diff --git a/packages/@n8n/mcp-browser-extension/src/documentPreparation.script.test.ts b/packages/@n8n/mcp-browser-extension/src/documentPreparation.script.test.ts
new file mode 100644
index 00000000000..6708e87c538
--- /dev/null
+++ b/packages/@n8n/mcp-browser-extension/src/documentPreparation.script.test.ts
@@ -0,0 +1,181 @@
+import { applyOptOutAttributes, OPT_OUT_SCRIPT } from './documentPreparation';
+
+const run = applyOptOutAttributes;
+
+function html(markup: string) {
+	document.body.innerHTML = markup;
+}
+
+beforeEach(() => {
+	Reflect.deleteProperty(window, '__n8nExtensionOptOut');
+	document.body.innerHTML = '';
+	document.body.removeAttribute('data-1p-ignore');
+});
+
+describe('OPT_OUT_SCRIPT', () => {
+	it('ships the function as a self-invoking source string', () => {
+		expect(OPT_OUT_SCRIPT.startsWith('(function')).toBe(true);
+		expect(OPT_OUT_SCRIPT.endsWith(')()')).toBe(true);
+		expect(OPT_OUT_SCRIPT).toContain('data-bwignore');
+	});
+
+	it('marks every form field with the password manager opt-out attributes', () => {
+		html('<input id="a"><textarea id="b"></textarea><select id="c"></select>');
+
+		run();
+
+		for (const id of ['a', 'b', 'c']) {
+			const field = document.getElementById(id);
+			expect(field?.hasAttribute('data-1p-ignore')).toBe(true);
+			expect(field?.getAttribute('data-lpignore')).toBe('true');
+			expect(field?.hasAttribute('data-bwignore')).toBe(true);
+			expect(field?.hasAttribute('data-protonpass-ignore')).toBe(true);
+			expect(field?.getAttribute('data-form-type')).toBe('other');
+		}
+	});
+
+	it('marks the body with the 1Password whole-page switch only', () => {
+		html('<input>');
+
+		run();
+
+		expect(document.body.hasAttribute('data-1p-ignore')).toBe(true);
+		expect(document.body.hasAttribute('data-lpignore')).toBe(false);
+		expect(document.body.hasAttribute('data-form-type')).toBe(false);
+	});
+
+	it('marks editable text with the writing assistant opt-out attributes', () => {
+		html('<textarea id="a"></textarea><div id="b" contenteditable></div>');
+
+		run();
+
+		for (const id of ['a', 'b']) {
+			const field = document.getElementById(id);
+			expect(field?.getAttribute('data-gramm')).toBe('false');
+			expect(field?.getAttribute('data-gramm_editor')).toBe('false');
+			expect(field?.getAttribute('data-enable-grammarly')).toBe('false');
+			expect(field?.getAttribute('data-lt-active')).toBe('false');
+		}
+	});
+
+	it('leaves plain inputs out of the writing assistant group', () => {
+		html('<input id="a">');
+
+		run();
+
+		expect(document.getElementById('a')?.hasAttribute('data-gramm')).toBe(false);
+	});
+
+	it('skips explicitly non-editable contenteditable elements', () => {
+		html('<div id="a" contenteditable="false"></div>');
+
+		run();
+
+		expect(document.getElementById('a')?.hasAttribute('data-gramm')).toBe(false);
+	});
+
+	it('leaves attributes the page already set untouched', () => {
+		html('<input id="a" data-form-type="password">');
+
+		run();
+
+		expect(document.getElementById('a')?.getAttribute('data-form-type')).toBe('password');
+	});
+
+	it('reaches fields inside open shadow roots', () => {
+		html('<div id="host"></div>');
+		const shadow = document.getElementById('host')!.attachShadow({ mode: 'open' });
+		shadow.innerHTML = '<input id="inner">';
+
+		run();
+
+		expect(shadow.getElementById('inner')?.hasAttribute('data-1p-ignore')).toBe(true);
+	});
+
+	it('does not throw on a page with no fields', () => {
+		html('<p>nothing here</p>');
+
+		expect(() => run()).not.toThrow();
+	});
+
+	it('marks a field inserted after it ran', async () => {
+		html('<div id="app"></div>');
+		run();
+
+		const input = document.createElement('input');
+		document.getElementById('app')!.appendChild(input);
+
+		await vi.waitFor(() => expect(input.hasAttribute('data-bwignore')).toBe(true));
+	});
+
+	it('marks a field nested inside an inserted subtree', async () => {
+		html('<div id="app"></div>');
