diff --git a/webui/src/lib/runtime.ts b/webui/src/lib/runtime.ts
index 64dbcb9f..3d327a3a 100644
--- a/webui/src/lib/runtime.ts
+++ b/webui/src/lib/runtime.ts
@@ -24,11 +24,11 @@ export interface HostRuntimeInfo {
 }
 
 export interface NanobotHostApi {
-  getRuntimeInfo(): Promise<HostRuntimeInfo>;
-  restartEngine(): Promise<void>;
-  pickFolder(): Promise<string | null>;
-  openLogs(): Promise<void>;
-  exportDiagnostics(): Promise<string>;
+  getRuntimeInfo?(): Promise<HostRuntimeInfo>;
+  restartEngine?(): Promise<void>;
+  pickFolder?(): Promise<string | null>;
+  openLogs?(): Promise<void>;
+  exportDiagnostics?(): Promise<string>;
   openSocket?(url: string): Promise<string>;
   sendSocket?(id: string, data: string): Promise<void>;
   closeSocket?(id: string): Promise<void>;
@@ -55,6 +55,16 @@ const HOST_WS_CONNECTING = 0;
 const HOST_WS_OPEN = 1;
 const HOST_WS_CLOSING = 2;
 const HOST_WS_CLOSED = 3;
+const LOOPBACK_HOST_PORT_PARAM = "nativeHostPort";
+const LOOPBACK_HOST_TOKEN_PARAM = "nativeHostToken";
+const LOOPBACK_HOST_STORAGE_KEY = "nanobot-webui.native-host";
+
+interface LoopbackHostConfig {
+  port: number;
+  token: string;
+}
+
+let loopbackHostApi: NanobotHostApi | null = null;
 
 declare global {
   interface Window {
@@ -64,7 +74,21 @@ declare global {
 
 function getHostApi(): NanobotHostApi | null {
   if (typeof window === "undefined") return null;
-  return window.nanobotHost ?? null;
+  return window.nanobotHost ?? loopbackHostApi;
+}
+
+/**
+ * Install the external native-host bridge advertised in the URL fragment.
+ *
+ * Only a loopback port is accepted; callers cannot redirect privileged host
+ * actions to an arbitrary origin. The short-lived bridge token is removed
+ * from the URL and retained only for the lifetime of this browser tab.
+ */
+export function initializeLoopbackRuntimeHost(): boolean {
+  if (typeof window === "undefined") return false;
+  const config = consumeLoopbackHostConfig() ?? loadLoopbackHostConfig();
+  loopbackHostApi = config ? createLoopbackHostApi(config) : null;
+  return loopbackHostApi !== null;
 }
 
 export function toRuntimeSurface(surface: string | null | undefined): RuntimeSurface {
@@ -88,10 +112,10 @@ export function createRuntimeHost(
     surface,
     capabilities: mergedCapabilities,
     socketFactory: bridge ? createHostWebSocket : undefined,
-    pickFolder: api ? () => api.pickFolder() : undefined,
-    restartEngine: api ? () => api.restartEngine() : undefined,
-    openLogs: api ? () => api.openLogs() : undefined,
-    exportDiagnostics: api ? () => api.exportDiagnostics() : undefined,
+    pickFolder: api?.pickFolder?.bind(api),
+    restartEngine: api?.restartEngine?.bind(api),
+    openLogs: api?.openLogs?.bind(api),
+    exportDiagnostics: api?.exportDiagnostics?.bind(api),
   };
 }
 
@@ -136,6 +160,96 @@ function getHostSocketBridge(): HostSocketBridge | null {
   };
 }
 
