fix(filesystem): resolve symlinked allowed directories to both forms

On macOS, /tmp is a symlink to /private/tmp. When users specify /tmp
as an allowed directory, the server was resolving it to /private/tmp
during startup but then rejecting paths like /tmp/file.txt because
they dont start with /private/tmp.

This fix stores BOTH the original normalized path AND the resolved
path in allowedDirectories, so users can access files through either
form. For example, with /tmp as allowed directory, both /tmp/file.txt
and /private/tmp/file.txt will now be accepted.

Fixes #3253

src/filesystem/__tests__/path-validation.test.ts

@@ -564,6 +564,53 @@ describe('Path Validation', () => {
       }
     });
 
+    // Test for macOS /tmp -> /private/tmp symlink issue (GitHub issue #3253)
+    // When allowed directories include BOTH original and resolved paths,
+    // paths through either form should be accepted
+    it('allows paths through both original and resolved symlink directories', async () => {
+      try {
+        // Setup: Create the actual target directory with content
+        const actualTargetDir = path.join(testDir, 'actual-target');
+        await fs.mkdir(actualTargetDir, { recursive: true });
+        const targetFile = path.join(actualTargetDir, 'file.txt');
+        await fs.writeFile(targetFile, 'FILE_CONTENT');
+
+        // Setup: Create symlink directory that points to target (simulates /tmp -> /private/tmp)
+        const symlinkDir = path.join(testDir, 'symlink-dir');
+        await fs.symlink(actualTargetDir, symlinkDir);
+
+        // Get the resolved path
+        const resolvedDir = await fs.realpath(symlinkDir);
+
+        // THE FIX: Store BOTH original symlink path AND resolved path in allowed directories
+        // This is what the server should do during startup to fix issue #3253
+        const allowedDirsWithBoth = [symlinkDir, resolvedDir];
+
+        // Test 1: Path through original symlink should pass validation
+        // (e.g., user requests /tmp/file.txt when /tmp is in allowed dirs)
+        const fileViaSymlink = path.join(symlinkDir, 'file.txt');
+        expect(isPathWithinAllowedDirectories(fileViaSymlink, allowedDirsWithBoth)).toBe(true);
+
+        // Test 2: Path through resolved directory should also pass validation
+        // (e.g., user requests /private/tmp/file.txt)
+        const fileViaResolved = path.join(resolvedDir, 'file.txt');
+        expect(isPathWithinAllowedDirectories(fileViaResolved, allowedDirsWithBoth)).toBe(true);
+
+        // Test 3: The resolved path of the symlink file should also pass
+        const resolvedFile = await fs.realpath(fileViaSymlink);
+        expect(isPathWithinAllowedDirectories(resolvedFile, allowedDirsWithBoth)).toBe(true);
+
+        // Verify both paths point to the same actual file
+        expect(resolvedFile).toBe(await fs.realpath(fileViaResolved));
+
+      } catch (error) {
+        // Skip if no symlink permissions on the system
+        if ((error as NodeJS.ErrnoException).code !== 'EPERM') {
+          throw error;
+        }
+      }
+    });
+
     it('resolves nested symlink chains completely', async () => {
       try {
         // Setup: Create target file in forbidden area

src/filesystem/index.ts

@@ -39,22 +39,32 @@ if (args.length === 0) {
 }
 
 // Store allowed directories in normalized and resolved form
-let allowedDirectories = await Promise.all(
+// We store BOTH the original path AND the resolved path to handle symlinks correctly
+// This fixes the macOS /tmp -> /private/tmp symlink issue where users specify /tmp
+// but the resolved path is /private/tmp
+let allowedDirectories = (await Promise.all(
   args.map(async (dir) => {
     const expanded = expandHome(dir);
     const absolute = path.resolve(expanded);
+    const normalizedOriginal = normalizePath(absolute);
     try {
       // Security: Resolve symlinks in allowed directories during startup
       // This ensures we know the real paths and can validate against them later
       const resolved = await fs.realpath(absolute);
-      return normalizePath(resolved);
+      const normalizedResolved = normalizePath(resolved);
+      // Return both original and resolved paths if they differ
+      // This allows matching against either /tmp or /private/tmp on macOS
+      if (normalizedOriginal !== normalizedResolved) {
+        return [normalizedOriginal, normalizedResolved];
+      }
+      return [normalizedResolved];
     } catch (error) {
       // If we can't resolve (doesn't exist), use the normalized absolute path
       // This allows configuring allowed dirs that will be created later
-      return normalizePath(absolute);
+      return [normalizedOriginal];
     }
   })
-);
+)).flat();
 
 // Validate that all directories exist and are accessible
 await Promise.all(allowedDirectories.map(async (dir) => {