+		run();
+
+		const modal = document.createElement('div');
+		modal.innerHTML = '<form><input id="nested"></form>';
+		document.getElementById('app')!.appendChild(modal);
+
+		await vi.waitFor(() =>
+			expect(document.getElementById('nested')?.hasAttribute('data-bwignore')).toBe(true),
+		);
+	});
+
+	it('marks a field inserted into a shadow root it has already walked', async () => {
+		html('<div id="host"></div>');
+		const shadow = document.getElementById('host')!.attachShadow({ mode: 'open' });
+		run();
+
+		const input = document.createElement('input');
+		shadow.appendChild(input);
+
+		await vi.waitFor(() => expect(input.hasAttribute('data-bwignore')).toBe(true));
+	});
+
+	it('installs one observer per root however often it runs', () => {
+		html('<div id="app"></div>');
+		let observers = 0;
+		const Native = window.MutationObserver;
+		class Counting extends Native {
+			constructor(onMutation: MutationCallback) {
+				super(onMutation);
+				observers++;
+			}
+		}
+		vi.stubGlobal('MutationObserver', Counting);
+
+		run();
+		run();
+		run();
+
+		expect(observers).toBe(1);
+		vi.unstubAllGlobals();
+	});
+
+	// As an init script this runs before the parser has created <html>, let alone <body>, so the
+	// empty document is the real entry state — not the populated one the cases above start from.
+	it('marks a body and field parsed after it ran against an empty document', async () => {
+		const documentElement = document.documentElement;
+		documentElement.remove();
+
+		expect(() => run()).not.toThrow();
+
+		const root = document.createElement('html');
+		const body = document.createElement('body');
+		const input = document.createElement('input');
+		body.appendChild(input);
+		root.appendChild(body);
+		document.appendChild(root);
+
+		try {
+			await vi.waitFor(() => {
+				expect(input.hasAttribute('data-bwignore')).toBe(true);
+				expect(body.hasAttribute('data-1p-ignore')).toBe(true);
+			});
+		} finally {
+			// jsdom is per file, not per test, so every later beforeEach needs a document back.
+			document.documentElement?.remove();
+			document.appendChild(documentElement);
+		}
+	});
+});
diff --git a/packages/@n8n/mcp-browser-extension/src/documentPreparation.test.ts b/packages/@n8n/mcp-browser-extension/src/documentPreparation.test.ts
new file mode 100644
index 00000000000..9a25c9a2cb8
--- /dev/null
+++ b/packages/@n8n/mcp-browser-extension/src/documentPreparation.test.ts
@@ -0,0 +1,108 @@
+import { DocumentPreparation, OPT_OUT_SCRIPT } from './documentPreparation';
+
+const { warn, debug } = vi.hoisted(() => ({ warn: vi.fn(), debug: vi.fn() }));
+
+vi.mock('./logger', () => ({
+	createLogger: () => ({ warn, debug, info: vi.fn(), error: vi.fn() }),
+}));
+
+const sendCommand = vi.fn();
+
+beforeEach(() => {
+	sendCommand.mockReset().mockResolvedValue({});
+	warn.mockReset();
+	debug.mockReset();
+	vi.stubGlobal('chrome', { debugger: { sendCommand } });
+});
+
+afterEach(() => {
+	vi.unstubAllGlobals();
+});
+
+function callsTo(method: string) {
+	return sendCommand.mock.calls.filter((call) => call[1] === method);
+}
+
+describe('DocumentPreparation.applyToTab', () => {
+	it('registers the script for every document the tab opens from now on', async () => {
+		await new DocumentPreparation().applyToTab(7);
+
+		expect(sendCommand).toHaveBeenCalledWith(
+			{ tabId: 7 },
+			'Page.addScriptToEvaluateOnNewDocument',
+			{ source: OPT_OUT_SCRIPT },
+		);
+	});
+
+	it('marks the document that already exists', async () => {
+		await new DocumentPreparation().applyToTab(7);
+
+		expect(sendCommand).toHaveBeenCalledWith(
+			{ tabId: 7 },
+			'Runtime.evaluate',
+			expect.objectContaining({ expression: OPT_OUT_SCRIPT }),
+		);
+	});
+
+	it('does not touch the Runtime domain state the client owns', async () => {
+		await new DocumentPreparation().applyToTab(7);
+
+		// A second Runtime.enable reports no contexts, which would starve the client's own wait.
