diff --git a/.changeset/serve-app-opening-call.md b/.changeset/serve-app-opening-call.md
new file mode 100644
index 000000000..b82dd18df
--- /dev/null
+++ b/.changeset/serve-app-opening-call.md
@@ -0,0 +1,5 @@
+---
+"agent-bundle": patch
+---
+
+Fix `agent-bundle serve-app` (and `serveApp`) showing the "ordinary tool result" fallback with `AB8010: Request body exceeds 64 KiB` instead of the App when the opening tool's result is large, and give a served App one scrollbar instead of three nested ones (the host page, the MCP App sandbox document, and the Runtime App surface proxy no longer scroll around it). (#565)
diff --git a/packages/agent-bundle/src/dev/mcp-apps/mcp-app-routes.ts b/packages/agent-bundle/src/dev/mcp-apps/mcp-app-routes.ts
index b844e7fa1..82643c86c 100644
--- a/packages/agent-bundle/src/dev/mcp-apps/mcp-app-routes.ts
+++ b/packages/agent-bundle/src/dev/mcp-apps/mcp-app-routes.ts
@@ -92,8 +92,23 @@ export interface McpAppRoutePreviewService {
   readonly runtime?: McpAppRuntimeRoutePreviewService;
 }
 
+/** The tool call a host already made for a session, which a page may bind without re-sending it. */
+export interface McpAppOpeningCall {
+  readonly input: McpAppJsonValue;
+  readonly result: McpAppJsonValue;
+}
+
 export interface McpAppRoutesOptions {
   readonly authorize: (request: IncomingMessage) => void;
+  /**
+   * The tool call the host performed itself when it opened a session (the
+   * standalone `serve-app` host calls the opening tool once and seeds its
+   * page with the result). A create request that omits `input` and `result`
+   * binds to this call, so a large result is never round-tripped through the
+   * browser and past the request-body bound (#562); without it, both fields
+   * are required, as the Workbench sends them.
+   */
+  readonly openingCall?: (sessionId: string, toolName: string) => McpAppOpeningCall | undefined;
   /**
    * Test-only override for the graceful-close receipt window. Production
    * callers must leave this unset so the window keeps dominating the frame
@@ -292,16 +307,30 @@ const hostContext = (value: unknown): McpAppPreviewHostContext => {
   });
 };
 
-const createRequest = (value: JsonObject, sessionId: string): Parameters<McpAppRoutePreviewService['create']>[0] => {
+const createRequest = (
+  value: JsonObject,
+  sessionId: string,
+  openingCall: McpAppRoutesOptions['openingCall'],
+): Parameters<McpAppRoutePreviewService['create']>[0] => {
   if (!hasOnly(value, ['host', 'input', 'previewProfile', 'result', 'toolName']) || !nonemptyString(value.toolName)
-    || !isJsonValue(value.input) || !isJsonValue(value.result) || (value.previewProfile !== 'portable' && value.previewProfile !== 'chatgpt' && value.previewProfile !== 'claude')) {
+    || (value.previewProfile !== 'portable' && value.previewProfile !== 'chatgpt' && value.previewProfile !== 'claude')) {
     return invalidShape();
   }
+  // A request carrying neither field binds the call the host already made;
+  // one carrying both is the Workbench's own tool run. Anything in between
+  // is malformed.
+  const carriesCall = Object.hasOwn(value, 'input') || Object.hasOwn(value, 'result');
+  const call = carriesCall
+    ? isJsonValue(value.input) && isJsonValue(value.result)
+      ? { input: cloneJson(value.input), result: cloneJson(value.result) }
+      : undefined
+    : openingCall?.(sessionId, value.toolName);
+  if (call === undefined) return invalidShape();
   return Object.freeze({
     host: hostContext(value.host),
-    input: cloneJson(value.input),
+    input: call.input,
     previewProfile: value.previewProfile,
-    result: cloneJson(value.result),
+    result: call.result,
     sessionId,
     toolName: value.toolName,
   });
@@ -385,6 +414,7 @@ const bridgeHostContext = (host: McpAppPreviewHostContext): McpAppBridgeJsonReco
 export class McpAppRoutes {
   readonly #authorize: (request: IncomingMessage) => void;
   readonly #gracefulCloseReceiptTimeoutMs: number;
+  readonly #openingCall: McpAppRoutesOptions['openingCall'];
   readonly #service: McpAppRoutePreviewService | undefined;
   readonly #tails = new Map<string, Promise<void>>();
   readonly #teardowns = new Map<string, ReturnType<typeof setTimeout>>();
@@ -393,6 +423,7 @@ export class McpAppRoutes {
   constructor(options: McpAppRoutesOptions) {
     this.#authorize = options.authorize;
     this.#gracefulCloseReceiptTimeoutMs = options.gracefulCloseReceiptTimeoutMs ?? gracefulCloseReceiptTimeoutMs;
+    this.#openingCall = options.openingCall;
     this.#service = options.service;
   }
 
