diff --git a/__tests__/security.test.ts b/__tests__/security.test.ts
index 75ac8432..5fc5df1a 100644
--- a/__tests__/security.test.ts
+++ b/__tests__/security.test.ts
@@ -12,7 +12,7 @@ import { describe, it, expect, beforeEach, afterEach } from 'vitest';
 import * as fs from 'fs';
 import * as path from 'path';
 import * as os from 'os';
-import { FileLock, validateProjectPath } from '../src/utils';
+import { FileLock, validateProjectPath, validatePathWithinRoot, isPathWithinRoot } from '../src/utils';
 import CodeGraph from '../src/index';
 import { ToolHandler, tools } from '../src/mcp/tools';
 import { scanDirectory, isSourceFile } from '../src/extraction';
@@ -206,6 +206,38 @@ describe('validateProjectPath — sensitive directory blocking', () => {
   );
 });
 
+describe('validatePathWithinRoot / isPathWithinRoot — containment check', () => {
+  // A drive root ("W:\") or POSIX root ("/") already ends with a separator.
+  // The check used to append another, comparing against "W:\\" / "//", which
+  // no real path starts with — so every file under a drive-root project was
+  // wrongly rejected and the whole index failed with "files could not be read".
+  it.runIf(process.platform === 'win32')('accepts files when the project root is a drive root', () => {
+    expect(validatePathWithinRoot('C:\\', 'project\\src\\app.ts')).toBe('C:\\project\\src\\app.ts');
+    expect(validatePathWithinRoot('W:\\', 'CyberMAX\\Source\\foo.pas')).toBe('W:\\CyberMAX\\Source\\foo.pas');
+    expect(isPathWithinRoot('W:\\CyberMAX\\Source\\foo.pas', 'W:\\')).toBe(true);
+  });
+
+  it.runIf(process.platform !== 'win32')('accepts files when the project root is the filesystem root', () => {
+    expect(validatePathWithinRoot('/', 'project/src/app.ts')).toBe('/project/src/app.ts');
+    expect(isPathWithinRoot('/project/src/app.ts', '/')).toBe(true);
+  });
+
+  it('still works for a normal nested project root', () => {
+    const root = path.join(os.tmpdir(), 'cg-root');
+    expect(validatePathWithinRoot(root, 'src/app.ts')).toBe(path.join(root, 'src', 'app.ts'));
+    expect(isPathWithinRoot(path.join(root, 'src', 'app.ts'), root)).toBe(true);
+  });
+
+  it('still rejects traversal escapes and sibling-prefix paths', () => {
+    const root = path.join(os.tmpdir(), 'cg-root');
+    // Classic traversal escape.
+    expect(validatePathWithinRoot(root, path.join('..', 'escape', 'x.ts'))).toBeNull();
+    // Sibling directory that shares a name prefix (cg-root vs cg-root-evil)
+    // must not be treated as inside — the trailing separator guards this.
+    expect(isPathWithinRoot(`${root}-evil${path.sep}x.ts`, root)).toBe(false);
+  });
+});
+
 describe('MCP Input Validation', () => {
   let testDir: string;
   let cg: CodeGraph;
diff --git a/src/utils.ts b/src/utils.ts
index 1ee1c937..b4d57181 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -46,6 +46,18 @@ const SENSITIVE_PATHS = new Set([
   'c:\\', 'c:\\windows', 'c:\\windows\\system32',
 ]);
 
+/**
+ * Build the prefix that a path must start with to be considered *inside* `root`.
+ *
+ * A drive or filesystem root ("W:\\", "C:\\", "/") already ends with a
+ * separator. Blindly appending `path.sep` would produce "W:\\\\", which no real
+ * path starts with — so every file under a drive-root project would be wrongly
+ * rejected as a traversal escape. Only append a separator when one is missing.
+ */
+function rootPrefix(root: string): string {
+  return root.endsWith(path.sep) ? root : root + path.sep;
+}
+
 /**
  * Validate that a resolved file path stays within the project root.
  * Prevents path traversal attacks (e.g. node.filePath = "../../etc/passwd").
@@ -58,7 +70,7 @@ export function validatePathWithinRoot(projectRoot: string, filePath: string): s
   const resolved = path.resolve(projectRoot, filePath);
   const normalizedRoot = path.resolve(projectRoot);
 
-  if (!resolved.startsWith(normalizedRoot + path.sep) && resolved !== normalizedRoot) {
+  if (!resolved.startsWith(rootPrefix(normalizedRoot)) && resolved !== normalizedRoot) {
     return null;
   }
   return resolved;
@@ -119,7 +131,7 @@ export function validateProjectPath(dirPath: string): string | null {
 export function isPathWithinRoot(filePath: string, rootDir: string): boolean {
   const resolvedPath = path.resolve(rootDir, filePath);
   const resolvedRoot = path.resolve(rootDir);
-  return resolvedPath.startsWith(resolvedRoot + path.sep) || resolvedPath === resolvedRoot;
+  return resolvedPath.startsWith(rootPrefix(resolvedRoot)) || resolvedPath === resolvedRoot;
 }
 
 /**
@@ -139,7 +151,7 @@ export function isPathWithinRootReal(filePath: string, rootDir: string): boolean
   try {
     const realPath = fs.realpathSync(path.resolve(rootDir, filePath));
     const realRoot = fs.realpathSync(rootDir);
-    return realPath.startsWith(realRoot + path.sep) || realPath === realRoot;
+    return realPath.startsWith(rootPrefix(realRoot)) || realPath === realRoot;
   } catch {
     // If realpath fails (broken symlink, permissions), fall back to logical check
     return true;