+		expect(callsTo('Runtime.enable')).toHaveLength(0);
+		expect(callsTo('Runtime.disable')).toHaveLength(0);
+	});
+
+	it('still marks the current document when registration fails', async () => {
+		sendCommand.mockImplementation(async (_d: unknown, method: string) => {
+			if (method === 'Page.addScriptToEvaluateOnNewDocument') throw new Error('detached');
+			return await Promise.resolve({});
+		});
+
+		await new DocumentPreparation().applyToTab(7);
+
+		expect(callsTo('Runtime.evaluate')).toHaveLength(1);
+	});
+
+	it('does not reject when the tab is gone', async () => {
+		sendCommand.mockRejectedValue(new Error('No tab with given id'));
+
+		await expect(new DocumentPreparation().applyToTab(7)).resolves.toBeUndefined();
+	});
+
+	it('gives up on a renderer that never answers, so attach is not held up', async () => {
+		vi.useFakeTimers();
+		try {
+			sendCommand.mockImplementation(async (_d: unknown, method: string) =>
+				method === 'Runtime.evaluate' ? await new Promise(() => {}) : await Promise.resolve({}),
+			);
+
+			const applied = new DocumentPreparation().applyToTab(7);
+			await vi.advanceTimersByTimeAsync(10_000);
+
+			await expect(applied).resolves.toBeUndefined();
+		} finally {
+			vi.useRealTimers();
+		}
+	});
+
+	it('reports a page that rejected the script instead of swallowing it', async () => {
+		sendCommand.mockImplementation(async (_d: unknown, method: string) =>
+			method === 'Runtime.evaluate'
+				? await Promise.resolve({ exceptionDetails: { text: 'blocked by policy' } })
+				: await Promise.resolve({}),
+		);
+
+		await new DocumentPreparation().applyToTab(7);
+
+		expect(warn).toHaveBeenCalledWith(expect.stringContaining('rejected'), 'blocked by policy');
+	});
+
+	it('keeps an expected race quiet', async () => {
+		sendCommand.mockRejectedValue(new Error('No tab with given id'));
+
+		await new DocumentPreparation().applyToTab(7);
+
+		expect(warn).not.toHaveBeenCalled();
+		expect(debug).toHaveBeenCalled();
+	});
+});
diff --git a/packages/@n8n/mcp-browser-extension/src/documentPreparation.ts b/packages/@n8n/mcp-browser-extension/src/documentPreparation.ts
new file mode 100644
index 00000000000..a3cf24c7860
--- /dev/null
+++ b/packages/@n8n/mcp-browser-extension/src/documentPreparation.ts
@@ -0,0 +1,140 @@
+// Lives here rather than server-side so a renamed vendor attribute ships on the extension's
+// cadence instead of waiting for an instance upgrade.
+
+import { createLogger } from './logger';
+
+const log = createLogger('document-prep');
+
+const EVALUATE_TIMEOUT_MS = 5_000;
+
+/**
+ * Runs in the page context, once per document. Not covered: a field that only becomes eligible
+ * via a later attribute change, a shadow root attached after its host was observed, and child
+ * frames of a document that was already loaded when we attached.
+ */
+export function applyOptOutAttributes(): void {
+	const groups: Array<{ selector: string; attributes: Array<[string, string]> }> = [
+		{
+			// 1Password honors this at <body> as a whole-page switch; the rest are per-field.