@@ -432,7 +463,7 @@ export class McpAppRoutes {
     if (isRuntimeRoute(parsed)) return this.#dispatchRuntime(parsed, request, response, service.runtime);
     if (parsed.kind === 'create') {
       if (method !== 'POST') return responseDiagnostic(response, diagnostic('AB8007', 'Route does not accept this method.', 405));
-      const preview = await service.create(createRequest(await jsonBody(request), parsed.sessionId));
+      const preview = await service.create(createRequest(await jsonBody(request), parsed.sessionId, this.#openingCall));
       return writeJsonResponse(response, { lifecycle: preview.bridge.lifecycle, preview: previewSnapshot(preview) });
     }
     if (parsed.kind === 'force-close') {
diff --git a/packages/agent-bundle/src/dev/mcp-apps/mcp-app-sandbox.ts b/packages/agent-bundle/src/dev/mcp-apps/mcp-app-sandbox.ts
index f602d8dd0..1ae71ee23 100644
--- a/packages/agent-bundle/src/dev/mcp-apps/mcp-app-sandbox.ts
+++ b/packages/agent-bundle/src/dev/mcp-apps/mcp-app-sandbox.ts
@@ -35,7 +35,7 @@ const SHELL = `<!doctype html>
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <title>MCP App sandbox</title>
-<style>html,body,iframe{border:0;height:100%;margin:0;width:100%}</style>
+<style>html,body{height:100%;margin:0;overflow:hidden}iframe{border:0;display:block;height:100%;width:100%}</style>
 <iframe id="app" sandbox="allow-scripts" referrerpolicy="no-referrer"></iframe>
 <script>
   'use strict';
diff --git a/packages/agent-bundle/src/dev/runtime-client-surface-proxy.ts b/packages/agent-bundle/src/dev/runtime-client-surface-proxy.ts
index 203a2e036..dbf38d172 100644
--- a/packages/agent-bundle/src/dev/runtime-client-surface-proxy.ts
+++ b/packages/agent-bundle/src/dev/runtime-client-surface-proxy.ts
@@ -182,7 +182,7 @@ const runtimeProxyShell = (
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <title>Runtime App surface</title>
-<style>html,body,iframe{border:0;height:100%;margin:0;width:100%}</style>
+<style>html,body{height:100%;margin:0;overflow:hidden}iframe{border:0;display:block;height:100%;width:100%}</style>
 <iframe id="app" sandbox="allow-scripts" referrerpolicy="no-referrer"></iframe>
 <script>
   'use strict';
diff --git a/packages/agent-bundle/src/serve-app/serve-app-page.ts b/packages/agent-bundle/src/serve-app/serve-app-page.ts
index b47ac3f8b..49ad360cc 100644
--- a/packages/agent-bundle/src/serve-app/serve-app-page.ts
+++ b/packages/agent-bundle/src/serve-app/serve-app-page.ts
@@ -109,8 +109,10 @@ const browserHostContext = () => ({
 
 const start = async () => {
   setStatus('Binding ' + seed.toolName + ' to the App…');
+  // The host already made the opening call; binding it by tool name keeps a
+  // large result from travelling back through the request-body bound (#562).
   const created = await api('POST', '/api/mcp/sessions/' + encodeURIComponent(seed.sessionId) + '/apps', {
-    host: browserHostContext(), input: seed.input, previewProfile: seed.previewProfile, result: seed.result, toolName: seed.toolName,
+    host: browserHostContext(), previewProfile: seed.previewProfile, toolName: seed.toolName,
   });
   const preview = created.preview;
   const bindingId = preview.bindingId;
@@ -273,7 +275,7 @@ start().catch((error) => {
 
