---
title: "12. Tags"
description: "MCP Description specification v0.7.0 — 12. Tags."
source: "https://mcpdesc.org/docs/specification/0.7.0/tags/"
---

Mirrored specification

This page mirrors **12\. Tags** of the MCP Description specification **v0.7.0**. The canonical source of truth is [cisco-open/mcptoolkit-contract](https://github.com/cisco-open/mcptoolkit-contract/blob/main/spec/sections/12-tags.md). Where this page differs from upstream, upstream wins.

## 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:

```json
{
  "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"]
    }
  ]
}
```