---
title: "Use file URIs for roots"
description: "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 "
source: "https://mcpdesc.org/docs/rules/0.9.0-rc.3/mcp/root-uri-file-scheme/"
---

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](https://mcpdesc.org/docs/specification/0.9.0-rc.3/tools#req-interaction-protocol-version-valid)
  * 1. Tools: [req-interaction-response-closed](https://mcpdesc.org/docs/specification/0.9.0-rc.3/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](https://github.com/modelcontextprotocol/modelcontextprotocol/blob/dae694f61a31f7a3ec2af3aff709e11f70f61e48/schema/2024-11-05/schema.ts#L1045-L1052)
  * [validation/mcp-schemas/2025-03-26/schema.ts#L1186-L1193](https://github.com/modelcontextprotocol/modelcontextprotocol/blob/dae694f61a31f7a3ec2af3aff709e11f70f61e48/schema/2025-03-26/schema.ts#L1186-L1193)
  * [validation/mcp-schemas/2025-06-18/schema.ts#L1421-L1428](https://github.com/modelcontextprotocol/modelcontextprotocol/blob/dae694f61a31f7a3ec2af3aff709e11f70f61e48/schema/2025-06-18/schema.ts#L1421-L1428)
  * [validation/mcp-schemas/2025-11-25/schema.ts#L2117-L2124](https://github.com/modelcontextprotocol/modelcontextprotocol/blob/dae694f61a31f7a3ec2af3aff709e11f70f61e48/schema/2025-11-25/schema.ts#L2117-L2124)
  * [validation/mcp-schemas/2026-07-28/schema.ts#L2759-L2766](https://github.com/modelcontextprotocol/modelcontextprotocol/blob/dae694f61a31f7a3ec2af3aff709e11f70f61e48/schema/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](https://github.com/mcpdesc/mcpdesc-specification/blob/v0.9.0-rc.3/spec/draft/fixtures/expected-invalid/invalid-tool-interaction-example-semantics.json) · Selected: `/tools/0/interactionExamples/broken/steps/3/response/roots/0`

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

## Rule Source Code

[interaction.yaml](https://github.com/mcpdesc/mcpdesc-specification/blob/v0.9.0-rc.3/spec/draft/validation/rules/interaction.yaml) at immutable tag [v0.9.0-rc.3](https://github.com/mcpdesc/mcpdesc-specification/blob/v0.9.0-rc.3/spec/draft).