diff --git a/packages/core/src/test-utils/mockWorkspaceContext.fallback.test.ts b/packages/core/src/test-utils/mockWorkspaceContext.fallback.test.ts
new file mode 100644
index 00000000000..18c88b84944
--- /dev/null
+++ b/packages/core/src/test-utils/mockWorkspaceContext.fallback.test.ts
@@ -0,0 +1,69 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import os from 'node:os';
+import path from 'node:path';
+import { mkdtempSync, rmSync } from 'node:fs';
+import { afterAll, afterEach, describe, expect, it, vi } from 'vitest';
+
+import * as fs from 'node:fs';
+
+const throwMissingPath = vi.hoisted(() => () => {
+  const error = new Error('mocked missing path') as NodeJS.ErrnoException;
+  error.code = 'ENOENT';
+  throw error;
+});
+
+vi.mock('node:fs', async () => {
+  const actual = await vi.importActual<typeof import('node:fs')>('node:fs');
+  return {
+    ...actual,
+    realpathSync: vi.fn(throwMissingPath),
+  };
+});
+
+import { createMockWorkspaceContext } from './mockWorkspaceContext.js';
+
+const rootDir = mkdtempSync(path.join(os.tmpdir(), 'qwen-workspace-'));
+
+describe('createMockWorkspaceContext filesystem fallback', () => {
+  it('uses lexical containment when canonicalization is unavailable', () => {
+    const workspace = createMockWorkspaceContext(rootDir);
+
+    expect(
+      workspace.isPathWithinWorkspace(path.join(rootDir, 'missing.txt')),
+    ).toBe(true);
+    expect(workspace.isPathWithinWorkspace(`${rootDir}-sibling`)).toBe(false);
+  });
+
+  it('uses lexical containment when ENOENT has no path', () => {
+    const workspace = createMockWorkspaceContext(rootDir);
+
+    expect(
+      workspace.isPathWithinWorkspace(path.join(rootDir, 'missing.txt')),
+    ).toBe(true);
+  });
+
+  it('uses lexical containment for non-Node filesystem stubs', () => {
+    vi.mocked(fs.realpathSync).mockImplementation(() => {
+      throw new TypeError('mocked filesystem call');
+    });
+    const workspace = createMockWorkspaceContext(rootDir);
+
+    expect(workspace.isPathWithinWorkspace(`${rootDir}-sibling`)).toBe(false);
+    expect(
+      workspace.isPathWithinWorkspace(path.join(rootDir, 'missing.txt')),
+    ).toBe(true);
+  });
+});
+
+afterEach(() => {
+  vi.mocked(fs.realpathSync).mockImplementation(throwMissingPath);
+});
+
+afterAll(() => {
+  rmSync(rootDir, { recursive: true, force: true });
+});
diff --git a/packages/core/src/test-utils/mockWorkspaceContext.test.ts b/packages/core/src/test-utils/mockWorkspaceContext.test.ts
new file mode 100644
index 00000000000..04f6b537454
--- /dev/null
+++ b/packages/core/src/test-utils/mockWorkspaceContext.test.ts
@@ -0,0 +1,170 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import os from 'node:os';
+import path from 'node:path';
+import { mkdtempSync, rmSync, symlinkSync, writeFileSync } from 'node:fs';
+import { describe, expect, it } from 'vitest';
+import { createMockWorkspaceContext } from './mockWorkspaceContext.js';
+
+describe('createMockWorkspaceContext', () => {
+  it('accepts missing descendants under a workspace root', () => {
+    const rootDir = mkdtempSync(path.join(os.tmpdir(), 'qwen-workspace-'));
+    try {
+      const workspace = createMockWorkspaceContext(rootDir);
+
+      expect(
+        workspace.isPathWithinWorkspace(path.join(rootDir, 'missing.txt')),
+      ).toBe(true);
+    } finally {
+      rmSync(rootDir, { recursive: true, force: true });
+    }
+  });
+
+  it('does not treat a similarly prefixed sibling as inside the workspace', () => {
+    const rootDir = mkdtempSync(path.join(os.tmpdir(), 'qwen-workspace-'));
+    try {
+      const workspace = createMockWorkspaceContext(rootDir);
+
+      expect(
+        workspace.isPathWithinWorkspace(`${rootDir}-sibling/file.txt`),
+      ).toBe(false);
+    } finally {
+      rmSync(rootDir, { recursive: true, force: true });
+    }
+  });
+
+  it('checks additional workspace directories', () => {
+    const rootDir = mkdtempSync(path.join(os.tmpdir(), 'qwen-workspace-'));
+    const additionalDir = mkdtempSync(
+      path.join(os.tmpdir(), 'qwen-workspace-'),
+    );
+    try {
+      const workspace = createMockWorkspaceContext(rootDir, [additionalDir]);
+
+      expect(
+        workspace.isPathWithinWorkspace(
+          path.join(additionalDir, 'missing.txt'),
+        ),
+      ).toBe(true);
+    } finally {
