MCP-020: Unknown Capability Declaration Key - MCP

Summary

• Rule ID: MCP-020
• Severity: MEDIUM
• Category: MCP
• Normative Level: SHOULD
• Auto-Fix: No
• Verified On: 2026-10-04

Applicability

• Tool: all
• Version Range: unspecified
• Spec Revision: 2025-11-25, 2026-07-28

Evidence Sources

• https://modelcontextprotocol.io/specification/2025-11-25/basic/lifecycle
• https://modelcontextprotocol.io/specification/2026-07-28/basic/versioning

Test Coverage Metadata

• Unit tests: true
• Fixture tests: true
• E2E tests: false

Examples

The following examples demonstrate what triggers this rule and how to fix it.

Invalid

{
  "capabilities": {
    "tools": {"listChanged": true},
    "unknownCapability": {}
  }
}

Valid

{
  "capabilities": {
    "tools": {"listChanged": true},
    "resources": {"subscribe": false}
  }
}