engine. Daniels AI Design Studio dogfoods the fourth engine it measures: its own MCP server, mirror-ai-citability, is both callable and on the registry — live at mcp.danielsdesignstudio.com/mcp and listed as com.danielsdesignstudio/mirror-ai-citability in the official MCP registry.