+      rmSync(additionalDir, { recursive: true, force: true });
+      rmSync(rootDir, { recursive: true, force: true });
+    }
+  });
+
+  it('canonicalizes workspace aliases for containment checks', () => {
+    const rootDir = mkdtempSync(path.join(os.tmpdir(), 'qwen-workspace-'));
+    const aliasDir = path.join(
+      os.tmpdir(),
+      `qwen-workspace-alias-${Date.now()}`,
+    );
+    symlinkSync(rootDir, aliasDir);
+
+    try {
+      const workspace = createMockWorkspaceContext(aliasDir);
+
+      expect(
+        workspace.isPathWithinWorkspace(path.join(rootDir, 'missing.txt')),
+      ).toBe(true);
+    } finally {
+      rmSync(aliasDir, { recursive: true, force: true });
+      rmSync(rootDir, { recursive: true, force: true });
+    }
+  });
+
+  it('does not collapse missing paths below a symlinked ancestor', () => {
+    const rootDir = mkdtempSync(path.join(os.tmpdir(), 'qwen-workspace-'));
+    const aliasDir = path.join(
+      os.tmpdir(),
+      `qwen-workspace-alias-${Date.now()}`,
+    );
+    symlinkSync(rootDir, aliasDir);
+
+    try {
+      const workspaceRoot = path.join(aliasDir, 'ghost', 'workspace');
+      const siblingPath = path.join(
+        aliasDir,
+        'ghost',
+        'completely-different',
+        'file.txt',
+      );
+      const workspace = createMockWorkspaceContext(workspaceRoot);
+
+      expect(workspace.isPathWithinWorkspace(siblingPath)).toBe(false);
+    } finally {
+      rmSync(aliasDir, { recursive: true, force: true });
+      rmSync(rootDir, { recursive: true, force: true });
+    }
+  });
+
+  it('rejects dangling leaf symlinks', () => {
+    const rootDir = mkdtempSync(path.join(os.tmpdir(), 'qwen-workspace-'));
+    const danglingPath = path.join(rootDir, 'dangling');
+    symlinkSync(path.join(rootDir, 'missing-target'), danglingPath);
+
+    try {
+      const workspace = createMockWorkspaceContext(rootDir);
+
+      expect(workspace.isPathWithinWorkspace(danglingPath)).toBe(false);
+    } finally {
+      rmSync(rootDir, { recursive: true, force: true });
+    }
+  });
+
+  it('resolves existing candidate paths before checking containment', () => {
+    const rootDir = mkdtempSync(path.join(os.tmpdir(), 'qwen-workspace-'));
+    const outsideDir = mkdtempSync(path.join(os.tmpdir(), 'qwen-outside-'));
+    const insidePath = path.join(rootDir, 'inside.txt');
+    const outsidePath = path.join(outsideDir, 'outside.txt');
+    const escapePath = path.join(rootDir, 'escape');
+    writeFileSync(insidePath, 'inside');
+    writeFileSync(outsidePath, 'outside');
+    symlinkSync(outsidePath, escapePath);
+
+    try {
+      const workspace = createMockWorkspaceContext(rootDir);
+
+      expect(workspace.isPathWithinWorkspace(insidePath)).toBe(true);
+      expect(workspace.isPathWithinWorkspace(escapePath)).toBe(false);
+    } finally {
+      rmSync(outsideDir, { recursive: true, force: true });
+      rmSync(rootDir, { recursive: true, force: true });
+    }
+  });
+
+  it('rejects paths through a symlink cycle', () => {
+    const rootDir = mkdtempSync(path.join(os.tmpdir(), 'qwen-workspace-'));
+    const cyclePath = path.join(rootDir, 'cycle');
+    symlinkSync('cycle', cyclePath);
+
+    try {
+      const workspace = createMockWorkspaceContext(rootDir);
+
+      expect(
+        workspace.isPathWithinWorkspace(path.join(cyclePath, 'file.txt')),
+      ).toBe(false);
+    } finally {
+      rmSync(rootDir, { recursive: true, force: true });
+    }
+  });
+
+  it('ignores an invalid workspace root when another root is valid', () => {
+    const rootDir = mkdtempSync(path.join(os.tmpdir(), 'qwen-workspace-'));
+    const cyclePath = path.join(rootDir, 'cycle');
+    const candidatePath = path.join(rootDir, 'inside.txt');
+    symlinkSync('cycle', cyclePath);
+    writeFileSync(candidatePath, 'inside');
+
+    try {
+      const workspace = createMockWorkspaceContext(cyclePath, [rootDir]);
+
+      expect(workspace.isPathWithinWorkspace(candidatePath)).toBe(true);
+    } finally {
+      rmSync(rootDir, { recursive: true, force: true });
+    }
+  });
+});
diff --git a/packages/core/src/test-utils/mockWorkspaceContext.ts b/packages/core/src/test-utils/mockWorkspaceContext.ts
index 67c614e9f53..7c8dbc096c2 100644
--- a/packages/core/src/test-utils/mockWorkspaceContext.ts
+++ b/packages/core/src/test-utils/mockWorkspaceContext.ts
@@ -5,6 +5,11 @@
  */
 