 const HOST_STYLE = `
 :root { color-scheme: light dark; font: 14px/1.4 system-ui, sans-serif; }
-html, body { height: 100%; margin: 0; }
+html, body { height: 100%; margin: 0; overflow: hidden; }
 body { display: flex; flex-direction: column; background: Canvas; color: CanvasText; }
 header { align-items: center; border-bottom: 1px solid color-mix(in srgb, CanvasText 15%, transparent); display: flex; gap: 12px; padding: 8px 16px; }
 header h1 { font-size: 15px; font-weight: 600; margin: 0; }
@@ -287,7 +289,7 @@ header h1 { font-size: 15px; font-weight: 600; margin: 0; }
 #consent li { align-items: center; display: flex; flex-wrap: wrap; gap: 8px; }
 #consent code { font-size: 12px; opacity: 0.8; overflow: hidden; text-overflow: ellipsis; max-width: 40ch; white-space: nowrap; }
 #frame-host { flex: 1; min-height: 0; }
-#frame-host iframe { border: 0; height: 100%; width: 100%; }
+#frame-host iframe { border: 0; display: block; height: 100%; width: 100%; }
 #fallback { overflow: auto; padding: 16px; }
 #fallback pre { background: color-mix(in srgb, CanvasText 6%, transparent); overflow: auto; padding: 8px; }
 `;
diff --git a/packages/agent-bundle/src/serve-app/serve-mcp-app.ts b/packages/agent-bundle/src/serve-app/serve-mcp-app.ts
index 8fbe28fac..8f6a5f65c 100644
--- a/packages/agent-bundle/src/serve-app/serve-mcp-app.ts
+++ b/packages/agent-bundle/src/serve-app/serve-mcp-app.ts
@@ -486,8 +486,15 @@ const serveProgram = (options: ServeMcpAppOptions): Effect.Effect<ServedMcpAppSh
       throw requestError(diagnostic('AB8004', 'A valid MCP App host token is required.', 403));
     }
   };
+  // The page binds the opening call this host already made instead of
+  // posting the result back: a large result would otherwise exceed the
+  // request-body bound and drop the App to the fallback panel (#562).
+  const openingCall = (sessionId: string, toolName: string) =>
+    sessionId === session.sessionId && toolName === selection.tool.name
+      ? Object.freeze({ input: selection.input, result: selection.result })
+      : undefined;
   const routes = yield* Effect.acquireRelease(
-    Effect.sync(() => new McpAppRoutes({ authorize, service: previews })),
+    Effect.sync(() => new McpAppRoutes({ authorize, openingCall, service: previews })),
     (created) => Effect.sync(() => { created.close(); }),
   );
   const page = renderServeAppPage({
diff --git a/packages/agent-bundle/tests/mcp-app-routes.test.ts b/packages/agent-bundle/tests/mcp-app-routes.test.ts
index 1d4af1fdc..fe0c2512a 100644
--- a/packages/agent-bundle/tests/mcp-app-routes.test.ts
+++ b/packages/agent-bundle/tests/mcp-app-routes.test.ts
@@ -7,6 +7,7 @@ import { expect, it } from '@rstest/core';
 import {
   McpAppRoutes,
   type McpAppRoutePreviewService,
+  type McpAppRoutesOptions,
 } from '../src/dev/mcp-apps/mcp-app-routes.ts';
 import { runtimeAppMessageLimits } from '../src/dev/runtime-app-message-limits.ts';
 import { McpAppRuntimePreviewError } from '../src/dev/mcp-app-runtime-preview-service.ts';
@@ -152,10 +153,12 @@ class RecordingPreviewService implements McpAppRoutePreviewService {
 const startRoutes = async (
   service = new RecordingPreviewService(),
   gracefulCloseReceiptTimeoutMs?: number,
+  openingCall?: McpAppRoutesOptions['openingCall'],
 ): Promise<StartedRoutes> => {
   const routes = new McpAppRoutes({
     authorize,
     ...(gracefulCloseReceiptTimeoutMs === undefined ? {} : { gracefulCloseReceiptTimeoutMs }),
+    ...(openingCall === undefined ? {} : { openingCall }),
     service,
   });
   const server = createServer((request, response) => {
@@ -604,6 +607,65 @@ it('creates an App preview from only session-scoped JSON data', async () => {
   }
 });
 
