Preserve unknown reserved metadata with a warning

An unrecognized key in an MCP-reserved namespace may belong to a newer protocol definition. It is retained with a warning rather than assigned invented semantics. Verify the key and protocol version before treating it as a recognized MCP feature.

Requirement

A _meta key under an MCP-reserved prefix that the MCP protocol version does not define is preserved with a warning.

Details

Rule ID: mcp.meta.reserved-prefix
Owner: mcp
Surface: _meta
Kind: semantic
Requirement level: must
Status: internal · Enforcement: internal

Protocol version applicability

2024-11-05	2025-03-26	2025-06-18	2025-11-25	2026-07-28
not-applicable	not-applicable	applies	applies	applies

Required by

• MCP Description 0.9.0, Section 3.5
  • 3. Document Structure: req-meta-reserved-prefix

Based on

• Tier 1 — MCP schema.ts and normative specification text: MCP 2025-06-18 to 2026-07-28 specification, Basic, General fields, _meta (a prefix whose second label is modelcontextprotocol or mcp is reserved for MCP use)
  • https://github.com/modelcontextprotocol/modelcontextprotocol/blob/dae694f61a31f7a3ec2af3aff709e11f70f61e48/docs/specification/2025-06-18/basic/index.mdx#L140-L142
  • https://github.com/modelcontextprotocol/modelcontextprotocol/blob/dae694f61a31f7a3ec2af3aff709e11f70f61e48/docs/specification/2025-11-25/basic/index.mdx#L208-L210
  • https://github.com/modelcontextprotocol/modelcontextprotocol/blob/dae694f61a31f7a3ec2af3aff709e11f70f61e48/docs/specification/2026-07-28/basic/index.mdx#L337-L339

Notes

A later MCP protocol version may define the key, so it is preserved rather than rejected.

Examples

Accepted with a warning

Source: expected-warning/unknown-reserved-meta-key.json · Selected: /prompts/0/_meta

{
  "io.modelcontextprotocol/future-key": {
    "preserved": true
  }
}

Rule Source Code

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