diff --git a/changelog.d/150.fixed.md b/changelog.d/150.fixed.md
new file mode 100644
index 0000000..7cb3d2d
--- /dev/null
+++ b/changelog.d/150.fixed.md
@@ -0,0 +1 @@
+Prompter popout windows now build same-origin `mode=prompter` URLs with the platform URL parser and isolate the opened window from `window.opener`.
diff --git a/src/system/presenter/prompter-window.ts b/src/system/presenter/prompter-window.ts
index 4016a7d..0e89714 100644
--- a/src/system/presenter/prompter-window.ts
+++ b/src/system/presenter/prompter-window.ts
@@ -12,10 +12,45 @@
 
 import type { PrompterRenderer, PrompterScript } from '../../runtime/prompter';
 
-const buildPrompterUrl = (baseUrl: string): string => {
-  if (baseUrl.includes('mode=')) return baseUrl.replace(/mode=[^&]*/, 'mode=prompter');
-  if (baseUrl.includes('?')) return `${baseUrl}&mode=prompter`;
-  return `${baseUrl}?mode=prompter`;
+const DEFAULT_PROMPTER_WINDOW_FEATURES = 'width=900,height=700,menubar=no,toolbar=no';
+const POPUP_ISOLATION_FEATURES = ['noopener', 'noreferrer'] as const;
+
+const buildPrompterUrl = (
+  baseUrl: string,
+  location: Pick<Location, 'href' | 'origin'>,
+): string | null => {
+  let url: URL;
+  try {
+    url = new URL(baseUrl, location.href);
+  } catch {
+    return null;
+  }
+
+  if (url.origin !== location.origin) return null;
+  url.searchParams.set('mode', 'prompter');
+  return url.href;
+};
+
+const withPopupIsolationFeatures = (features: string): string => {
+  const tokens = features
+    .split(',')
+    .map((feature) => feature.trim())
+    .filter((feature) => feature.length > 0);
+  const present = new Set(tokens.map((feature) => feature.toLowerCase()));
+  for (const feature of POPUP_ISOLATION_FEATURES) {
+    if (!present.has(feature)) tokens.push(feature);
+  }
+  return tokens.join(',');
+};
+
+const isolateOpenedWindow = (opened: Window | null): Window | null => {
+  if (opened === null) return null;
+  try {
+    opened.opener = null;
+  } catch {
+    // Some browsers return a WindowProxy that rejects opener mutation.
+  }
+  return opened;
 };
 
 /**
@@ -26,10 +61,16 @@ const buildPrompterUrl = (baseUrl: string): string => {
  */
 export const openPrompterWindow = (
   baseUrl: string,
-  features = 'width=900,height=700,menubar=no,toolbar=no',
+  features = DEFAULT_PROMPTER_WINDOW_FEATURES,
 ): Window | null => {
-  const url = buildPrompterUrl(baseUrl);
-  return globalThis.window?.open(url, '_blank', features) ?? null;
+  const win = globalThis.window;
+  if (win === undefined) return null;
+
+  const url = buildPrompterUrl(baseUrl, win.location);
+  if (url === null) return null;
+
+  const opened = win.open(url, '_blank', withPopupIsolationFeatures(features));
+  return isolateOpenedWindow(opened);
 };
 
 /**
diff --git a/tests/system/prompter-window-url.test.ts b/tests/system/prompter-window-url.test.ts
index 1417a0f..2d03646 100644
--- a/tests/system/prompter-window-url.test.ts
+++ b/tests/system/prompter-window-url.test.ts
@@ -7,16 +7,21 @@ describe('openPrompterWindow', () => {
   const realOpen = globalThis.window?.open ?? null;
   const originalWindow = globalThis.window;
   let opened: Array<{ url: string; target: string; features: string }> = [];
+  let openedWindow: Window & { opener: unknown };
 
   beforeEach(() => {
     opened = [];
+    openedWindow = { closed: false, opener: { source: 'presenter' } } as Window & {
+      opener: unknown;
+    };
     // Stub `window.open`. Vitest's default Node env has no window;
     // attach one minimally for this test.
     Object.defineProperty(globalThis, 'window', {
       value: {
+        location: new URL('https://pulsar.test/workbench/current?composition=current'),
         open: (url: string, target: string, features: string) => {
           opened.push({ url, target, features });
-          return { closed: false } as unknown as Window;
+          return openedWindow;
         },
       },
       configurable: true,
@@ -37,21 +42,61 @@ describe('openPrompterWindow', () => {
 
   it('replaces an existing mode= param', () => {
     openPrompterWindow('/?composition=demo&mode=present');
-    expect(opened[0]?.url).toBe('/?composition=demo&mode=prompter');
+    expect(opened[0]?.url).toBe('https://pulsar.test/?composition=demo&mode=prompter');
   });
 
   it('appends mode=prompter to a URL with other params', () => {
     openPrompterWindow('/?composition=demo');
-    expect(opened[0]?.url).toBe('/?composition=demo&mode=prompter');
+    expect(opened[0]?.url).toBe('https://pulsar.test/?composition=demo&mode=prompter');
   });
 
   it('uses ?mode=prompter for a URL with no query', () => {
     openPrompterWindow('/');
-    expect(opened[0]?.url).toBe('/?mode=prompter');
+    expect(opened[0]?.url).toBe('https://pulsar.test/?mode=prompter');
+  });
+
+  it('does not rewrite query values containing mode=', () => {
+    openPrompterWindow('/?composition=demo&note=has-mode=inside');
+    expect(opened[0]?.url).toBe(
+      'https://pulsar.test/?composition=demo&note=has-mode%3Dinside&mode=prompter',
+    );
+  });
+
+  it('preserves hashes after the prompter mode query parameter', () => {
+    openPrompterWindow('/deck?composition=demo#speaker-notes');
+    expect(opened[0]?.url).toBe(
+      'https://pulsar.test/deck?composition=demo&mode=prompter#speaker-notes',
+    );
+  });
+
+  it('resolves relative URLs against the current window location', () => {
+    openPrompterWindow('./notes?scene=intro');
+    expect(opened[0]?.url).toBe('https://pulsar.test/workbench/notes?scene=intro&mode=prompter');
+  });
+
+  it('accepts absolute same-origin URLs', () => {
+    openPrompterWindow('https://pulsar.test/presenter?composition=demo&mode=loop#notes');
+    expect(opened[0]?.url).toBe(
+      'https://pulsar.test/presenter?composition=demo&mode=prompter#notes',
+    );
+  });
+
+  it('does not open cross-origin URLs', () => {
+    const result = openPrompterWindow('https://external.example/?composition=demo');
+    expect(result).toBeNull();
+    expect(opened).toEqual([]);
   });
 
   it('forwards custom features arg', () => {
     openPrompterWindow('/', 'width=400');
-    expect(opened[0]?.features).toBe('width=400');
+    const features = opened[0]?.features.split(',').map((feature) => feature.trim());
+    expect(features).toEqual(expect.arrayContaining(['width=400', 'noopener', 'noreferrer']));
+  });
+
+  it('isolates the opened window from the opener', () => {
+    const result = openPrompterWindow('/');
+    expect(result).toBe(openedWindow);
+    expect(opened[0]?.target).toBe('_blank');
+    expect(openedWindow.opener).toBeNull();
   });
 });