feat: download ADK Web assets dynamically and serve from shared folder

.gitignore

@@ -9,3 +9,4 @@ tests/**/package-lock.json
 coverage/
 .env
 go.sum
+dev/.cache/

dev/build.js

@@ -4,11 +4,23 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 import esbuild from 'esbuild';
-import {cp, writeFile} from 'node:fs/promises';
+import {execSync} from 'node:child_process';
+import {createWriteStream, existsSync, unlink} from 'node:fs';
+import {mkdir, readFile, rm, writeFile} from 'node:fs/promises';
+import * as https from 'node:https';
+import * as path from 'node:path';
+import {fileURLToPath} from 'node:url';
 // eslint-disable-next-line @typescript-eslint/ban-ts-comment
 // @ts-ignore
 import {shimPlugin} from 'esbuild-shim-plugin';
 
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = path.dirname(__filename);
+
+const ADK_WEB_VERSION = 'v1.0.0';
+const cacheDir = path.join(__dirname, '.cache');
+const zipCachePath = path.join(cacheDir, `adk-web-${ADK_WEB_VERSION}.zip`);
+
 const licenseHeaderText = `/**
   * @license
   * Copyright 2026 Google LLC
@@ -26,6 +38,101 @@ const commonOptions = {
   sourcemap: false,
 };
 
+function downloadFile(url, dest) {
+  return new Promise((resolve, reject) => {
+    const request = https.get(url, (response) => {
+      if (
+        response.statusCode &&
+        response.statusCode >= 300 &&
+        response.statusCode < 400 &&
+        response.headers.location
+      ) {
+        // Follow redirect
+        downloadFile(response.headers.location, dest)
+          .then(resolve)
+          .catch(reject);
+        return;
+      }
+
+      if (response.statusCode !== 200) {
+        reject(
+          new Error(
+            `Failed to get '${url}' (status code: ${response.statusCode})`,
+          ),
+        );
+        return;
+      }
+
+      const file = createWriteStream(dest);
+      response.pipe(file);
+      file.on('finish', () => {
+        file.close();
+        resolve(undefined);
+      });
+      file.on('error', (err) => {
+        unlink(dest, () => reject(err));
+      });
+    });
+
+    request.on('error', (err) => {
+      unlink(dest, () => reject(err));
+    });
+  });
+}
+
+function unzipFile(zipPath, destDir) {
+  if (process.platform === 'win32') {
+    execSync(
+      `powershell -Command "Expand-Archive -Path '${zipPath}' -DestinationPath '${destDir}' -Force"`,
+    );
+  } else {
+    execSync(`unzip -o "${zipPath}" -d "${destDir}"`);
+  }
+}
+
+async function ensureBrowserAssets() {
+  const browserOutputDir = path.join(__dirname, 'dist/browser');
+  const versionFile = path.join(browserOutputDir, '.version');
+
+  let needPopulate = false;
+
+  if (!existsSync(browserOutputDir)) {
+    needPopulate = true;
+  } else {
+    try {
+      const version = (await readFile(versionFile, 'utf8')).trim();
+      if (version !== ADK_WEB_VERSION) needPopulate = true;
+    } catch {
+      needPopulate = true;
+    }
+  }
+
+  if (needPopulate) {
+    if (!existsSync(zipCachePath)) {
+      console.log(
+        `[ADK Build] ADK Web zip not cached. Fetching ${ADK_WEB_VERSION} from GitHub...`,
+      );
+      if (!existsSync(cacheDir)) {
+        await mkdir(cacheDir, {recursive: true});
+      }
+      const url = `https://github.com/google/adk-web/releases/download/${ADK_WEB_VERSION}/adk-web-browser.zip`;
+      await downloadFile(url, zipCachePath);
+      console.log(
+        `[ADK Build] Downloaded and cached ADK Web ${ADK_WEB_VERSION}.`,
+      );
+    }
+
+    console.log(`[ADK Build] Extracting ADK Web assets to dist/browser...`);
+    if (existsSync(browserOutputDir)) {
+      await rm(browserOutputDir, {recursive: true, force: true});
+    }
+    await mkdir(browserOutputDir, {recursive: true});
+    unzipFile(zipCachePath, browserOutputDir);
+    await writeFile(versionFile, ADK_WEB_VERSION, 'utf8');
+    console.log(`[ADK Build] ADK Web assets successfully populated.`);
+  }
+}
+
 /**
  * Builds the ADK devtools library.
  */
@@ -52,8 +159,7 @@ async function main() {
 
   // Run file operations sequentially to avoid race conditions
   await writeFile('./dist/cjs/package.json', '{"type": "commonjs"}');
-  await cp('./src/browser', './dist/esm/browser', {recursive: true});
-  await cp('./src/browser', './dist/cjs/browser', {recursive: true});
+  await ensureBrowserAssets();
 }
 
 main().catch((err) => {