13. Specification Extensions

MCP Description documents support vendor-specific metadata through specification extensions.

13.1 Extension Naming

Specification extension properties MUST match the pattern ^x-. The RECOMMENDED naming convention is:

x-{organization}-{feature}

Examples:

x-cisco-metadata
x-acme-deployment
x-myorg-governance

13.2 Extension Placement

Specification extensions MAY appear at the root level of an MCP Description document. Extensions MUST NOT appear within objects defined by this specification (e.g., within info, transports items, or tool objects) unless the object explicitly allows additional properties.

13.3 Extension Values

Extension values MAY be of any JSON type: object, array, string, number, boolean, or null.

13.4 Processing Rules

Implementations that do not recognize a specification extension MUST ignore it and MUST NOT reject the document.

Implementations SHOULD preserve unrecognized extensions when processing and re-serializing MCP Description documents.

13.5 Extension Documentation

Extension authors SHOULD publish a specification for their extension, including:

A JSON Schema defining the extension’s structure
Documentation of the extension’s purpose and semantics
Versioning information

13.6 Example

{
  "mcpdesc": "0.7.0",
  "info": {
    "name": "chess-coach",
    "version": "2.1.0"
  },
  "transports": [
    { "type": "stdio", "command": "chess-coach", "args": ["mcp"] }
  ],
  "tools": [
    {
      "name": "analyze_game",
      "description": "Analyze a chess game from PGN notation"
    }
  ],
  "x-cisco-metadata": {
    "version": "0.2.0",
    "dump": {
      "toolName": "mcpcontract",
      "toolVersion": "0.8.0",
      "createdAt": "2026-03-15T14:30:00Z"
    }
  },
  "x-acme-deployment": {
    "region": "us-west-2",
    "tier": "production"
  }
}