diff --git a/packages/core/src/tools/shell.test.ts b/packages/core/src/tools/shell.test.ts
index 03c1bc8767b..cd066198ce9 100644
--- a/packages/core/src/tools/shell.test.ts
+++ b/packages/core/src/tools/shell.test.ts
@@ -331,6 +331,29 @@ describe('ShellTool', () => {
       );
     });
 
+    it('should reject sibling-prefix directories outside the workspace', async () => {
+      const workspaceContext = createMockWorkspaceContext('/test/dir', [
+        '/tmp/project',
+      ]);
+      vi.mocked(workspaceContext.isPathWithinWorkspace).mockReturnValue(false);
+      (mockConfig.getWorkspaceContext as Mock).mockReturnValue(
+        workspaceContext,
+      );
+
+      expect(() =>
+        shellTool.build({
+          command: 'ls',
+          directory: '/tmp/project-other',
+          is_background: false,
+        }),
+      ).toThrow(
+        "Directory '/tmp/project-other' is not within any of the registered workspace directories.",
+      );
+      expect(workspaceContext.isPathWithinWorkspace).toHaveBeenCalledWith(
+        '/tmp/project-other',
+      );
+    });
+
     it('should throw an error for a directory within the user skills directory', async () => {
       expect(() =>
         shellTool.build({
diff --git a/packages/core/src/tools/shell.ts b/packages/core/src/tools/shell.ts
index f8a2030771c..da57d5096b1 100644
--- a/packages/core/src/tools/shell.ts
+++ b/packages/core/src/tools/shell.ts
@@ -4809,12 +4809,8 @@ export class ShellTool extends BaseDeclarativeTool<
         return `Explicitly running shell commands from within the user skills directory is not allowed. Please use absolute paths for command parameter instead.`;
       }
 
-      const workspaceDirs = this.config.getWorkspaceContext().getDirectories();
-      const isWithinWorkspace = workspaceDirs.some((wsDir) =>
-        params.directory!.startsWith(wsDir),
-      );
-
-      if (!isWithinWorkspace) {
+      const workspaceContext = this.config.getWorkspaceContext();
+      if (!workspaceContext.isPathWithinWorkspace(params.directory)) {
         return `Directory '${params.directory}' is not within any of the registered workspace directories.`;
       }
     }