+function consumeLoopbackHostConfig(): LoopbackHostConfig | null {
+  const hash = window.location.hash || "";
+  const queryStart = hash.indexOf("?");
+  if (queryStart < 0) return null;
+
+  const path = hash.slice(0, queryStart) || "#/";
+  const params = new URLSearchParams(hash.slice(queryStart + 1));
+  const hasBridgeParams = params.has(LOOPBACK_HOST_PORT_PARAM)
+    || params.has(LOOPBACK_HOST_TOKEN_PARAM);
+  if (!hasBridgeParams) return null;
+
+  const config = validateLoopbackHostConfig({
+    port: Number(params.get(LOOPBACK_HOST_PORT_PARAM)),
+    token: params.get(LOOPBACK_HOST_TOKEN_PARAM) ?? "",
+  });
+  params.delete(LOOPBACK_HOST_PORT_PARAM);
+  params.delete(LOOPBACK_HOST_TOKEN_PARAM);
+  const nextQuery = params.toString();
+  const nextHash = `${path}${nextQuery ? `?${nextQuery}` : ""}`;
+  window.history.replaceState(
+    null,
+    "",
+    `${window.location.pathname}${window.location.search}${nextHash}`,
+  );
+
+  try {
+    if (config) {
+      window.sessionStorage.setItem(LOOPBACK_HOST_STORAGE_KEY, JSON.stringify(config));
+    } else {
+      window.sessionStorage.removeItem(LOOPBACK_HOST_STORAGE_KEY);
+    }
+  } catch {
+    // The current page can still use the bridge when session storage is unavailable.
+  }
+  return config;
+}
+
+function loadLoopbackHostConfig(): LoopbackHostConfig | null {
+  try {
+    const raw = window.sessionStorage.getItem(LOOPBACK_HOST_STORAGE_KEY);
+    if (!raw) return null;
+    return validateLoopbackHostConfig(JSON.parse(raw) as Partial<LoopbackHostConfig>);
+  } catch {
+    return null;
+  }
+}
+
+function validateLoopbackHostConfig(
+  value: Partial<LoopbackHostConfig>,
+): LoopbackHostConfig | null {
+  const port = Number(value.port);
+  const token = typeof value.token === "string" ? value.token : "";
+  if (!Number.isInteger(port) || port < 1 || port > 65_535) return null;
+  if (!/^[A-Za-z0-9_-]{32,128}$/.test(token)) return null;
+  return { port, token };
+}
+
+function createLoopbackHostApi(config: LoopbackHostConfig): NanobotHostApi {
+  return {
+    async pickFolder(): Promise<string | null> {
+      let response: Response;
+      try {
+        response = await fetch(`http://127.0.0.1:${config.port}/v1/pick-folder`, {
+          method: "POST",
+          cache: "no-store",
+          credentials: "omit",
+          referrerPolicy: "no-referrer",
+          headers: { Authorization: `Bearer ${config.token}` },
+        });
+      } catch {
+        throw new Error("Native folder picker is unavailable. Reopen Nanobot and try again.");
+      }
+
+      const body = await response.json().catch(() => null) as {
+        error?: unknown;
+        path?: unknown;
+      } | null;
+      if (!response.ok) {
+        const detail = typeof body?.error === "string" ? body.error : `HTTP ${response.status}`;
+        throw new Error(`Native folder picker failed: ${detail}`);
+      }
+      if (body?.path === null) return null;
+      if (typeof body?.path !== "string" || !body.path) {
+        throw new Error("Native folder picker returned an invalid path.");
+      }
+      return body.path;
+    },
+  };
+}
+
 class HostWebSocket {
   binaryType: BinaryType = "blob";
   onclose: ((this: WebSocket, ev: CloseEvent) => unknown) | null = null;
diff --git a/webui/src/main.tsx b/webui/src/main.tsx
index f385ff61..2fc96fd8 100644
--- a/webui/src/main.tsx
+++ b/webui/src/main.tsx
@@ -3,6 +3,7 @@ import ReactDOM from "react-dom/client";
 import App from "./App";
 import "./globals.css";
 import "./i18n";
+import { initializeLoopbackRuntimeHost } from "./lib/runtime";
 
 // `crypto.randomUUID` is only defined in secure contexts (HTTPS or localhost).
 // LAN access over plain HTTP leaves it undefined, which crashes components that
@@ -23,5 +24,7 @@ if (typeof globalThis.crypto !== "undefined" && !("randomUUID" in globalThis.cry
 const root = document.getElementById("root");
 if (!root) throw new Error("root element missing");
 
+initializeLoopbackRuntimeHost();
+
 /* StrictMode disabled: dev double-invokes state updaters; delta accumulation must stay pure — see useNanobotStream. */
 ReactDOM.createRoot(root).render(<App />);
diff --git a/webui/src/tests/runtime.test.ts b/webui/src/tests/runtime.test.ts
index 73198a38..0c9d7052 100644
--- a/webui/src/tests/runtime.test.ts
+++ b/webui/src/tests/runtime.test.ts
@@ -1,9 +1,17 @@
 import { afterEach, describe, expect, it, vi } from "vitest";
 
-import { getRuntimeHost, isNativeRuntime } from "@/lib/runtime";
+import {
+  getRuntimeHost,
+  initializeLoopbackRuntimeHost,
+  isNativeRuntime,
+} from "@/lib/runtime";
 
 afterEach(() => {
   Reflect.deleteProperty(window, "nanobotHost");
+  window.sessionStorage.clear();
+  window.history.replaceState(null, "", "/");
+  initializeLoopbackRuntimeHost();
+  vi.unstubAllGlobals();
 });
 
 describe("runtime host facade", () => {
@@ -42,4 +50,46 @@ describe("runtime host facade", () => {
   it("treats server-reported native surface as native for UI labels", () => {
     expect(isNativeRuntime("native")).toBe(true);
   });
+
+  it("installs an authenticated loopback folder picker from the URL fragment", async () => {
+    const token = "a".repeat(43);
+    const fetchMock = vi.fn(async () => new Response(
+      JSON.stringify({ path: "/Users/test/project" }),
+      { status: 200, headers: { "Content-Type": "application/json" } },
+    ));
+    vi.stubGlobal("fetch", fetchMock);
+    window.history.replaceState(
+      null,
+      "",
+      `/#/new?bootstrapSecret=secret&nativeHostPort=43123&nativeHostToken=${token}`,
+    );
+
+    expect(initializeLoopbackRuntimeHost()).toBe(true);
+    expect(window.location.hash).toBe("#/new?bootstrapSecret=secret");
+    expect(isNativeRuntime()).toBe(true);
+    await expect(getRuntimeHost().pickFolder?.()).resolves.toBe("/Users/test/project");
+    expect(fetchMock).toHaveBeenCalledWith(
+      "http://127.0.0.1:43123/v1/pick-folder",
+      expect.objectContaining({
+        method: "POST",
+        headers: { Authorization: `Bearer ${token}` },
+      }),
+    );
+
+    window.history.replaceState(null, "", "/#/new");
+    expect(initializeLoopbackRuntimeHost()).toBe(true);
+    await expect(getRuntimeHost().pickFolder?.()).resolves.toBe("/Users/test/project");
+  });
+
+  it("rejects invalid loopback bridge bootstrap values", () => {
+    window.history.replaceState(
+      null,
+      "",
+      "/#/new?nativeHostPort=70000&nativeHostToken=too-short",
+    );
+
+    expect(initializeLoopbackRuntimeHost()).toBe(false);
+    expect(window.location.hash).toBe("#/new");
+    expect(getRuntimeHost().pickFolder).toBeUndefined();
+  });
 });