+			selector: 'body',
+			attributes: [['data-1p-ignore', '']],
+		},
+		{
+			selector: 'input, textarea, select',
+			attributes: [
+				['data-1p-ignore', ''], // 1Password
+				['data-lpignore', 'true'], // LastPass
+				['data-bwignore', ''], // Bitwarden
+				['data-protonpass-ignore', ''], // Proton Pass
+				['data-form-type', 'other'], // Dashlane
+			],
+		},
+		{
+			selector: 'textarea, [contenteditable]:not([contenteditable="false"])',
+			attributes: [
+				['data-gramm', 'false'], // Grammarly
+				['data-gramm_editor', 'false'], // Grammarly, legacy
+				['data-enable-grammarly', 'false'], // Grammarly, current
+				['data-lt-active', 'false'], // LanguageTool
+			],
+		},
+	];
+
+	function isElement(node: Node): node is Element {
+		return node.nodeType === 1;
+	}
+
+	function isRegistry(value: unknown): value is WeakSet<Node> {
+		return value instanceof WeakSet;
+	}
+
+	// Keyed by root, so a document nobody has observed yet still gets an observer.
+	const REGISTRY = '__n8nExtensionOptOut';
+	const existing: unknown = Reflect.get(window, REGISTRY);
+	const observed = isRegistry(existing) ? existing : new WeakSet<Node>();
+	if (!isRegistry(existing)) Reflect.set(window, REGISTRY, observed);
+
+	function setAttrs(element: Element, attributes: Array<[string, string]>): void {
+		for (const [name, value] of attributes) {
+			if (!element.hasAttribute(name)) element.setAttribute(name, value);
+		}
+	}
+
+	function mark(root: Document | ShadowRoot | Element): void {
+		for (const { selector, attributes } of groups) {
+			if (isElement(root) && root.matches(selector)) setAttrs(root, attributes);
+			for (const element of root.querySelectorAll(selector)) setAttrs(element, attributes);
+		}
+
+		// Never reach into a frame from here: measured, it stops that frame's own run from marking
+		// anything, and every frame runs this script itself.
+		if (isElement(root)) descendShadow(root);
+		for (const element of root.querySelectorAll('*')) descendShadow(element);
+	}
+
+	function descendShadow(element: Element): void {
+		if (!element.shadowRoot) return;
+		mark(element.shadowRoot);
+		observe(element.shadowRoot);
+	}
+
+	function observe(root: Document | ShadowRoot): void {
+		if (observed.has(root)) return;
+		observed.add(root);
+		const observer = new MutationObserver((records) => {
+			for (const record of records) {
+				for (const node of record.addedNodes) {
+					if (isElement(node)) mark(node);
+				}
+			}
+		});
+		observer.observe(root, { childList: true, subtree: true });
+	}
+
+	mark(document);
+	observe(document);
+}
+
+export const OPT_OUT_SCRIPT = `(${applyOptOutAttributes.toString()})()`;
+
+export class DocumentPreparation {
+	/** Covers every document the tab opens from now on, in every frame, plus the current one. */
+	async applyToTab(chromeTabId: number): Promise<void> {
+		const debuggee = { tabId: chromeTabId };
+		try {
+			await chrome.debugger.sendCommand(debuggee, 'Page.addScriptToEvaluateOnNewDocument', {
+				source: OPT_OUT_SCRIPT,
+			});
+		} catch (e) {
+			log.debug(`could not register for tab ${chromeTabId}:`, e);
+		}
+
+		// Bounded: awaited during attach, so a renderer blocked on a dialog must not stall it.
