Use valid metadata key names

MCP metadata keys follow a grammar for an optional dotted prefix and a local name. Check separators and permitted characters when a key is rejected. Reserved-name context and value checks are separate from this key-format check.

Requirement

A _meta key has an optional prefix of dot-separated labels followed by a slash, and a name that begins and ends with an alphanumeric character.

Details

Rule ID: mcp.meta.key-format
Owner: mcp
Surface: _meta
Kind: structural
Requirement level: must
Protocol versions: independent of the MCP protocol version
Document schema locations: #/$defs/metaObject
Status: internal · Enforcement: internal

Required by

• MCP Description 0.9.0, Section 3.5 (the same grammar applies in every protocol version)
  • 3. Document Structure: req-meta-keys

Based on

• Tier 1 — MCP schema.ts and normative specification text: MCP 2025-06-18 to 2026-07-28 specification, Basic, General fields, _meta key name format
  • https://github.com/modelcontextprotocol/modelcontextprotocol/blob/dae694f61a31f7a3ec2af3aff709e11f70f61e48/docs/specification/2025-06-18/basic/index.mdx#L134-L147
  • https://github.com/modelcontextprotocol/modelcontextprotocol/blob/dae694f61a31f7a3ec2af3aff709e11f70f61e48/docs/specification/2025-11-25/basic/index.mdx#L201-L215
  • https://github.com/modelcontextprotocol/modelcontextprotocol/blob/dae694f61a31f7a3ec2af3aff709e11f70f61e48/docs/specification/2026-07-28/basic/index.mdx#L330-L446

Notes

The document schema rejects malformed keys first; the semantic key check applies only to direct callers of the semantic validator.

Examples

Invalid example

Source: expected-invalid/meta-key-invalid.json · Selected: /tools/0

{
  "name": "inspect",
  "inputSchema": {
    "type": "object"
  },
  "_meta": {
    "com.example/bad key": true
  }
}

Rule Source Code

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