diff --git a/dev/src/utils/agent_loader.ts b/dev/src/utils/agent_loader.ts
index d5097c482..15775c498 100644
--- a/dev/src/utils/agent_loader.ts
+++ b/dev/src/utils/agent_loader.ts
@@ -49,6 +49,31 @@ const FILE_MODULE_TYPE_EXTENSION_MAP = {
   [FileModuleType.ESM]: '.mjs',
 };
 
+/**
+ * Packages that must never be inlined into the bundled agent file.
+ *
+ * See http://mikro-orm.io/docs/deployment#deploy-a-bundle-of-entities-and-dependencies-with-esbuild for more details
+ */
+const EXTERNAL_PACKAGES = [
+  'sqlite3',
+  'better-sqlite3',
+  'mysql',
+  'mysql2',
+  // Native addons must remain external so Node can resolve their
+  // platform-specific assets at runtime.
+  'onnxruntime-node',
+  'oracledb',
+  'pg-native',
+  'pg-query-stream',
+  'tedious',
+  'libsql',
+  // Optional peer dependencies of vite and eslint that are not
+  // installed and MUST NOT be bundled.
+  'lightningcss',
+  'jiti',
+  'jiti/package.json',
+];
+
 /**
  * Metadata for a file.
  */
@@ -187,26 +212,9 @@ export class AgentFile {
         bundle: this.options.bundle,
         minify: this.options.bundle,
         plugins: [replaceDirnamePlugin(filePath, originalDir), shimPlugin()],
-        // See http://mikro-orm.io/docs/deployment#deploy-a-bundle-of-entities-and-dependencies-with-esbuild for more details
-        external: [
-          'sqlite3',
-          'better-sqlite3',
-          'mysql',
-          'mysql2',
-          // Native addons must remain external so Node can resolve their
-          // platform-specific assets at runtime.
-          'onnxruntime-node',
-          'oracledb',
-          'pg-native',
-          'pg-query-stream',
-          'tedious',
-          'libsql',
-          // Optional peer dependencies of vite and eslint that are not
-          // installed and MUST NOT be bundled.
-          'lightningcss',
-          'jiti',
-          'jiti/package.json',
-        ],
+        // esbuild rejects `external` unless `bundle` is enabled, so the
+        // allowlist is only passed when the agent file is actually bundled.
+        ...(this.options.bundle ? {external: EXTERNAL_PACKAGES} : {}),
       });
 
       this.cleanupDirPath = outputDir;
diff --git a/dev/test/utils/agent_loader_test.ts b/dev/test/utils/agent_loader_test.ts
index 2285df895..fef6f1cc7 100644
--- a/dev/test/utils/agent_loader_test.ts
+++ b/dev/test/utils/agent_loader_test.ts
@@ -500,6 +500,70 @@ describe('AgentLoader', () => {
       await agentFile.dispose();
     });
 
+    it('omits esbuild "external" when bundling is disabled', async () => {
+      const agentPath = path.join(tempAgentsDir, 'agent2.ts');
+      await fs.writeFile(agentPath, agent2TsContent);
+
+      (esbuild.build as Mock).mockImplementation(async () => {
+        await fs.writeFile(compiledPath('agent2.cjs'), agent2CjsContentMocked);
+        return Promise.resolve();
+      });
+
+      const agentFile = new AgentFile(agentPath, {
+        compile: true,
+        bundle: false,
+      });
+      const agent = await agentFile.load();
+
+      expect(agent.name).toEqual('agent2');
+      const buildOptions = (esbuild.build as Mock).mock.calls[0][0];
+      expect(buildOptions).not.toHaveProperty('external');
+      expect(buildOptions).toMatchObject({bundle: false, minify: false});
+
+      await agentFile.dispose();
+    });
+
+    it('omits esbuild "external" when bundle option is not provided', async () => {
+      const agentPath = path.join(tempAgentsDir, 'agent2.ts');
+      await fs.writeFile(agentPath, agent2TsContent);
+
+      (esbuild.build as Mock).mockImplementation(async () => {
+        await fs.writeFile(compiledPath('agent2.cjs'), agent2CjsContentMocked);
+        return Promise.resolve();
+      });
+
+      const agentFile = new AgentFile(agentPath, {compile: true});
+      const agent = await agentFile.load();
+
+      expect(agent.name).toEqual('agent2');
+      expect((esbuild.build as Mock).mock.calls[0][0]).not.toHaveProperty(
+        'external',
+      );
+
+      await agentFile.dispose();
+    });
+
+    it('compiles and loads a .ts agent with real esbuild when bundle is disabled', async () => {
+      const {build: realEsbuildBuild} =
+        await vi.importActual<typeof import('esbuild')>('esbuild');
+      (esbuild.build as Mock).mockImplementationOnce(realEsbuildBuild);
+
+      const agentPath = path.join(tempAgentsDir, 'agent2.ts');
+      await fs.writeFile(agentPath, agent2TsContent);
+
+      const agentFile = new AgentFile(agentPath, {
+        compile: true,
+        bundle: false,
+      });
+      const agent = await agentFile.load();
+
+      expect(agent.name).toEqual('agent2');
+      const compiled = await fs.readFile(compiledPath('agent2.cjs'), 'utf8');
+      expect(compiled).toContain('require("@google/adk")');
+
+      await agentFile.dispose();
+    });
+
     it('throws specific error if file does not exist', async () => {
       const agentPath = path.join(tempAgentsDir, 'non_existent.js');
       const agentFile = new AgentFile(agentPath);