Claude Debugs and Fixes Its Own MCP Filesystem Server UNC Path Bug on Windows

A developer documented using Claude Opus to debug and fix a bug in the MCP Filesystem Server (Desktop Extension version) on Windows. The issue affected UNC network share paths (e.g., \\server\share\).

The Problem

When using a UNC share as an allowed directory, list_directory on the share root worked, but any operation on subdirectories or files failed with an "Access denied - path outside allowed directories" error. Writing files to the root also failed, despite the share being fully accessible from Windows Explorer and mapped drives.

The Debugging Process

Claude helped identify that:

• The configuration for the Desktop Extension version is stored in %APPDATA%\Claude\Claude Extensions Settings\ant.dir.ant.anthropic.filesystem.json, not claude_desktop_config.json.
• Switching to a mapped drive letter didn't work because Node.js fs.realpath() resolves mapped drives back to UNC paths during server startup.
• The server source files (index.js → lib.js → path-validation.js) were examined to find the root cause.

Root Cause

In path-validation.js, the function isPathWithinAllowedDirectories() checks subdirectory membership with:

return normalizedPath.startsWith(normalizedDir + path.sep);

UNC share roots are filesystem roots (like C:\) and retain their trailing backslash after normalization: \\server\share\. This creates a double trailing backslash (\\server\share\\) that never matches real paths. The code had special handling for drive roots like C:\ but not for UNC roots.

The Fix

Replace the problematic line with:

const dirWithSep = normalizedDir.endsWith(path.sep) ? normalizedDir : normalizedDir + path.sep;
return normalizedPath.startsWith(dirWithSep);

Apply the patch with this PowerShell one-liner:

$file = "$env:APPDATA\Claude\Claude Extensions\ant.dir.ant.anthropic.filesystem\node_modules\@modelcontextprotocol\server-filesystem\dist\path-validation.js"
Copy-Item $file "$HOME\Desktop\path-validation.js.backup"
$content = Get-Content $file -Raw
$content = $content.Replace(
  'return normalizedPath.startsWith(normalizedDir + path.sep);',
  'const dirWithSep = normalizedDir.endsWith(path.sep) ? normalizedDir : normalizedDir + path.sep; return normalizedPath.startsWith(dirWithSep);'
)
[System.IO.File]::WriteAllText($file, $content)

Then fully quit and restart Claude Desktop. Claude tested the fix itself using MCP tools after restarting—listing subdirectories and writing a test file to confirm functionality.

Note: This patch will be overwritten if the extension auto-updates. The fix should be implemented upstream in @modelcontextprotocol/server-filesystem. Related GitHub issues: #1838, #470.