ScriptedAlchemy · ScriptedAlchemy · Sep 1, 2026 · Sep 1, 2026
diff --git a/.changeset/browser-bridge-close-on-rejected-publish.md b/.changeset/browser-bridge-close-on-rejected-publish.md
@@ -0,0 +1,5 @@
+---
+"agent-bundle": patch
+---
+
+Force-close the MCP App bridge when the initial tool result publication is rejected in mountBrowserApp, so the test binding is released instead of leaking.
diff --git a/packages/agent-bundle/src/test/browser.ts b/packages/agent-bundle/src/test/browser.ts
@@ -364,6 +364,8 @@ export const mountBrowserApp = async (
   window.addEventListener('message', onMessage);
   if (!bridge.publishToolResult(options.toolResult)) {
     window.removeEventListener('message', onMessage);
+    await bridge.forceClose().catch(() => undefined);
+    iframe.remove();
     throw new BrowserAppTestError('The initial MCP App tool result was rejected.', provenance);
   }
 

diff --git a/packages/agent-bundle/tests/test-browser-mount.test.ts b/packages/agent-bundle/tests/test-browser-mount.test.ts
@@ -0,0 +1,120 @@
+import { afterEach, describe, expect, it } from '@rstest/core';
+
+import {
+  AGENT_BROWSER_TEST_REGISTRY_SYMBOL_KEY,
+  AGENT_BROWSER_TEST_REGISTRY_VERSION,
+  BROWSER_APP_PROOF_LEVEL,
+} from '../src/test/browser-registry.ts';
+import { mountBrowserApp } from '../src/test/browser.ts';
+
+const registrySymbol = Symbol.for(AGENT_BROWSER_TEST_REGISTRY_SYMBOL_KEY);
+const queuedHostMessageLimitBytes = 1_048_576;
+
+interface TestIframe {
+  contentDocument: null;
+  contentWindow: null;
+  referrerPolicy: string;
+  remove(): void;
+  setAttribute(name: string, value: string): void;
+  srcdoc: string;
+}
+
+interface TestDocument {
+  body: { append(node: TestIframe): void };
+  createElement(tagName: string): TestIframe;
+}
+
+interface TestWindow {
+  addEventListener(type: string, listener: (event: unknown) => void): void;
+  removeEventListener(type: string, listener: (event: unknown) => void): void;
+}
+
+type MutableRealm = {
+  [registrySymbol]?: unknown;
+  document?: TestDocument;
+  window?: TestWindow;
+};
+
+const realmOf = (): MutableRealm => globalThis as unknown as MutableRealm;
+
+const installRegistry = (): (() => void) => {
+  const realm = realmOf();
+  const previous = realm[registrySymbol];
+  realm[registrySymbol] = {
+    apps: {
+      panel: {
+        html: '<!DOCTYPE html><html><body></body></html>',
+        name: 'panel',
+        output: '/tmp/panel.html',
+        proofLevel: BROWSER_APP_PROOF_LEVEL,
+        resourceUri: 'ui://test/panel',
+        serverIds: ['mcp:test'],
+        target: 'portable',
+      },
+    },
+    version: AGENT_BROWSER_TEST_REGISTRY_VERSION,
+  };
+  return () => {
+    if (previous === undefined) delete realm[registrySymbol];
+    else realm[registrySymbol] = previous;
+  };
+};
+
+const installBrowserGlobals = (): (() => void) => {
+  const realm = realmOf();
+  const previous = { document: realm.document, window: realm.window };
+  const iframe: TestIframe = {
+    contentDocument: null,
+    contentWindow: null,
+    referrerPolicy: '',
+    remove() {},
+    setAttribute() {},
+    srcdoc: '',
+  };
+  realm.document = {
+    body: { append() {} },
+    createElement: (tagName: string) => {
+      if (tagName !== 'iframe') throw new Error(`unexpected element: ${tagName}`);
+      return iframe;
+    },
+  };
+  realm.window = {
+    addEventListener() {},
+    removeEventListener() {},
+  };
+  return () => {
+    if (previous.document === undefined) delete realm.document;
+    else realm.document = previous.document;
+    if (previous.window === undefined) delete realm.window;
+    else realm.window = previous.window;
+  };
+};
+
+const restoreFns: Array<() => void> = [];
+
+afterEach(() => {
+  while (restoreFns.length > 0) restoreFns.pop()?.();
+});
+
+describe('mountBrowserApp', () => {
+  it('force-closes the bridge when the initial tool result publication is rejected', async () => {
+    restoreFns.push(installRegistry(), installBrowserGlobals());
+    const closes: string[] = [];
+
+    await expect(mountBrowserApp('panel', {
+      operations: {
+        callTool: async () => ({ content: [{ text: 'unused', type: 'text' }] }),
+        closeBinding: async (bindingId) => {
+          closes.push(bindingId);
+          return true;
+        },
+        readResource: async () => ({ contents: [] }),
+      },
+      toolResult: {
+        content: [{ text: 'x'.repeat(queuedHostMessageLimitBytes), type: 'text' }],
+      },
+    })).rejects.toThrow('The initial MCP App tool result was rejected.');
+
+    expect(closes).toHaveLength(1);
+  });
+});