diff --git a/code/core/src/cli/AddonVitestService.test.ts b/code/core/src/cli/AddonVitestService.test.ts
index f457886880ba..6c2f9fa19d2a 100644
--- a/code/core/src/cli/AddonVitestService.test.ts
+++ b/code/core/src/cli/AddonVitestService.test.ts
@@ -601,5 +601,48 @@ describe('AddonVitestService', () => {
       expect(result.reasons).toBeDefined();
       expect(result.reasons!.length).toBe(2);
     });
+
+    it('should validate mergeConfig with plain object literal', async () => {
+      vi.mocked(find.any)
+        .mockReturnValueOnce(undefined) // workspace
+        .mockReturnValueOnce('vitest.config.ts'); // config
+      vi.mocked(fs.readFile).mockResolvedValue(
+        'export default mergeConfig(viteConfig, { test: { name: "node" } })'
+      );
+      const result = await service.validateConfigFiles('.storybook');
+      expect(result.compatible).toBe(true);
+    });
+
+    it('should validate mergeConfig with defineConfig call', async () => {
+      vi.mocked(find.any)
+        .mockReturnValueOnce(undefined) // workspace
+        .mockReturnValueOnce('vitest.config.ts'); // config
+      vi.mocked(fs.readFile).mockResolvedValue(
+        'export default mergeConfig(viteConfig, defineConfig({ test: { name: "node" } }))'
+      );
+      const result = await service.validateConfigFiles('.storybook');
+      expect(result.compatible).toBe(true);
+    });
+
+    it('should validate mergeConfig with multiple plain objects', async () => {
+      vi.mocked(find.any)
+        .mockReturnValueOnce(undefined) // workspace
+        .mockReturnValueOnce('vitest.config.ts'); // config
+      vi.mocked(fs.readFile).mockResolvedValue(
+        'export default mergeConfig({ test: {} }, { plugins: [] })'
+      );
+      const result = await service.validateConfigFiles('.storybook');
+      expect(result.compatible).toBe(true);
+    });
+
+    it('should reject mergeConfig with invalid object (non-object argument)', async () => {
+      vi.mocked(find.any)
+        .mockReturnValueOnce(undefined) // workspace
+        .mockReturnValueOnce('vitest.config.ts'); // config
+      vi.mocked(fs.readFile).mockResolvedValue('export default mergeConfig(viteConfig, "string")');
+      const result = await service.validateConfigFiles('.storybook');
+      expect(result.compatible).toBe(false);
+      expect(result.reasons!.some((r) => r.includes('invalid Vitest config'))).toBe(true);
+    });
   });
 });
diff --git a/code/core/src/cli/AddonVitestService.ts b/code/core/src/cli/AddonVitestService.ts
index a8375d0fa5f0..a9771ca53438 100644
--- a/code/core/src/cli/AddonVitestService.ts
+++ b/code/core/src/cli/AddonVitestService.ts
@@ -326,9 +326,7 @@ export class AddonVitestService {
     babel.traverse(parsedConfig, {
       ExportDefaultDeclaration: (path: any) => {
         if (this.isDefineConfigExpression(path.node.declaration)) {
-          isValidVitestConfig = this.isSafeToExtendWorkspace(
-            path.node.declaration as CallExpression
-          );
+          isValidVitestConfig = this.isSafeToExtendWorkspace(path.node.declaration);
         } else if (this.isMergeConfigExpression(path.node.declaration)) {
           // the config could be anywhere in the mergeConfig call, so we need to check each argument
           const mergeCall = path.node.declaration as CallExpression;
@@ -372,20 +370,34 @@ export class AddonVitestService {
     return babel.types.isCallExpression(path) && (path.callee as any)?.name === 'mergeConfig';
   }
 
-  private isSafeToExtendWorkspace(node: CallExpression): boolean {
-    return (
-      babel.types.isCallExpression(node) &&
-      node.arguments.length > 0 &&
-      babel.types.isObjectExpression(node.arguments?.[0]) &&
-      node.arguments[0]?.properties.every(
-        (p: any) =>
-          p.key?.name !== 'test' ||
-          (babel.types.isObjectExpression(p.value) &&
-            p.value.properties.every(
-              ({ key, value }: any) =>
-                key?.name !== 'workspace' || babel.types.isArrayExpression(value)
-            ))
-      )
+  private isSafeToExtendWorkspace(node: babel.types.Node): boolean {
+    // Extract the object expression to validate
+    let objectToValidate: babel.types.ObjectExpression | null = null;
+
+    if (babel.types.isCallExpression(node)) {
+      // Handle function calls like defineConfig({...})
+      if (node.arguments.length > 0 && babel.types.isObjectExpression(node.arguments[0])) {
+        objectToValidate = node.arguments[0];
+      }
+    } else if (babel.types.isObjectExpression(node)) {
+      // Handle plain object literals like {...}
+      objectToValidate = node;
+    }
+
+    // If we couldn't extract a valid object, it's not safe
+    if (!objectToValidate) {
+      return false;
+    }
+
+    // Check that the object doesn't have problematic test.workspace properties
+    return objectToValidate.properties.every(
+      (p: any) =>
+        p.key?.name !== 'test' ||
+        (babel.types.isObjectExpression(p.value) &&
+          p.value.properties.every(
+            ({ key, value }: any) =>
+              key?.name !== 'workspace' || babel.types.isArrayExpression(value)
+          ))
     );
   }
 }