diff --git a/code/addons/vitest/src/updateVitestFile.test.ts b/code/addons/vitest/src/updateVitestFile.test.ts
index 6e08aa611eef..c316e0f281e2 100644
--- a/code/addons/vitest/src/updateVitestFile.test.ts
+++ b/code/addons/vitest/src/updateVitestFile.test.ts
@@ -854,6 +854,95 @@ describe('updateConfigFile', () => {
     `);
   });
 
+  it('appends storybook project to existing test.projects array (no double nesting)', async () => {
+    const source = babel.babelParse(
+      await loadTemplate('vitest.config.3.2.template.ts', {
+        CONFIG_DIR: '.storybook',
+        BROWSER_CONFIG: "{ provider: 'playwright' }",
+        SETUP_FILE: '../.storybook/vitest.setup.ts',
+      })
+    );
+    const target = babel.babelParse(`
+      import { mergeConfig, defineConfig } from 'vitest/config'
+      import viteConfig from './vite.config'
+
+      export default mergeConfig(
+        viteConfig,
+        defineConfig({
+          test: {
+            expect: { requireAssertions: true },
+            projects: [
+              {
+                extends: "./vite.config.ts",
+                test: { name: "client" },
+              },
+              {
+                extends: "./vite.config.ts",
+                test: { name: "server" },
+              },
+            ],
+          },
+        })
+      )
+    `);
+
+    const before = babel.generate(target).code;
+    const updated = updateConfigFile(source, target);
+    expect(updated).toBe(true);
+
+    const after = babel.generate(target).code;
+
+    // check if the code was updated at all
+    expect(after).not.toBe(before);
+
+    // check if the code was updated correctly (storybook project appended to existing projects, no double nesting)
+    expect(getDiff(before, after)).toMatchInlineSnapshot(`
+      "  import { mergeConfig, defineConfig } from 'vitest/config';
+        import viteConfig from './vite.config';
+        
+      + import path from 'node:path';
+      + import { fileURLToPath } from 'node:url';
+      + import { storybookTest } from '@storybook/addon-vitest/vitest-plugin';
+      + const dirname = typeof __dirname !== 'undefined' ? __dirname : path.dirname(fileURLToPath(import.meta.url));
+      + 
+      + // More info at: https://storybook.js.org/docs/next/writing-tests/integrations/vitest-addon
+      + 
+        export default mergeConfig(viteConfig, defineConfig({
+          test: {
+            expect: {
+              requireAssertions: true
+      ...
+              test: {
+                name: "server"
+              }
+        
+      +     }, {
+      +       extends: true,
+      +       plugins: [
+      +       // The plugin will run tests for the stories defined in your Storybook config
+      +       // See options at: https://storybook.js.org/docs/next/writing-tests/integrations/vitest-addon#storybooktest
+      +       storybookTest({
+      +         configDir: path.join(dirname, '.storybook')
+      +       })],
+      +       test: {
+      +         name: 'storybook',
+      +         browser: {
+      +           enabled: true,
+      +           headless: true,
+      +           provider: 'playwright',
+      +           instances: [{
+      +             browser: 'chromium'
+      +           }]
+      +         },
+      +         setupFiles: ['../.storybook/vitest.setup.ts']
+      +       }
+      + 
+            }]
+          }
+        }));"
+    `);
+  });
+
   it('extracts coverage config and keeps it at top level when using workspace', async () => {
     const source = babel.babelParse(
       await loadTemplate('vitest.config.template.ts', {
diff --git a/code/addons/vitest/src/updateVitestFile.ts b/code/addons/vitest/src/updateVitestFile.ts
index 0f544dde3482..567df3de2114 100644
--- a/code/addons/vitest/src/updateVitestFile.ts
+++ b/code/addons/vitest/src/updateVitestFile.ts
@@ -232,8 +232,64 @@ export const updateConfigFile = (source: BabelFile['ast'], target: BabelFile['as
                 p.type === 'ObjectProperty' && p.key.type === 'Identifier' && p.key.name === 'test'
             ) as t.ObjectProperty | undefined;
 
-            if (templateTestProp && templateTestProp.value.type === 'ObjectExpression') {
-              // Find the workspace/projects array in the template
+            const hasProjectsProp = (
+              p: t.ObjectMethod | t.ObjectProperty | t.SpreadElement
+            ): p is t.ObjectProperty =>
+              p.type === 'ObjectProperty' &&
+              p.key.type === 'Identifier' &&
+              p.key.name === 'projects' &&
+              p.value.type === 'ArrayExpression';
+
+            // Check if the existing config already uses a projects array (multi-project setup).
+            // If so, we must append the storybook project to that array instead of wrapping
+            // the entire test config as a single project (which would cause double nesting).
+            const existingProjectsProp = existingTestProp.value.properties.find(hasProjectsProp);
+
+            if (existingProjectsProp) {
+              // Existing config already has test.projects: append storybook project(s) to it
+              if (templateTestProp && templateTestProp.value.type === 'ObjectExpression') {
+                const templateProjectsProp =
+                  templateTestProp.value.properties.find(hasProjectsProp);
+                if (templateProjectsProp && templateProjectsProp.value.type === 'ArrayExpression') {
+                  const templateElements = (templateProjectsProp.value as t.ArrayExpression)
+                    .elements;
+                  (existingProjectsProp.value as t.ArrayExpression).elements.push(
+                    ...templateElements
+                  );
+                }
+                // Merge other test-level options from template (e.g. coverage) into existing test
+                for (const templateProp of templateTestProp.value.properties) {
+                  if (
+                    templateProp.type === 'ObjectProperty' &&
+                    templateProp.key.type === 'Identifier' &&
+                    (templateProp.key as t.Identifier).name !== 'projects'
+                  ) {
+                    const existingProp = existingTestProp.value.properties.find(
+                      (p) =>
+                        p.type === 'ObjectProperty' &&
+                        p.key.type === 'Identifier' &&
+                        (p.key as t.Identifier).name === (templateProp.key as t.Identifier).name
+                    );
+                    if (!existingProp && templateProp.type === 'ObjectProperty') {
+                      existingTestProp.value.properties.push(templateProp);
+                    }
+                  }
+                }
+              }
+              // Merge only non-test properties from template to avoid re-adding storybook project
+              const otherTemplateProps = properties.filter(
+                (p) =>
+                  !(
+                    p.type === 'ObjectProperty' &&
+                    p.key.type === 'Identifier' &&
+                    p.key.name === 'test'
+                  )
+              );
+              if (otherTemplateProps.length > 0) {
+                mergeProperties(otherTemplateProps, targetConfigObject.properties);
+              }
+            } else if (templateTestProp && templateTestProp.value.type === 'ObjectExpression') {
+              // Existing test has no projects array: wrap entire test config as one project
               const workspaceOrProjectsProp = templateTestProp.value.properties.find(
                 (p) =>
                   p.type === 'ObjectProperty' &&