diff --git a/.changeset/fix-webview-route-masking.md b/.changeset/fix-webview-route-masking.md
new file mode 100644
index 00000000..6f542d7a
--- /dev/null
+++ b/.changeset/fix-webview-route-masking.md
@@ -0,0 +1,18 @@
+---
+"aicodeman": patch
+---
+
+fix(webview): let a proxied single-page app route on its own path, and recover a frame that reloads
+
+A dashboard served through a web tab saw `/webview/<cap>/` as its `location.pathname`, and
+no app has a route for that: a React Router, Vue Router or Vite dev-server page painted its
+HTML and CSS and then replaced them with its own "page not found" the moment its script ran.
+The proxy's runtime shim now rewrites the history entry to the path the page would see on its
+own origin before any page script runs, while every URL the page emits still goes through
+the existing rewrite layers (plus `Worker`, `sendBeacon` and `window.open`, which the masked
+Referer can no longer rescue). A navigation the page starts itself afterwards — a dev
+server's full-reload HMR, a root-absolute `location.href` — lands on Codeman's root with no
+capability; it is recognised by shape (an iframe navigation asking for HTML for a path Codeman
+does not serve), answered with a static page that tells the owning tab which path was lost,
+and the tab remounts the frame inside the prefix at that path. That answer is served before
+the credential checks, so it never counts as a failed login.
diff --git a/docs/web-tabs.md b/docs/web-tabs.md
index 753a607d..7f6be80e 100644
--- a/docs/web-tabs.md
+++ b/docs/web-tabs.md
@@ -147,6 +147,21 @@ layers cooperate so a dashboard talking to its own backend just works:
    using its `Referer` to identify the dashboard. This only fires for a request
    that already missed every Codeman route, and never for one that resolves to a
    real route, which is what keeps it from being an authentication bypass.
+5. The same script **masks the proxy prefix off the page's own URL** before any
+   of the page's code runs (`history.replaceState` to the path the page would see
+   on its own origin). A single-page app routes on `location.pathname` at boot,
+   and `/webview/<cap>/` is a path no app has a route for: without this, a React
+   Router / Vue Router / Next dev server painted its HTML and CSS and then replaced
+   them with its own "page not found" the moment its script ran. The page only
+   *reads* the masked path; every URL it emits still goes through the layers above.
+6. A navigation the page starts **itself** after that — `location.reload()` (a dev
+   server's full-reload HMR), a root-absolute `location.href = '/login'` — now
+   targets Codeman's root with no capability anywhere on it. Codeman recognises
+   that request by shape (a top-level `<iframe>` navigation asking for HTML, for a
+   path it does not serve) and answers a static page that does nothing but tell
+   the owning tab which path was lost; the tab remounts the frame inside the
+   prefix at that path. It never counts as a failed login, so a dev server that
+   reloads on every save cannot rate-limit its user out of Codeman.
 
 On top of that, the proxy answers those requests with CORS headers. That sounds
 wrong for same-host requests, but a sandboxed iframe has an *opaque* origin, so the
@@ -160,10 +175,12 @@ then every API call fails, which looks like the dashboard being broken.
   EventSource, normal markup, the DOM sinks a page uses to build markup at runtime,
   and `url()` inside stylesheets. Something that constructs requests by an unusual
   route can still slip through. Symptom: the page renders but a panel stays empty.
-- **Root-absolute `location` navigation.** A dashboard that navigates itself with
-  `location.href = '/login'` escapes the prefix, because `Location.href` is
-  unforgeable and cannot be patched the way the other sinks are. A relative
-  `location.href = 'login'` is fine (`<base>` covers it).
+- **Root-absolute `location` navigation is recovered, not prevented.** `Location`
+  is unforgeable, so `location.href = '/login'` or `location.reload()` really does
+  leave the prefix; the frame comes back through the recovery hop in layer 6 above,
+  which needs a browser that sends `Sec-Fetch-Dest` (every current one; iOS Safari
+  since 16.4). Older browsers show Codeman's 404 in the frame; the tab's **Reload**
+  button puts it back.
 - **Cross-origin redirects are not followed.** If a dashboard bounces to a different
   host (an external SSO provider, say), the proxy hands the redirect back unchanged
   rather than relaying it, because relaying would make this an open proxy. Use
diff --git a/src/web/middleware/auth.ts b/src/web/middleware/auth.ts
index adbd9461..f7df69d2 100644
--- a/src/web/middleware/auth.ts
+++ b/src/web/middleware/auth.ts
@@ -23,7 +23,13 @@ import { getHookSecret, HOOK_SECRET_HEADER } from '../../config/hook-secret.js';
 import { isMultiUserMode } from '../../config/multiuser.js';
 import { findUser, setPassword, touchLastLogin, verifyPassword } from '../../user-store.js';
 import { webviewCapabilities } from '../../webview-capabilities.js';
-import { capabilityFromProxyPath, capabilityFromReferer } from '../webview-proxy.js';
+import {
+  capabilityFromProxyPath,
+  capabilityFromReferer,
+  isLostWebviewFrameNavigation,
+  lostWebviewFramePage,
+  LOST_FRAME_PAGE_CSP,
+} from '../webview-proxy.js';
 import { ApiErrorCode, createErrorResponse, type AuthUser } from '../../types.js';
 
