Resource annotations contain only MCP hints

Resource annotations describe audience, priority, and modification time, not Tool behavior. Unknown properties are invalid even when their names start with x-. Remove those properties and keep Tool-specific hints on Tool annotations instead.

Requirement

Resource Annotations on resources, resource templates, and content blocks contain only audience, priority, and lastModified.

Details

Rule ID: mcp.annotations.members
Owner: mcp
Surface: Annotations
Kind: structural
Requirement level: must
Document schema locations: #/$defs/resourceAnnotations
Status: internal · Enforcement: internal

Protocol version applicability

2024-11-05	2025-03-26	2025-06-18	2025-11-25	2026-07-28
applies	applies	applies	applies	applies

Required by

• MCP Description 0.9.0, Section 10.3
  • 10. Resources and Resource Templates: req-resource-annotations-closed
  • 10. Resources and Resource Templates: req-content-annotations-distinct
  • 15. Serialization: req-serialization-value-formats

Based on

• Tier 1 — MCP schema.ts and normative specification text: MCP 2024-11-05 schema.ts, inline annotations on Resource, ResourceTemplate, and content types; MCP 2025-03-26 to 2026-07-28 schema.ts, interface Annotations (no index signature)
  • validation/mcp-schemas/2024-11-05/schema.ts#L815-L840
  • validation/mcp-schemas/2025-03-26/schema.ts#L921-L944
  • validation/mcp-schemas/2025-06-18/schema.ts#L1093-L1128
  • validation/mcp-schemas/2025-11-25/schema.ts#L1700-L1735
  • validation/mcp-schemas/2026-07-28/schema.ts#L2265-L2300

Notes

lastModified is defined from MCP 2025-06-18; that boundary is checked separately.

Examples

Valid example

Source: expected-valid/distinct-tool-and-resource-annotations.json · Selected: /resources/0/annotations

{
  "audience": [
    "user",
    "assistant"
  ],
  "priority": 0.8,
  "lastModified": "2026-08-24T10:00:00Z"
}

Invalid example

Source: expected-invalid/resource-annotations-unknown-property.json · Selected: /resources/0/annotations

{
  "priority": 0.5,
  "x-acme-tier": "gold"
}

Invalid example

Source: expected-invalid/content-annotations-unknown-property.json · Selected: /tools/0/examples/basic/result/content/0/annotations

{
  "audience": [
    "user"
  ],
  "readOnlyHint": true
}

Rule Source Code

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