+it('binds the host\'s own opening call when a create request omits input and result (#562)', async () => {
+  // A result far past the 64 KiB request-body bound: the host holds it, so the
+  // page never sends it back.
+  const large = { structuredContent: { rows: Array.from({ length: 4000 }, (_, index) => ({ index, text: 'x'.repeat(24) })) } };
+  expect(Buffer.byteLength(JSON.stringify(large))).toBeGreaterThan(64 * 1024);
+  const service = new RecordingPreviewService();
+  const started = await startRoutes(service, undefined, (sessionId, toolName) =>
+    sessionId === 'session-a' && toolName === 'show-weather' ? { input: { city: 'Oslo' }, result: large } : undefined);
+  try {
+    const bound = await fetch(`${started.url}/api/mcp/sessions/session-a/apps`, {
+      body: JSON.stringify({ host, previewProfile: 'portable', toolName: 'show-weather' }),
+      headers: { ...headers(), 'content-type': 'application/json' },
+      method: 'POST',
+    });
+    expect(bound.status).toBe(200);
+    expect(service.calls).toEqual([{
+      kind: 'create',
+      options: { host, input: { city: 'Oslo' }, previewProfile: 'portable', result: large, sessionId: 'session-a', toolName: 'show-weather' },
+    }]);
+
+    // Another tool, or a session the host did not open, has no call to bind.
+    for (const body of [
+      { host, previewProfile: 'portable', toolName: 'other-tool' },
+      { host, input: { city: 'Oslo' }, previewProfile: 'portable', toolName: 'show-weather' },
+    ]) {
+      const response = await fetch(`${started.url}/api/mcp/sessions/session-a/apps`, {
+        body: JSON.stringify(body),
+        headers: { ...headers(), 'content-type': 'application/json' },
+        method: 'POST',
+      });
+      expect(response.status).toBe(400);
+      await expect(response.json()).resolves.toEqual({ diagnostic: { code: 'AB8021', message: 'MCP App request has an invalid shape.' } });
+    }
+    const otherSession = await fetch(`${started.url}/api/mcp/sessions/session-b/apps`, {
+      body: JSON.stringify({ host, previewProfile: 'portable', toolName: 'show-weather' }),
+      headers: { ...headers(), 'content-type': 'application/json' },
+      method: 'POST',
+    });
+    expect(otherSession.status).toBe(400);
+    expect(service.calls).toHaveLength(1);
+  } finally {
+    await started.close();
+  }
+
+  // Without a host-made call, the Workbench shape stays required.
+  const plain = await startRoutes();
+  try {
+    const response = await fetch(`${plain.url}/api/mcp/sessions/session-a/apps`, {
+      body: JSON.stringify({ host, previewProfile: 'portable', toolName: 'show-weather' }),
+      headers: { ...headers(), 'content-type': 'application/json' },
+      method: 'POST',
+    });
+    expect(response.status).toBe(400);
+    expect(plain.service.calls).toEqual([]);
+  } finally {
+    await plain.close();
+  }
+});
+
 it('rejects obsolete browser-created document consent on preview creation', async () => {
   const started = await startRoutes();
   try {
diff --git a/packages/agent-bundle/tests/serve-app.test.ts b/packages/agent-bundle/tests/serve-app.test.ts
index 128b6e9f0..a3459e1bd 100644
--- a/packages/agent-bundle/tests/serve-app.test.ts
+++ b/packages/agent-bundle/tests/serve-app.test.ts
@@ -196,9 +196,12 @@ it('serves the MCP App example standalone over its packed server and relays the
       return json;
     };
 
-    // Binding the App through the same preview service the Workbench uses.
+    // Binding the App through the same preview service the Workbench uses —
+    // by tool name alone, as the page does: the host already made the opening
+    // call, so its result never crosses the request-body bound (#562).
+    expect(html).not.toContain('result: seed.result, toolName');
     const created = await api('POST', `/api/mcp/sessions/${encodeURIComponent(seed.sessionId)}/apps`, {
-      host: browserHost, input: seed.input, previewProfile: seed.previewProfile, result: seed.result, toolName: seed.toolName,
+      host: browserHost, previewProfile: seed.previewProfile, toolName: seed.toolName,
     });
     const preview = created.preview as {
       readonly bindingId: string;
diff --git a/packages/workbench/tests/mcp-app-real.e2e.test.ts b/packages/workbench/tests/mcp-app-real.e2e.test.ts
index 64919100b..d335b4c87 100644
--- a/packages/workbench/tests/mcp-app-real.e2e.test.ts
+++ b/packages/workbench/tests/mcp-app-real.e2e.test.ts
@@ -334,6 +334,15 @@ e2e('runs a generated SDK-v2 App through the real foreground session and separat
     if (initialAppFrame === undefined) throw new Error('Expected the sandbox proxy to create the App srcdoc frame.');
     const initialAppState = initialAppFrame.getByTestId('app-state');
     await expect(initialAppState).toContainText('real-sdk-v2', { timeout: browserTimeout });
+    // The sandbox proxy document owns no scrollbar of its own (#565): its App
+    // frame is a block filling the document, so only the App itself scrolls.
+    const proxyFrame = page.frames().find((frame) => frame.url().startsWith(sandboxOrigin));
+    if (proxyFrame === undefined) throw new Error('Expected the sandbox proxy frame on the sandbox origin.');
+    await expect.poll(() => proxyFrame.evaluate(() => ({
+      bodyOverflow: getComputedStyle(document.body).overflow,
+      frameDisplay: getComputedStyle(document.getElementById('app')!).display,
+      scrolls: document.documentElement.scrollHeight > document.documentElement.clientHeight,
+    })), { timeout: browserTimeout }).toEqual({ bodyOverflow: 'hidden', frameDisplay: 'block', scrolls: false });
     const consentDecisions = () => consentSnapshots.filter((snapshot) =>
       snapshot !== null && typeof snapshot === 'object' && Object.hasOwn(snapshot, 'approved'));
     const consentDecisionRequests = () => appRequests.filter((request) =>