 import { vi } from 'vitest';
+import { isNodeError } from '../utils/errors.js';
+import {
+  isPathWithinRoot,
+  resolveWorkspacePath,
+} from '../utils/workspaceContext.js';
 import type { WorkspaceContext } from '../utils/workspaceContext.js';
 
 /**
@@ -22,12 +27,41 @@ export function createMockWorkspaceContext(
   const mockWorkspaceContext = {
     addDirectory: vi.fn(),
     getDirectories: vi.fn().mockReturnValue(allDirs),
-    isPathWithinWorkspace: vi
-      .fn()
-      .mockImplementation((path: string) =>
-        allDirs.some((dir) => path.startsWith(dir)),
-      ),
+    isPathWithinWorkspace: vi.fn().mockImplementation((path: string) => {
+      try {
+        const canonicalPath = canonicalizeForContainment(path);
+        return allDirs.some((dir) => {
+          try {
+            return isPathWithinRoot(
+              canonicalPath,
+              canonicalizeForContainment(dir),
+            );
+          } catch {
+            return false;
+          }
+        });
+      } catch {
+        return false;
+      }
+    }),
   } as unknown as WorkspaceContext;
 
   return mockWorkspaceContext;
 }
+
+function canonicalizeForContainment(inputPath: string): string {
+  try {
+    return resolveWorkspacePath(inputPath);
+  } catch (error: unknown) {
+    if (isNodeError(error)) {
+      if (error.code === 'ENOENT' && !error.path) {
+        return inputPath;
+      }
+      throw error;
+    }
+
+    // Some tests stub filesystem calls; retain lexical behavior for those
+    // mocked environments.
+    return inputPath;
+  }
+}
diff --git a/packages/core/src/utils/workspaceContext.test.ts b/packages/core/src/utils/workspaceContext.test.ts
index 7b41be841ea..83ce1c80ffb 100644
--- a/packages/core/src/utils/workspaceContext.test.ts
+++ b/packages/core/src/utils/workspaceContext.test.ts
@@ -8,7 +8,7 @@ import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';
 import * as fs from 'node:fs';
 import * as os from 'node:os';
 import * as path from 'node:path';
-import { WorkspaceContext } from './workspaceContext.js';
+import { resolveWorkspacePath, WorkspaceContext } from './workspaceContext.js';
 
 describe('WorkspaceContext with real filesystem', () => {
   let tempDir: string;
@@ -158,6 +158,16 @@ describe('WorkspaceContext with real filesystem', () => {
       );
     });
 