 // Request-scoped identity (multi-user). Single-user leaves it undefined and the
@@ -176,6 +182,30 @@ function hasValidWebviewCapability(req: FastifyRequest, basePath = ''): boolean
   return !!fromReferer && webviewCapabilities.resolve(fromReferer) !== undefined;
 }
 
+/**
+ * A web-tab frame that navigated itself off the proxy prefix (see
+ * isLostWebviewFrameNavigation). It cannot authenticate: opaque origin, no cookie,
+ * no capability left in the URL. Answer with the static recovery page here, BEFORE
+ * the credential checks, so the reload of a proxied dashboard neither shows a
+ * login challenge inside the tab nor counts as a failed attempt against the
+ * caller's IP — a dev server that full-reloads on every save would otherwise
+ * rate-limit its own user out of Codeman. Fenced like the Referer exemption: a
+ * path that resolves to a real route (the app shell, /api, /q) is never answered
+ * this way, so a genuine unauthenticated navigation still gets the 401.
+ *
+ * @returns true when the reply was sent.
+ */
+function serveLostWebviewFrame(req: FastifyRequest, reply: FastifyReply): boolean {
+  if (!isLostWebviewFrameNavigation(req)) return false;
+  const url = (req.url ?? '').split('?')[0];
+  if (url === '/' || url.startsWith('/api/') || url.startsWith('/ws/') || url.startsWith('/q/')) return false;
+  if (matchesRegisteredRoute(req, url)) return false;
+  reply.header('content-security-policy', LOST_FRAME_PAGE_CSP);
+  reply.header('cache-control', 'no-store');
+  reply.type('text/html; charset=utf-8').send(lostWebviewFramePage());
+  return true;
+}
+
 /**
  * Whether `url` resolves to a route Codeman actually registered.
  *
@@ -302,6 +332,8 @@ export function registerAuthMiddleware(app: FastifyInstance, https: boolean, bas
       done();
       return;
     }
+    // A web-tab frame that lost its prefix: hand it back to its tab, no credentials involved.
+    if (serveLostWebviewFrame(req, reply)) return;
 
     const clientIp = req.ip;
 
@@ -439,6 +471,8 @@ function registerMultiUserAuthHook(
     // ownership against the identity BOUND TO THE CAPABILITY, which is stricter
     // than re-deriving it from a request that carries no credentials.
     if (hasValidWebviewCapability(req, basePath)) return;
+    // A web-tab frame that lost its prefix: hand it back to its tab, no credentials involved.
+    if (serveLostWebviewFrame(req, reply)) return;
 
     const clientIp = req.ip;
 
diff --git a/src/web/public/webview-tabs.js b/src/web/public/webview-tabs.js
index d17ae582..5a8a993e 100644
--- a/src/web/public/webview-tabs.js
+++ b/src/web/public/webview-tabs.js
@@ -138,6 +138,7 @@ Object.assign(CodemanApp.prototype, {
     this._webviewFrameLru = this._webviewFrameLru || [];
 
     await this.refreshWebviews();
+    this._installWebviewLostListener();
 
