Review reserved extension identifiers

Extension identifiers in an MCP-reserved namespace are checked against the pinned catalogue. Unknown or experimental identifiers are preserved with a warning. Verify the identifier and its maturity; an unrecognized identifier is not silently assigned known extension semantics.

Requirement

An extension identifier under an MCP-reserved prefix that the pinned extension catalogue lists as experimental, or does not list, is preserved with a warning.

Details

Rule ID: mcpdesc.extensions.catalogue-maturity
Owner: mcpdesc
Surface: Extension identifier · Path: capabilities.extensions, clientRequirements.extensions
Kind: mcpdesc-semantic
Requirement level: should
Protocol versions: independent of the MCP protocol version
Status: internal · Enforcement: internal

Required by

• MCP Description 0.9.0, Sections 8.4 and 8.5; validator extension catalogue (effective 2026-09-04)
  • 8. Capabilities: req-extensions-reserved-identifiers
  • 8. Capabilities: req-extensions-catalogue-maturity
  • 8. Capabilities: req-extensions-official-no-warning

Based on

• Tier 1 — MCP schema.ts and normative specification text: MCP 2026-07-28 specification, Basic, General fields, _meta key name format (second label modelcontextprotocol or mcp is reserved)
  • https://github.com/modelcontextprotocol/modelcontextprotocol/blob/dae694f61a31f7a3ec2af3aff709e11f70f61e48/docs/specification/2026-07-28/basic/index.mdx#L337-L339

Notes

The pinned catalogue lists no experimental identifiers, so no document currently produces the experimental-identifier warning.

Examples

Accepted with a warning

Source: expected-warning/unknown-reserved-extension-identifier.json · Selected: /capabilities/0/extensions

{
  "io.modelcontextprotocol/future-capability": {
    "enabled": true
  }
}

Accepted with a warning

Source: expected-warning/deprecated-client-capability-requirements.json · Selected: /prompts/0/clientRequirements/extensions

{
  "io.modelcontextprotocol/future-capability": {}
}

Rule Source Code

capabilities.yaml at immutable tag v0.9.0-rc.3.