+    it('should preserve paths with missing intermediate components', () => {
+      const workspaceContext = new WorkspaceContext(cwd);
+      const nonExistentPath = path.join(cwd, 'missing', 'nested.txt');
+
+      expect(resolveWorkspacePath(nonExistentPath)).toBe(nonExistentPath);
+      expect(workspaceContext.isPathWithinWorkspace(nonExistentPath)).toBe(
+        true,
+      );
+    });
+
     describe('with symbolic link', () => {
       describe('in the workspace', () => {
         let realDir: string;
diff --git a/packages/core/src/utils/workspaceContext.ts b/packages/core/src/utils/workspaceContext.ts
index fcb5fa91ae1..f287be6d514 100755
--- a/packages/core/src/utils/workspaceContext.ts
+++ b/packages/core/src/utils/workspaceContext.ts
@@ -273,25 +273,7 @@ export class WorkspaceContext {
     if (cached !== undefined) {
       return cached;
     }
-    let resolved: string;
-    try {
-      resolved = fs.realpathSync(pathToCheck);
-    } catch (e: unknown) {
-      if (
-        isNodeError(e) &&
-        e.code === 'ENOENT' &&
-        e.path &&
-        // realpathSync does not set e.path correctly for symlinks to
-        // non-existent files.
-        !this.isFileSymlink(e.path)
-      ) {
-        // If it doesn't exist, e.path contains the fully resolved path.
-        resolved = e.path;
-      } else {
-        // Don't cache exceptions — the path may exist on retry.
-        throw e;
-      }
-    }
+    const resolved = resolveWorkspacePath(pathToCheck);
     if (
       this.resolvedPathCache.size >= WorkspaceContext.RESOLVED_PATH_CACHE_MAX
     ) {
@@ -302,19 +284,70 @@ export class WorkspaceContext {
     this.resolvedPathCache.set(pathToCheck, resolved);
     return resolved;
   }
+}
 
-  /**
-   * Checks if a file path is a symbolic link that points to a file.
-   */
-  private isFileSymlink(filePath: string): boolean {
+/**
+ * Resolves a workspace path using the same missing-path and symlink semantics
+ * used by WorkspaceContext containment checks.
+ */
+export function resolveWorkspacePath(pathToCheck: string): string {
+  try {
+    const resolved = fs.realpathSync(pathToCheck);
+    return typeof resolved === 'string' ? resolved : pathToCheck;
+  } catch (error: unknown) {
+    if (isResolvableMissingPathError(error)) {
+      return resolveMissingPath(pathToCheck);
+    }
+
+    throw error;
+  }
+}
+
+function resolveMissingPath(pathToCheck: string): string {
+  const missingTail: string[] = [];
+  let ancestor = pathToCheck;
+
+  while (true) {
     try {
-      return !fs.readlinkSync(filePath).endsWith('/');
-    } catch (_error) {
-      return false;
+      const resolvedAncestor = fs.realpathSync(ancestor);
+      return path.join(resolvedAncestor, ...missingTail);
+    } catch (error: unknown) {
+      if (!isResolvableMissingPathError(error)) {
+        throw error;
+      }
+
+      const parent = path.dirname(ancestor);
+      if (parent === ancestor) {
+        return pathToCheck;
+      }
+      missingTail.unshift(path.basename(ancestor));
+      ancestor = parent;
     }
   }
 }
 
+function isResolvableMissingPathError(error: unknown): boolean {
+  return (
+    isNodeError(error) &&
+    error.code === 'ENOENT' &&
+    !!error.path &&
+    // realpathSync does not set error.path correctly for symlinks to
+    // non-existent files.
+    !isFileSymlink(error.path)
+  );
+}
+
+/**
+ * Checks if a file path is a symbolic link that points to a file.
+ */
+function isFileSymlink(filePath: string): boolean {
+  try {
+    return !fs.readlinkSync(filePath).endsWith('/');
+  } catch (_error) {
+    return false;
+  }
+}
+
 /**
  * Checks if a path is within a given root directory.
  * @param pathToCheck The absolute path to check