     // Restore the previously open web tabs (per device: which dashboards you keep
     // open is a workspace-layout choice, not something to sync across machines).
@@ -167,6 +168,48 @@ Object.assign(CodemanApp.prototype, {
     this.renderSessionTabs();
   },
 
+  /**
+   * Take back a frame that navigated itself off its proxy prefix.
+   *
+   * The proxy's runtime shim masks `/webview/<cap>/` off the document URL so a
+   * single-page app routes on the path it expects. A navigation the page then
+   * starts itself — `location.reload()` (a dev server's full-reload HMR), a
+   * root-absolute `location.href = '/login'` — lands on Codeman's root with no
+   * capability, where the server answers a static page that does nothing but
+   * post `{type:'codeman:webview-lost', path}` here. The frame is identified by
+   * `event.source` against the iframes this tab mounted (never by the payload),
+   * and remounted inside the prefix at that path. Bounded per frame so a page
+   * that reloads itself on every boot cannot spin.
+   */
+  _installWebviewLostListener() {
+    if (this._webviewLostListener) return;
+    this._webviewLostListener = (event) => {
+      const data = event.data;
+      if (!data || typeof data !== 'object' || data.type !== 'codeman:webview-lost') return;
+      if (typeof data.path !== 'string' || !event.source) return;
+      const layer = document.getElementById('webviewLayer');
+      if (!layer) return;
+      for (const wrap of layer.querySelectorAll('.webview-frame')) {
+        const frame = wrap.querySelector('iframe');
+        if (!frame || frame.contentWindow !== event.source) continue;
+        const id = wrap.dataset.webviewId;
+        if (!id || !this.webviews?.has(id)) return;
+        const now = Date.now();
+        this._webviewRecoveries = this._webviewRecoveries || new Map();
+        const recent = (this._webviewRecoveries.get(id) || []).filter((at) => now - at < 60000);
+        if (recent.length >= 5) return;
+        recent.push(now);
+        this._webviewRecoveries.set(id, recent);
+        // Path only, never an origin: a `//host/x` here would jump the frame off
+        // the proxy (resolveUpstreamUrl refuses it server-side as well).
+        const path = data.path.replace(/^\/+/, '/');
+        void this.openWebview(id, { path: path.startsWith('/') && !path.startsWith('//') ? path : '/' });
+        return;
+      }
+    };
+    window.addEventListener('message', this._webviewLostListener);
+  },
+
   _persistWebviewOrder() {
     try {
       localStorage.setItem('codeman-webview-order', JSON.stringify(this.webviewOrder || []));
@@ -264,13 +307,15 @@ Object.assign(CodemanApp.prototype, {
     if (data.webview) this.webviews.set(id, data.webview);
 
     let src = data.embedUrl || data.webview?.url || webview.url;
-    const path = typeof options.path === 'string' ? options.path : '';
+    // A string `path` (even '') means "go there": the proxy prefix is
+    // `/webview/<cap>/` and the wildcard rides after it; in direct mode the deep
+    // link resolves against the dashboard's own origin. No `path` means "show
+    // the tab", leaving a mounted frame on whatever page it reached.
+    const path = typeof options.path === 'string' ? options.path : null;
     if (path) {
-      // The proxy prefix is `/webview/<cap>/`; a wildcard rides after it. In
-      // direct mode the deep link resolves against the dashboard's own origin.
       src = data.embedUrl ? `${data.embedUrl.replace(/\/?$/, '/')}${path.replace(/^\//, '')}` : new URL(path, src).href;
     }
-    this._mountWebviewFrame(id, src, data.webview || webview, { navigate: !!path });
+    this._mountWebviewFrame(id, src, data.webview || webview, { navigate: path !== null });
     this.activeWebviewId = id;
     this.hideWelcome?.();
     document.querySelector('.main')?.classList.add('webview-active');
diff --git a/src/web/server.ts b/src/web/server.ts
index b554ada5..1287b4f5 100644
--- a/src/web/server.ts
+++ b/src/web/server.ts
@@ -181,6 +181,7 @@ import {
   registerTabLayoutRoutes,
   tryWebviewRefererFallback,
 } from './routes/index.js';
+import { isLostWebviewFrameNavigation, lostWebviewFramePage, LOST_FRAME_PAGE_CSP } from './webview-proxy.js';
 import { CronService } from '../cron/cron-service.js';
 
