MCP: Protocol Hub

When AI assistants and agent runtimes cannot discover what your systems can do, they cannot reliably serve your catalog or complete transactions. This protocol hub is for engineering teams and technical product managers implementing the Model Context Protocol (MCP) in production agentic commerce stacks. It provides a precise definition, scope, primitives, architecture and API model, an implementation checklist, common pitfalls and best practices, and pointers to related protocols. It aligns with eLLMo AI's protocol-plural strategy to make brands discoverable, trusted, and transactable without replatforming.

The Model Context Protocol (MCP) is a JSON-RPC 2.0 based protocol that standardizes how AI clients and servers negotiate capabilities and exchange messages. Server features include tools (callable actions), resources (readable artifacts), and prompts (template bundles). Client features include roots (virtual file trees), sampling (LLM interactions), and elicitation (schema-driven forms and URLs).

MCP's lifecycle includes initialization, capability negotiation, operation, and shutdown. It is transport-agnostic with common deployments over HTTP(s), WebSocket, or stdio. MCP is complementary to commerce protocols; it does not define catalogs or orders, but provides a clean way to expose them as tools, resources, and prompts.

References: MCP Base Overview (2025-11-25); MCP Lifecycle (2025-11-25); Agentic Commerce Protocol Landscape 2025-2026 (eLLMo AI); API-first vs protocol-first for agentic commerce (eLLMo AI).

MCP focuses on capability negotiation and structured message exchange between a client (e.g., an agentic runtime) and a server (e.g., a brand gateway). Scope includes connection lifecycle and version negotiation; capability negotiation (what the server and client can do in-session); server feature exposure (tools, resources, prompts); client feature exposure (roots, sampling, elicitation); and utilities such as pagination, logging, completion, and tasks (experimental).

Default dialect is 2020-12 when $schema is absent. Clients and servers must validate schemas against the declared or default dialect. References: MCP Base; MCP Server Tools (2025-06-18).

Interaction model: the client sends initialize; the server returns capabilities; the client sends initialized; normal operations (tools, resources, prompts, logging, completions) follow; shutdown occurs via transport closure. Transport options are HTTP(s), WebSocket, or stdio. For HTTP, clients include the MCP-Protocol-Version header after negotiation.

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "initialize",
  "params": {
    "protocolVersion": "2025-11-25",
    "capabilities": {
      "roots": { "listChanged": true },
      "sampling": {},
      "elicitation": { "form": {}, "url": {} },
      "tasks": { "requests": { "elicitation": { "create": {} }, "sampling": { "createMessage": {} } } }
    },
    "clientInfo": {
      "name": "ExampleClient",
      "title": "Example Client Display Name",
      "version": "1.0.0",
      "description": "An example MCP client application"
    }
  }
}

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "protocolVersion": "2025-11-25",
    "capabilities": {
      "logging": {},
      "prompts": { "listChanged": true },
      "resources": { "subscribe": true, "listChanged": true },
      "tools": { "listChanged": true }
    },
    "serverInfo": {
      "name": "ExampleServer",
      "title": "Example Server Display Name",
      "version": "1.0.0",
      "description": "An example MCP server providing tools and resources"
    }
  }
}

Client requests tools/list (optionally with cursor for pagination). Server returns a list of tools with name, title, description, and inputSchema. Client then calls tools/call with name and arguments. Server returns result with content (e.g., text) and optionally structuredContent for machine-friendly parsing. Server may send notifications/tools/list_changed when the tool list changes.

Standards: JSON-RPC 2.0 (request/response/notification envelope); JSON Schema 2020-12 (parameter and output validation); MCP Spec (latest base, lifecycle, server tools). For stack guides (e.g., Shopify), see Integrations.

Parent hub: Agentic Commerce Protocols overview. Related protocols: UCP, ACP, A2A. Integrations hub (implementation guides by stack). Glossary: Agentic commerce, Answer Engine Optimization.