+		let timer: ReturnType<typeof setTimeout> | undefined;
+		const deadline = new Promise<void>((resolve) => {
+			timer = setTimeout(() => {
+				log.debug(`evaluate timed out for tab ${chromeTabId}`);
+				resolve();
+			}, EVALUATE_TIMEOUT_MS);
+		});
+		try {
+			await Promise.race([this.markCurrentDocument(debuggee), deadline]);
+		} finally {
+			clearTimeout(timer);
+		}
+	}
+
+	private async markCurrentDocument(debuggee: chrome.debugger.Debuggee): Promise<void> {
+		try {
+			const result = (await chrome.debugger.sendCommand(debuggee, 'Runtime.evaluate', {
+				expression: OPT_OUT_SCRIPT,
+				awaitPromise: false,
+			})) as { exceptionDetails?: { text?: string } } | undefined;
+			if (result?.exceptionDetails) {
+				log.warn('page rejected the opt-out:', result.exceptionDetails.text);
+			}
+		} catch (e) {
+			log.debug('evaluate failed:', e);
+		}
+	}
+}
diff --git a/packages/@n8n/mcp-browser-extension/src/relayConnection.test.ts b/packages/@n8n/mcp-browser-extension/src/relayConnection.test.ts
index b97ad5e6185..c41aeb17dd6 100644
--- a/packages/@n8n/mcp-browser-extension/src/relayConnection.test.ts
+++ b/packages/@n8n/mcp-browser-extension/src/relayConnection.test.ts
@@ -1126,6 +1126,41 @@ describe('RelayConnection', () => {
 			expect(ws.closeReason).toBe('extension_disconnected');
 		});
 	});
+	describe('document preparation', () => {
+		/** Both attach paths must prepare the tab; only one of them used to. */
+		function preparedTabs() {
+			return chrome.debugger.sendCommand.mock.calls
+				.filter((call) => call[1] === 'Page.addScriptToEvaluateOnNewDocument')
+				.map((call) => (call[0] as { tabId: number }).tabId);
+		}
+
+		it('prepares a lazily attached tab', async () => {
+			chrome.debugger.getTargets.mockResolvedValueOnce([mockTarget(31)]);
+			await relay.registerSelectedTabs([31]);
+
+			ws.onmessage?.({
+				data: JSON.stringify({
+					id: 1,
+					method: 'forwardCDPCommand',
+					params: { method: 'Page.reload' },
+				}),
+			});
+			await tick();
+
+			expect(preparedTabs()).toContain(31);
+		});
+
+		it('prepares an eagerly attached agent-created tab', async () => {
+			chrome.tabs.create.mockResolvedValueOnce({ id: 32, title: 'New', url: 'https://new.com' });
+
+			ws.onmessage?.({
+				data: JSON.stringify({ id: 1, method: 'createTab', params: { url: 'https://new.com' } }),
+			});
+			await tick();
+
+			expect(preparedTabs()).toContain(32);
+		});
+	});
 });
 
 describe('RelayConnection keepalive', () => {
diff --git a/packages/@n8n/mcp-browser-extension/src/relayConnection.ts b/packages/@n8n/mcp-browser-extension/src/relayConnection.ts
index a34097e6d05..283151a6c0a 100644
--- a/packages/@n8n/mcp-browser-extension/src/relayConnection.ts
+++ b/packages/@n8n/mcp-browser-extension/src/relayConnection.ts
@@ -7,6 +7,7 @@
  * All communication with the relay uses these CDP target IDs.
  */
 
+import { DocumentPreparation } from './documentPreparation';
 import { ForeignFrames } from './foreignFrames';
 import { createLogger } from './logger';
 import type { TabManagementSettings } from './types';
@@ -92,6 +93,7 @@ export class RelayConnection {
 		params?: object,
 	) => void;
 	private readonly detachListener: (source: chrome.debugger.Debuggee, reason: string) => void;
+	private readonly documentPrep = new DocumentPreparation();
 	private closed = false;
 	private keepaliveInterval: ReturnType<typeof setInterval> | undefined;
 	private settings: TabManagementSettings = DEFAULT_SETTINGS;
@@ -313,6 +315,7 @@ export class RelayConnection {
 		)) as { targetInfo: { targetId: string } };
 
 		await this.applyFocusEmulation(chromeTabId);
+		await this.documentPrep.applyToTab(chromeTabId);
 
 		const targetId = result.targetInfo.targetId;
 		log.debug(`attached: chromeTabId=${chromeTabId} → targetId=${targetId}`);
@@ -368,6 +371,7 @@ export class RelayConnection {
 			log.debug(`ensureAttached: attached ${id}`);
 
 			await this.applyFocusEmulation(entry.chromeTabId);
+			await this.documentPrep.applyToTab(entry.chromeTabId);
 
 			// Reapply cached auto-attach so new tabs report iframes immediately
 			if (this.autoAttachParams) {