 const __dirname = dirname(fileURLToPath(import.meta.url));
@@ -976,6 +977,15 @@ export class WebServer extends EventEmitter {
       // and the relay declines unless the Referer carries a live capability, so
       // genuinely unknown `/api` paths still get the envelope below.
       if (await tryWebviewRefererFallback(req, reply, this.basePath)) return reply;
+      // An authenticated web-tab frame (Basic auth, or trusted mode with a cookie)
+      // that navigated itself off its proxy prefix: the runtime shim masks the
+      // prefix so the page's router sees its own path, and a reload of that page
+      // lands here. The unauthenticated form is answered in the auth middleware.
+      if (!req.url.startsWith('/api') && isLostWebviewFrameNavigation(req)) {
+        reply.header('content-security-policy', LOST_FRAME_PAGE_CSP);
+        reply.header('cache-control', 'no-store');
+        return reply.type('text/html; charset=utf-8').send(lostWebviewFramePage());
+      }
       if (req.url.startsWith('/api')) {
         return reply.code(404).send(createErrorResponse(ApiErrorCode.NOT_FOUND, notFound));
       }
diff --git a/src/web/webview-proxy.ts b/src/web/webview-proxy.ts
index d94626f7..133be3a8 100644
--- a/src/web/webview-proxy.ts
+++ b/src/web/webview-proxy.ts
@@ -38,6 +38,7 @@
  * that everything else here works to preserve.
  */
 
+import { createHash } from 'node:crypto';
 import { WEBVIEW_PROXY_PREFIX } from '../config/webview-limits.js';
 import { stripBasePath } from '../config/base-path.js';
 
