Use file URIs for roots

Root entries in interaction responses use file:// URIs. An arbitrary resource URI is not a valid root merely because the resource is accessible to the server. Correct the root URI without fetching or probing the referenced filesystem location.

Requirement

A root URI in a roots response starts with file://.

Details

Rule ID: mcp.root.uri.file-scheme
Owner: mcp
Surface: Root · Path: uri
Kind: semantic
Requirement level: must
Protocol versions: independent of the MCP protocol version
Status: internal · Enforcement: internal

Required by

• MCP Description 0.9.0, Section 9.4
  • 9. Tools: req-interaction-protocol-version-valid
  • 9. Tools: req-interaction-response-closed

Based on

• Tier 1 — MCP schema.ts and normative specification text: MCP 2024-11-05 to 2026-07-28 schema.ts, interface Root, uri (“This must start with file:// for now”)
  • validation/mcp-schemas/2024-11-05/schema.ts#L1045-L1052
  • validation/mcp-schemas/2025-03-26/schema.ts#L1186-L1193
  • validation/mcp-schemas/2025-06-18/schema.ts#L1421-L1428
  • validation/mcp-schemas/2025-11-25/schema.ts#L2117-L2124
  • validation/mcp-schemas/2026-07-28/schema.ts#L2759-L2766
• Tier 1 — MCP schema.ts and normative specification text: MCP 2026-07-28 specification, Client Features, Roots, Data Types (uri MUST be a file:// URI)
  • https://github.com/modelcontextprotocol/modelcontextprotocol/blob/dae694f61a31f7a3ec2af3aff709e11f70f61e48/docs/specification/2026-07-28/client/roots.mdx#L100-L102

Examples

Invalid example

Source: expected-invalid/invalid-tool-interaction-example-semantics.json · Selected: /tools/0/interactionExamples/broken/steps/3/response/roots/0

{
  "uri": "https://example.com/not-a-file-root"
}

Rule Source Code

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