Extensions

The extensions object provides a namespace for domain-specific metadata not covered by the core specification.

Namespace format

Extension keys MUST use reverse-domain notation:


{
  "extensions": {
    "com.example": { "custom_field": "value" },
    "org.c2pa": { "manifest_uri": "https://example.com/c2pa/manifest.cbor" },
    "io.modelcontextprotocol": { "server_name": "my-server", "tool_name": "generate" }
  }
}

Rules

Extension keys MUST be valid reverse-domain strings
Extensions MUST NOT redefine core schema fields
Consumers MUST ignore unrecognised extensions (fail-open)
Extensions SHOULD be documented by their namespace owners

Well-known extensions

Namespace	Owner	Purpose
`org.c2pa`	C2PA	Links to Content Credentials manifests
`io.modelcontextprotocol`	MCP	Model Context Protocol integration metadata
`org.schema`	Schema.org	Schema.org CreativeWork property mappings

C2PA integration example


{
  "extensions": {
    "org.c2pa": {
      "manifest_uri": "https://example.com/c2pa/manifest.cbor",
      "claim_generator": "example-platform/1.0"
    }
  }
}

MCP integration example


{
  "extensions": {
    "io.modelcontextprotocol": {
      "server_name": "product-description-server",
      "server_version": "1.0.0",
      "tool_name": "generate_description",
      "session_id": "mcp-session-abc123"
    }
  }
}

Registering a namespace

There is no formal registration process. By convention, use the reverse-domain notation of a domain you control. Document your extension schema publicly to enable interoperability.