@@ -425,6 +426,21 @@ export function runtimeUrlShim(prefix: string): string {
   // and a throw here would break the dashboard rather than fix it.
   return `<script>(function(){try{
 var P=${JSON.stringify(prefix)};
+// Route masking. A single-page app reads location.pathname on boot and routes
+// on it; through the proxy that path starts with /webview/<cap>/, which no app
+// has a route for, so it rendered its own "page not found" the moment its
+// script ran — after the HTML and CSS had already painted. Replace the entry
+// with the path the page would see on its own origin. The base element still resolves
+// relative URLs inside the prefix, and every root-absolute sink below is
+// rewritten back into it, so only what the page READS changes. The parent
+// tab remounts the frame if the page ever navigates itself off the prefix
+// (see lostWebviewFramePage), which is what makes a masked reload survivable.
+try{
+  var L=location.pathname;
+  if(L.indexOf(P)===0&&window.history&&typeof history.replaceState==='function'){
+    history.replaceState(history.state,'',L.slice(P.length-1)+location.search+location.hash);
+  }
+}catch(e){}
 function rw(u){
   try{
     if(u==null)return u;
@@ -457,13 +473,24 @@ if(window.XMLHttpRequest&&XMLHttpRequest.prototype.open){
     var a=[].slice.call(arguments);a[1]=rw(u);return oo.apply(this,a);
   };
 }
-['WebSocket','EventSource'].forEach(function(k){
+['WebSocket','EventSource','Worker','SharedWorker'].forEach(function(k){
   var C=window[k];if(!C)return;
   function W(u,p){return p===undefined?new C(rw(u)):new C(rw(u),p);}
   W.prototype=C.prototype;
   ['CONNECTING','OPEN','CLOSING','CLOSED'].forEach(function(s){if(s in C)W[s]=C[s];});
   window[k]=W;
 });
+// With the document URL masked, a request the shim misses can no longer be
+// rescued by its Referer (that carried the prefix), so the remaining
+// URL-taking entry points are covered here rather than left to the fallback.
+if(window.navigator&&typeof navigator.sendBeacon==='function'){
+  var ob=navigator.sendBeacon;
+  navigator.sendBeacon=function(u,d){return ob.call(navigator,rw(u),d);};
+}
+if(typeof window.open==='function'){
+  var ow=window.open;
+  window.open=function(u){var a=[].slice.call(arguments);a[0]=rw(u);return ow.apply(this,a);};
+}
 var A=['src','href','action','poster','data','formaction','srcset'];
 function rwSet(v){
   try{
@@ -680,3 +707,60 @@ export function upstreamWebSocketUrl(target: URL): string {
   ws.protocol = ws.protocol === 'https:' ? 'wss:' : 'ws:';
   return ws.href;
 }
+
+// ───────────────────────── Lost-frame recovery ─────────────────────────
+
+/**
+ * The script the recovery page runs. Kept as a constant so its CSP hash below
+ * is computed from the exact bytes that are served.
+ */
+const LOST_FRAME_SCRIPT = `(function(){try{
+var path=location.pathname+location.search+location.hash;
+if(window.parent&&window.parent!==window){window.parent.postMessage({type:'codeman:webview-lost',path:path},'*');}
+}catch(e){}})();`;
+
+const LOST_FRAME_SCRIPT_HASH = createHash('sha256').update(LOST_FRAME_SCRIPT, 'utf8').digest('base64');
+
+/** CSP for the recovery page: nothing but its own hashed inline script. */
+export const LOST_FRAME_PAGE_CSP = `default-src 'none'; script-src 'sha256-${LOST_FRAME_SCRIPT_HASH}'; style-src 'unsafe-inline'`;
+
+/**
+ * Whether this request is a web-tab frame that has navigated off its proxy prefix.
+ *
+ * The runtime shim masks `/webview/<cap>/` off the document URL so a single-page
+ * app routes on the path it expects. The price is that a navigation the page
+ * starts ITSELF — `location.reload()` (a dev server's full-reload HMR), a
+ * root-absolute `location.href = '/login'` — now targets Codeman's own root with
+ * no capability anywhere on it: no prefix in the path, no cookie in an
+ * opaque-origin frame, and a Referer that names the masked page. Such a request
+ * is recognisable by shape alone: a top-level navigation of an `<iframe>`
+ * (`Sec-Fetch-Dest`), asking for HTML, for a path Codeman does not serve.
+ *
+ * The answer is `lostWebviewFramePage()`, a static page whose only content is a
+ * `postMessage` to the parent naming the path; the Codeman tab that owns the
