12. Tags

The root-level tags array defines a flat list of tags for the MCP server. It is OPTIONAL.

When present, tags declares all valid tags that MAY be referenced by tools, resources, resource templates, and prompts. The array order determines display priority — tags appearing earlier in the array SHOULD be presented first in UIs and documentation.

12.1 Tag Object

Property	Type	Required	Description
`name`	string	Yes	Tag identifier. MUST be unique across all tags.
`description`	string	No	Human-readable description of the tag’s purpose.

12.2 Tag Uniqueness

Tag names MUST be unique across all tags in the array. Implementations MUST reject documents containing duplicate tag names.

12.3 Tag References

Per-entity tags arrays (on tools, resources, resource templates, and prompts) contain plain strings referencing tag names. When a root-level tags array is present:

Every tag referenced by an entity MUST be declared in the root tags array.
Implementations MUST treat a reference to an undeclared tag as a validation error.
Per-entity tag arrays MUST NOT contain duplicate values.

When the root-level tags array is absent, per-entity tags are unconstrained strings (backward-compatible behavior).

12.4 Example

Flat tag list with entity references:

{
  "tags": [
    { "name": "analysis", "description": "Game analysis tools" },
    { "name": "rating", "description": "Player and game rating tools" },
    { "name": "history", "description": "Game history and records" },
    { "name": "leaderboard", "description": "Ranking leaderboards" },
    { "name": "player", "description": "Player-specific data" }
  ],
  "tools": [
    {
      "name": "analyze_game",
      "tags": ["analysis"]
    },
    {
      "name": "get_player_rating",
      "tags": ["rating", "player"]
    }
  ],
  "resources": [
    {
      "uri": "chess://leaderboards/classical",
      "name": "classical_leaderboard",
      "tags": ["leaderboard", "rating"]
    }
  ]
}