Encode binary Resource Contents as base64

A Resource example’s blob must be valid base64. Check the encoding rather than placing raw binary or a URL in the field. Encoded content remains untrusted, and a valid encoding does not establish that the resource exists or is safe to open.

Requirement

A blob in Resource Contents is canonical base64.

Details

Rule ID: mcp.resource-contents.blob.base64
Owner: mcp
Surface: BlobResourceContents · Path: blob
Kind: structural
Requirement level: must
Protocol versions: independent of the MCP protocol version
Status: internal · Enforcement: internal

Required by

• MCP Description 0.9.0, Section 10.4.4
  • 10. Resources and Resource Templates: req-resource-example-blob-base64

Based on

• Tier 1 — MCP schema.ts and normative specification text: MCP 2024-11-05 to 2026-07-28 schema.ts, interface BlobResourceContents, blob (@format byte)
  • validation/mcp-schemas/2024-11-05/schema.ts#L507-L512
  • validation/mcp-schemas/2025-03-26/schema.ts#L545-L550
  • validation/mcp-schemas/2025-06-18/schema.ts#L637-L642
  • validation/mcp-schemas/2025-11-25/schema.ts#L913-L918
  • validation/mcp-schemas/2026-07-28/schema.ts#L1549-L1554
• Tier 2 — Normative referenced standards: RFC 4648, Section 4
  • https://www.rfc-editor.org/rfc/rfc4648#section-4

Examples

Invalid example

Source: expected-invalid/resource-example-result-semantics.json · Selected: /resources/0/examples/invalid-result/result/contents/0

{
  "uri": "asset:///invalid.bin",
  "blob": "not base64"
}

Rule Source Code

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