+ * frame remounts it inside the prefix at that path. Nothing is exempted from
+ * auth by this except that static page, which carries no data.
+ */
+export function isLostWebviewFrameNavigation(req: {
+  method: string;
+  headers: Record<string, string | string[] | undefined>;
+}): boolean {
+  if (req.method !== 'GET' && req.method !== 'HEAD') return false;
+  const dest = req.headers['sec-fetch-dest'];
+  if (dest !== 'iframe' && dest !== 'frame') return false;
+  const mode = req.headers['sec-fetch-mode'];
+  if (mode !== undefined && mode !== 'navigate') return false;
+  const accept = req.headers.accept;
+  return typeof accept === 'string' && accept.includes('text/html');
+}
+
+/** The static page that hands a lost frame back to its owning tab. */
+export function lostWebviewFramePage(): string {
+  return (
+    '<!doctype html><html><head><meta charset="utf-8"><title>Reconnecting</title>' +
+    '<meta name="referrer" content="no-referrer"></head>' +
+    '<body style="margin:0;font:14px system-ui,sans-serif;color:#888;padding:16px">' +
+    'Reconnecting this web tab…' +
+    `<script>${LOST_FRAME_SCRIPT}</script></body></html>`
+  );
+}
diff --git a/test/webview-auth-exemption.test.ts b/test/webview-auth-exemption.test.ts
index 0ecdb9a5..c93a48af 100644
--- a/test/webview-auth-exemption.test.ts
+++ b/test/webview-auth-exemption.test.ts
@@ -236,3 +236,41 @@ describe('authenticated access is unaffected', () => {
     expect((await app.inject({ method: 'GET', url: '/', headers: { authorization: wrong } })).statusCode).toBe(401);
   });
 });
+
+/**
+ * A web-tab frame that navigated itself off its proxy prefix. The runtime shim
+ * masks `/webview/<cap>/` off the document URL so a single-page app routes on its
+ * own path; a reload of that page (a dev server's full-reload HMR) then targets
+ * Codeman's root with no capability, no cookie (opaque origin) and a Referer that
+ * names the masked page. It gets the static recovery page, not a login challenge,
+ * and it must not count as an auth failure.
+ */
+describe('a lost web-tab frame', () => {
+  const lostFrame = { 'sec-fetch-dest': 'iframe', 'sec-fetch-mode': 'navigate', accept: 'text/html,*/*;q=0.8' };
+
+  it('gets the recovery page instead of a 401', async () => {
+    const res = await app.inject({ method: 'GET', url: '/about?tab=2', headers: lostFrame });
+    expect(res.statusCode).toBe(200);
+    expect(res.headers['content-type']).toContain('text/html');
+    expect(res.headers['content-security-policy']).toContain("default-src 'none'");
+    expect(res.body).toContain('codeman:webview-lost');
+  });
+
+  it('never for a path Codeman actually serves, and never for a plain navigation', async () => {
+    expect((await app.inject({ method: 'GET', url: '/', headers: lostFrame })).statusCode).toBe(401);
+    expect((await app.inject({ method: 'GET', url: '/api/sessions/abc', headers: lostFrame })).statusCode).toBe(401);
+    expect((await app.inject({ method: 'GET', url: '/about' })).statusCode).toBe(401);
+    expect(
+      (await app.inject({ method: 'GET', url: '/about', headers: { ...lostFrame, 'sec-fetch-dest': 'document' } }))
+        .statusCode
+    ).toBe(401);
+  });
+
+  it('does not count against the auth failure limit', async () => {
+    for (let i = 0; i < 20; i += 1) {
+      expect((await app.inject({ method: 'GET', url: `/reload-${i}`, headers: lostFrame })).statusCode).toBe(200);
+    }
+    // A genuinely unauthenticated request afterwards is still a plain 401, not a 429.
+    expect((await app.inject({ method: 'GET', url: '/static/app.js' })).statusCode).toBe(401);
+  });
+});
diff --git a/test/webview-loopback-links.test.ts b/test/webview-loopback-links.test.ts
index 6c1376e4..82464a1e 100644
--- a/test/webview-loopback-links.test.ts
+++ b/test/webview-loopback-links.test.ts
@@ -40,6 +40,7 @@ interface AppLike {
   openWebview(id: string, options?: { path?: string }): Promise<void>;
   _apiJson(path: string, opts?: { method?: string; body?: unknown }): Promise<unknown>;
   _updateActiveWebviewTab(): void;
+  _installWebviewLostListener(): void;
   showToast?: (msg: string, kind: string) => void;
 }
 
