fix: allow file tree access on Windows cross-drive project paths

FileTree component was not passing baseDir to the file API, causing
it to fall back to restricting access to the home directory. On
Windows, projects on a different drive (e.g., D:\) were blocked
with 403.

Changes:
- FileTree now passes workingDirectory as baseDir parameter
- File API routes no longer require baseDir to be under homeDir
  (allows cross-drive access on Windows)
- Still prevents using filesystem root as baseDir for safety
- Add isRootPath() utility for root path detection

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/app/api/files/preview/route.ts

@@ -1,7 +1,7 @@
 import { NextRequest, NextResponse } from 'next/server';
 import path from 'path';
 import os from 'os';
-import { readFilePreview, isPathSafe } from '@/lib/files';
+import { readFilePreview, isPathSafe, isRootPath } from '@/lib/files';
 import type { FilePreviewResponse, ErrorResponse } from '@/types';
 
 export async function GET(request: NextRequest) {
@@ -20,17 +20,14 @@ export async function GET(request: NextRequest) {
   const homeDir = os.homedir();
 
   // Validate that the file is within the session's working directory.
-  // The baseDir parameter should be the session's working directory,
-  // which acts as the trust boundary for file access.
-  // The baseDir itself must be under the user's home directory to prevent
-  // attackers from setting baseDir=/ to bypass all restrictions.
+  // baseDir may be on a different drive than homeDir on Windows.
+  // Only reject root paths as baseDir to prevent full-disk access.
   const baseDir = searchParams.get('baseDir');
   if (baseDir) {
     const resolvedBase = path.resolve(baseDir);
-    // Ensure baseDir is within the home directory (prevent baseDir=/ bypass)
-    if (!isPathSafe(homeDir, resolvedBase)) {
+    if (isRootPath(resolvedBase)) {
       return NextResponse.json<ErrorResponse>(
-        { error: 'Base directory is outside the allowed scope' },
+        { error: 'Cannot use filesystem root as base directory' },
         { status: 403 }
       );
     }

src/app/api/files/route.ts

@@ -1,7 +1,7 @@
 import { NextRequest, NextResponse } from 'next/server';
 import path from 'path';
 import os from 'os';
-import { scanDirectory, isPathSafe } from '@/lib/files';
+import { scanDirectory, isPathSafe, isRootPath } from '@/lib/files';
 import type { FileTreeResponse, ErrorResponse } from '@/types';
 
 export async function GET(request: NextRequest) {
@@ -20,17 +20,17 @@ export async function GET(request: NextRequest) {
   const homeDir = os.homedir();
 
   // Use baseDir (the session's working directory) as the trust boundary.
-  // The baseDir itself must be under the user's home directory to prevent
-  // attackers from setting baseDir=/ to bypass all restrictions.
-  // If no baseDir is provided, fall back to the user's home directory
-  // to prevent scanning arbitrary system directories.
+  // baseDir is the project root the user explicitly chose — it may be on
+  // a different drive than the home directory on Windows (e.g., D:\projects).
+  // We only reject root paths (/, C:\) as baseDir to prevent full-disk scans.
+  // If no baseDir is provided, fall back to the user's home directory.
   const baseDir = searchParams.get('baseDir');
   if (baseDir) {
     const resolvedBase = path.resolve(baseDir);
-    // Ensure baseDir is within the home directory (prevent baseDir=/ bypass)
-    if (!isPathSafe(homeDir, resolvedBase)) {
+    // Prevent using a filesystem root as baseDir (e.g., /, C:\)
+    if (isRootPath(resolvedBase)) {
       return NextResponse.json<ErrorResponse>(
-        { error: 'Base directory is outside the allowed scope' },
+        { error: 'Cannot use filesystem root as base directory' },
         { status: 403 }
       );
     }

src/components/project/FileTree.tsx

@@ -118,7 +118,7 @@ export function FileTree({ workingDirectory, onFileSelect, onFileAdd }: FileTree
     setLoading(true);
     try {
       const res = await fetch(
-        `/api/files?dir=${encodeURIComponent(workingDirectory)}&depth=4`
+        `/api/files?dir=${encodeURIComponent(workingDirectory)}&baseDir=${encodeURIComponent(workingDirectory)}&depth=4`
       );
       if (res.ok) {
         const data = await res.json();

src/lib/files.ts

@@ -74,6 +74,15 @@ export function isPathSafe(basePath: string, targetPath: string): boolean {
   return resolvedTarget.startsWith(resolvedBase + path.sep) || resolvedTarget === resolvedBase;
 }
 
+/**
+ * Check if a path is a filesystem root (e.g., `/`, `C:\`, `D:\`).
+ * Used to prevent using root as a baseDir for file browsing.
+ */
+export function isRootPath(p: string): boolean {
+  const resolved = path.resolve(p);
+  return resolved === path.parse(resolved).root;
+}
+
 export async function scanDirectory(dir: string, depth: number = 3): Promise<FileTreeNode[]> {
   const resolvedDir = path.resolve(dir);