diff --git a/.github/workflows/pre-release.yml b/.github/workflows/pre-release.yml
index 5518483d2..f4d9cb921 100644
--- a/.github/workflows/pre-release.yml
+++ b/.github/workflows/pre-release.yml
@@ -23,6 +23,22 @@ jobs:
         with:
           cache: npm
           node-version-file: '.nvmrc'
+          registry-url: 'https://registry.npmjs.org'
+
+      # Ensure npm 11.5.1 or later is installed
+      - name: Update npm
+        run: npm install -g npm@latest
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Build and bundle
+        run: npm run bundle
+        env:
+          NODE_ENV: 'production'
 
       - name: Verify server.json
         run: npm run verify-server-json-version
+
+      - name: Verify npm package
+        run: npm run verify-npm-package
diff --git a/package.json b/package.json
index a55107464..5bcfd2ef5 100644
--- a/package.json
+++ b/package.json
@@ -22,12 +22,12 @@
     "test:update-snapshots": "npm run build && node scripts/test.mjs --test-update-snapshots",
     "prepare": "node --experimental-strip-types scripts/prepare.ts",
     "verify-server-json-version": "node --experimental-strip-types scripts/verify-server-json-version.ts",
+    "verify-npm-package": "node scripts/verify-npm-package.mjs",
     "eval": "npm run build && CHROME_DEVTOOLS_MCP_NO_USAGE_STATISTICS=true node --experimental-strip-types scripts/eval_gemini.ts",
     "count-tokens": "node --experimental-strip-types scripts/count_tokens.ts"
   },
   "files": [
     "build/src",
-    "build/node_modules",
     "LICENSE",
     "!*.tsbuildinfo"
   ],
diff --git a/scripts/verify-npm-package.mjs b/scripts/verify-npm-package.mjs
new file mode 100644
index 000000000..44c74663e
--- /dev/null
+++ b/scripts/verify-npm-package.mjs
@@ -0,0 +1,41 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import {execSync} from 'node:child_process';
+
+// Checks that the select build files are present using `npm publish --dry-run`.
+function verifyPackageContents() {
+  try {
+    const output = execSync('npm publish --dry-run --json --silent', {
+      encoding: 'utf8',
+    });
+    // skip non-JSON output from prepare.
+    const data = JSON.parse(output.substring(output.indexOf('{')));
+    const files = data.files.map(f => f.path);
+    // Check some important files.
+    const requiredPaths = [
+      'build/src/index.js',
+      'build/src/third_party/index.js',
+    ];
+    for (const requiredPath of requiredPaths) {
+      const hasBuildFolder = files.some(path => path.startsWith(requiredPath));
+      if (!hasBuildFolder) {
+        console.error(
+          `Assertion Failed: "${requiredPath}" not found in tarball.`,
+        );
+        process.exit(1);
+      }
+    }
+    console.log(
+      `npm publish --dry-run contained ${JSON.stringify(requiredPaths)}`,
+    );
+  } catch (err) {
+    console.error('failed to parse npm publish output', err);
+    process.exit(1);
+  }
+}
+
+verifyPackageContents();