@@ -226,3 +227,73 @@ describe('callers consult the hook first', () => {
     expect(section.indexOf('openLinkThroughWebTabIfLoopback?.(urlLink.href)')).toBeGreaterThan(urlHandler);
   });
 });
+
+/**
+ * Lost-frame recovery: the server's recovery page posts `{type, path}` to the
+ * parent; the tab that owns the frame remounts it inside the prefix at that path.
+ */
+describe('lost-frame recovery', () => {
+  const lost = (win: Window, source: unknown, path: unknown) =>
+    win.dispatchEvent(
+      new (win as unknown as { MessageEvent: typeof MessageEvent }).MessageEvent('message', {
+        data: { type: 'codeman:webview-lost', path },
+        source: source as Window,
+      })
+    );
+  const frameOf = (win: Window, id: string) =>
+    win.document.querySelector(`.webview-frame[data-webview-id="${id}"] iframe`) as HTMLIFrameElement;
+
+  it('remounts the frame that sent the message at the path it lost, inside the prefix', async () => {
+    const { win, app } = boot();
+    app._installWebviewLostListener();
+    await app.openWebview('dev');
+    const frame = frameOf(win, 'dev');
+    lost(win, frame.contentWindow, '/about?tab=2#top');
+    await vi.waitFor(() => expect(frameSrc(win, 'dev')).toBe('/webview/cap-dev/about?tab=2#top'));
+    expect(frameOf(win, 'dev')).toBe(frame);
+  });
+
+  it('recovers to the landing page for a bare reload', async () => {
+    const { win, app } = boot();
+    app._installWebviewLostListener();
+    await app.openWebview('dev');
+    await app.openUrlInWebTab('http://localhost:5173/deep');
+    expect(frameSrc(win, 'dev')).toBe('/webview/cap-dev/deep');
+    lost(win, frameOf(win, 'dev').contentWindow, '/');
+    await vi.waitFor(() => expect(frameSrc(win, 'dev')).toBe('/webview/cap-dev/'));
+  });
+
+  it('ignores a message that did not come from one of its frames, or is malformed', async () => {
+    const { win, app, calls } = boot();
+    app._installWebviewLostListener();
+    await app.openWebview('dev');
+    const before = calls.length;
+    lost(win, win, '/elsewhere');
+    lost(win, frameOf(win, 'dev').contentWindow, 42);
+    win.dispatchEvent(
+      new (win as unknown as { MessageEvent: typeof MessageEvent }).MessageEvent('message', {
+        data: 'codeman:webview-lost',
+        source: frameOf(win, 'dev').contentWindow as Window,
+      })
+    );
+    await new Promise((r) => setTimeout(r, 20));
+    expect(calls.length).toBe(before);
+    expect(frameSrc(win, 'dev')).toBe('/webview/cap-dev/');
+  });
+
+  it('never lets the path jump the frame off the proxy, and bounds a reload loop', async () => {
+    const { win, app, calls } = boot();
+    app._installWebviewLostListener();
+    await app.openWebview('dev');
+    lost(win, frameOf(win, 'dev').contentWindow, '//evil.example/x');
+    await vi.waitFor(() => expect(calls.filter((c) => c.path.endsWith('/open')).length).toBe(2));
+    expect(frameSrc(win, 'dev')).toBe('/webview/cap-dev/');
+
+    const opensBefore = calls.filter((c) => c.path.endsWith('/open')).length;
+    for (let i = 0; i < 10; i += 1) lost(win, frameOf(win, 'dev').contentWindow, `/spin-${i}`);
+    await new Promise((r) => setTimeout(r, 50));
+    const opens = calls.filter((c) => c.path.endsWith('/open')).length - opensBefore;
+    expect(opens).toBeLessThanOrEqual(5);
+    expect(opens).toBeGreaterThan(0);
+  });
+});
diff --git a/test/webview-proxy.test.ts b/test/webview-proxy.test.ts
index e4b4c52f..b27e9ffc 100644
--- a/test/webview-proxy.test.ts
+++ b/test/webview-proxy.test.ts
@@ -27,6 +27,9 @@ import {
   runtimeUrlShim,
   stripFrameAncestors,
   upstreamWebSocketUrl,
+  isLostWebviewFrameNavigation,
+  lostWebviewFramePage,
+  LOST_FRAME_PAGE_CSP,
 } from '../src/web/webview-proxy.js';
 
