Add optional subfolder field to Repository schema (#363)

Adds an optional `subfolder` field to the Repository object in
server.json schema to support MCP servers located within subdirectories
of monorepo structures.

## Motivation and Context

Addresses issue #360 .

The current MCP registry schema doesn't explicitly define how to
reference servers that are located within a subdirectory of a Git
repository (a "monorepo" structure). This is a common pattern (e.g.,
[microsoft/mcp](https://github.com/microsoft/mcp) &
[awslabs/mcp](https://github.com/awslabs/mcp).

## How Has This Been Tested?

- **Validator tests**: Added 6 new test cases covering valid subfolders
and security validation (path traversal, absolute paths, invalid
characters, etc.)
- **Schema validation**: All 11 examples in `examples.md` validate
successfully against both `server.schema.json` and
`registry-schema.json`
- **End-to-end validation**: Publisher tool (`mcp-publisher init`) works
correctly and generates valid templates without subfolder
- **Backward compatibility**: Existing server.json files without
subfolder continue to work unchanged

## Breaking Changes

**None.** This is a fully backward-compatible change:
- The `subfolder` field is optional (uses `omitempty` JSON tag)
- Existing server.json files work unchanged
- Publisher tools generate valid templates without subfolder by default
- All existing validation and schemas remain compatible

## Types of changes
- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [x] Documentation update

## Checklist
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [x] I have added appropriate error handling
- [x] I have added or updated documentation as needed


### Implementation Details

**Schema Changes:**
- Added `subfolder` property to Repository definition in
`server.schema.json` and `openapi.yaml`
- Field is optional with description and example (`"src/everything"`)

**Go Model:**
- Added `Subfolder string` field to Repository struct with
`json:"subfolder,omitempty"` tag
- Maintains backward compatibility through omitempty behavior

**Security-Focused Validation:**
- Validates subfolder is a clean relative path (no leading `/`)
- Prevents path traversal attacks (no `..` sequences)
- Blocks malformed paths (no trailing `/`, empty segments, invalid
characters)
- Uses regex validation for allowed characters: `^[a-zA-Z0-9\-_./]+$`

**Documentation:**
- Added monorepo example to `examples.md` demonstrating subfolder usage
- Updated `repository_references.md` with concise subfolder
documentation
- Maintained consistency across all documentation with same example path

**Testing:**
- 6 new integration tests in validator covering security and format
validation
- Updated example count validation tool for new example
- All existing tests continue to pass

### Design Decisions

1. **Optional by design**: Most servers don't need subfolder
(single-repo pattern is common)
2. **Clean separation**: Repository URL stays clean, subfolder is
separate metadata
3. **Security first**: Comprehensive validation prevents common
path-based attacks
4. **Publisher tool unchanged**: Default templates don't include
subfolder to avoid confusion for typical use cases

This enables publishers with monorepo structures to clearly specify
server locations while maintaining simplicity for the common
single-repository case.

docs/server-json/examples.md

@@ -46,6 +46,47 @@
 }
 ```
 
+## Server in a Monorepo with Subfolder
+
+For MCP servers located within a subdirectory of a larger repository (monorepo structure), use the `subfolder` field to specify the relative path:
+
+```json
+{
+  "$schema": "https://static.modelcontextprotocol.io/schemas/2025-07-09/server.schema.json",
+  "name": "io.modelcontextprotocol/everything",
+  "description": "MCP server that exercises all the features of the MCP protocol",
+  "status": "active",
+  "repository": {
+    "url": "https://github.com/modelcontextprotocol/servers",
+    "source": "github",
+    "subfolder": "src/everything"
+  },
+  "version_detail": {
+    "version": "0.6.2"
+  },
+  "packages": [
+    {
+      "registry_type": "npm",
+      "registry_base_url": "https://registry.npmjs.org",
+      "identifier": "@modelcontextprotocol/everything",
+      "version": "0.6.2",
+      "transport": {
+        "type": "stdio"
+      }
+    }
+  ],
+  "_meta": {
+    "publisher": {
+      "tool": "npm-publisher",
+      "version": "1.0.1",
+      "build_info": {
+        "timestamp": "2023-12-01T10:30:00Z"
+      }
+    }
+  }
+}
+```
+
 ## Constant (fixed) arguments needed to start the MCP server
 
 Suppose your MCP server application requires a `mcp start` CLI arguments to start in MCP server mode. Express these as positional arguments like this:

docs/server-json/repository_references.md

@@ -8,6 +8,8 @@ Consumers of the `server.json` metadata MAY use the `source` property to determi
 
 The `url` property MAY be used to browse the source code. Some source forges, such as GitHub, support `git clone <url>` on the URL, which also works for web browsing. For the purposes of the Official MCP Registry, the URL MUST be accessible in a web browser.
 
+The optional `subfolder` property MAY be used to specify a relative path from the repository root to the location of the MCP server within a monorepo structure. The value MUST be a clean relative path.
+
 The `id` property is owned and determined by the source forge, such as GitHub. This value SHOULD be stable across repository renames and, if applicable on the source forge, MAY be used to detect repository resurrection attacks. If a repository is renamed, the `id` value SHOULD remain constant. If the repository is deleted and then recreated later, the `id` value SHOULD change.
 
 Determining the `id` is specific to the source forge. For GitHub, the following [GitHub CLI](https://cli.github.com/) command MAY be used (works for both public and private repositories):

docs/server-json/server.schema.json

@@ -24,6 +24,11 @@
         "id": {
           "type": "string",
           "example": "b94b5f7e-c7c6-d760-2c78-a5e9b8a5b8c9"
+        },
+        "subfolder": {
+          "type": "string",
+          "description": "Optional relative path from repository root to the server location within a monorepo or nested package structure",
+          "example": "src/everything"
         }
       }
     },
@@ -448,7 +453,7 @@
                   "additionalProperties": true
                 },
                 "io.modelcontextprotocol.registry": {
-                  "type": "object", 
+                  "type": "object",
                   "description": "Official MCP registry metadata (read-only, added by registry)",
                   "additionalProperties": true
                 }

docs/server-registry-api/openapi.yaml

@@ -195,6 +195,10 @@ components:
         id:
           type: string
           example: "b94b5f7e-c7c6-d760-2c78-a5e9b8a5b8c9"
+        subfolder:
+          type: string
+          description: "Optional relative path from repository root to the server location within a monorepo structure"
+          example: "src/everything"
 
     Server:
       type: object