 const CAP = 'A'.repeat(32);
@@ -720,3 +723,97 @@ describe('reverse-proxy base path', () => {
     expect(capabilityFromReferer(`https://box.ts.net${BASED_PREFIX}page`)).toBeNull();
   });
 });
+
+/**
+ * Route masking. A single-page app routes on `location.pathname` at boot; through
+ * the proxy that path starts with `/webview/<cap>/`, which no app has a route for,
+ * so it rendered its own "page not found" the moment its script ran (measured
+ * against a minimal history-routed page: HTML and CSS painted, then the router
+ * replaced them). The shim rewrites the history entry to the path the page would
+ * see on its own origin, before any page script runs.
+ */
+describe('runtimeUrlShim route masking', () => {
+  const body = runtimeUrlShim(PREFIX)
+    .replace(/^<script>/, '')
+    .replace(/<\/script>$/, '');
+
+  function boot(url: string) {
+    const dom = new JSDOM('<!doctype html><html><head></head><body></body></html>', {
+      url,
+      runScripts: 'outside-only',
+    });
+    dom.window.eval(body);
+    return dom.window;
+  }
+
+  it('masks the proxy prefix off the document URL so the router sees its own path', () => {
+    const win = boot(`https://codeman.local${PREFIX}pages/report?tab=2#top`);
+    expect(win.location.pathname).toBe('/pages/report');
+    expect(win.location.search).toBe('?tab=2');
+    expect(win.location.hash).toBe('#top');
+  });
+
+  it('maps the landing page to /', () => {
+    expect(boot(`https://codeman.local${PREFIX}`).location.pathname).toBe('/');
+  });
+
+  it('keeps rewriting root-absolute URLs into the prefix after masking', () => {
+    const win = boot(`https://codeman.local${PREFIX}`);
+    const calls: string[] = [];
+    // Patched fetch is a closure over the ORIGINAL window.fetch; jsdom has none,
+    // so exercise the same rewrite through an <a> href, which is a patched sink.
+    const a = win.document.createElement('a');
+    a.href = '/api/data';
+    calls.push(a.getAttribute('href') ?? '');
+    expect(calls).toEqual([`${PREFIX}api/data`]);
+  });
+
+  it('leaves a document that is not under the prefix alone', () => {
+    const win = boot('https://codeman.local/somewhere/else');
+    expect(win.location.pathname).toBe('/somewhere/else');
+  });
+
+  it('patches window.open so a popup stays inside the prefix', () => {
+    const win = boot(`https://codeman.local${PREFIX}`);
+    const opened: string[] = [];
+    // The shim wrapped the original; swap the original out from under it by
+    // re-running against a stub is not possible, so verify the wrapper shape.
+    const original = win.open;
+    expect(typeof original).toBe('function');
+    Object.defineProperty(win, 'open', { value: (u: string) => opened.push(u), configurable: true });
+    // A second shim run wraps the stub (the __cmrw guard applies to setters, not
+    // to window.open, which has no marker of its own to preserve).
+    win.eval(body);
+    win.open('/print');
+    expect(opened).toEqual([`${PREFIX}print`]);
+  });
+});
+
+describe('lost-frame recovery', () => {
+  const nav = (headers: Record<string, string>, method = 'GET') => isLostWebviewFrameNavigation({ method, headers });
+
+  it('recognises a top-level iframe navigation asking for HTML', () => {
+    expect(nav({ 'sec-fetch-dest': 'iframe', 'sec-fetch-mode': 'navigate', accept: 'text/html,*/*' })).toBe(true);
+    expect(nav({ 'sec-fetch-dest': 'frame', accept: 'text/html' })).toBe(true);
+  });
+
+  it('refuses subresource fetches, non-HTML accepts, and writes', () => {
+    expect(nav({ 'sec-fetch-dest': 'script', 'sec-fetch-mode': 'no-cors', accept: '*/*' })).toBe(false);
+    expect(nav({ 'sec-fetch-dest': 'iframe', 'sec-fetch-mode': 'cors', accept: 'text/html' })).toBe(false);
+    expect(nav({ 'sec-fetch-dest': 'iframe', accept: 'application/json' })).toBe(false);
+    expect(nav({ accept: 'text/html' })).toBe(false);
+    expect(nav({ 'sec-fetch-dest': 'iframe', accept: 'text/html' }, 'POST')).toBe(false);
+  });
+
+  it('serves a page whose only script posts the lost path to the parent, and hashes it in the CSP', async () => {
+    const page = lostWebviewFramePage();
+    const script = /<script>([\s\S]*?)<\/script>/.exec(page)?.[1] ?? '';
+    expect(script).toContain("postMessage({type:'codeman:webview-lost',path:path},'*')");
+    const { createHash } = await import('node:crypto');
+    const hash = createHash('sha256').update(script, 'utf8').digest('base64');
+    expect(LOST_FRAME_PAGE_CSP).toContain(`'sha256-${hash}'`);
+    expect(LOST_FRAME_PAGE_CSP).toContain("default-src 'none'");
+    // The page must never carry a Referer that would leak anything about the tab.
+    expect(page).toContain('name="referrer" content="no-referrer"');
+  });
+});