diff --git a/frontend/src/lib/utils/deepLinkScroll.spec.ts b/frontend/src/lib/utils/deepLinkScroll.spec.ts
new file mode 100644
index 00000000..20f0cf19
--- /dev/null
+++ b/frontend/src/lib/utils/deepLinkScroll.spec.ts
@@ -0,0 +1,129 @@
+import { describe, expect, it, vi } from 'vitest';
+import { scrollToCommentFromQuery, type DeepLinkScrollOptions } from './deepLinkScroll';
+
+const COMMENT_ID = 'cccc1111-1111-1111-1111-111111111111';
+const ANNOTATION_ID = 'aaaa2222-2222-2222-2222-222222222222';
+
+function fakeElement() {
+	return {
+		scrollIntoView: vi.fn(),
+		focus: vi.fn()
+	} as unknown as HTMLElement;
+}
+
+type Overrides = Partial<DeepLinkScrollOptions>;
+
+function buildOpts(overrides: Overrides = {}): DeepLinkScrollOptions {
+	const el = overrides.getElement ? null : fakeElement();
+	return {
+		transcribeMode: true,
+		setTranscribeMode: vi.fn(),
+		loadBlocks: vi.fn().mockResolvedValue(undefined),
+		setActiveAnnotationId: vi.fn(),
+		flashAnnotation: vi.fn(),
+		prefersReducedMotion: false,
+		afterTick: vi.fn().mockResolvedValue(undefined),
+		getElement: vi.fn().mockReturnValue(el),
+		onStripUrl: vi.fn(),
+		...overrides
+	};
+}
+
+describe('scrollToCommentFromQuery', () => {
+	it('is a no-op when commentId query param is absent', async () => {
+		const url = new URL('https://app/documents/doc-1');
+		const opts = buildOpts();
+
+		await scrollToCommentFromQuery(url, opts);
+
+		expect(opts.setActiveAnnotationId).not.toHaveBeenCalled();
+		expect(opts.getElement).not.toHaveBeenCalled();
+		expect(opts.onStripUrl).not.toHaveBeenCalled();
+	});
+
+	it('is a no-op when annotationId query param is absent even if commentId is present', async () => {
+		const url = new URL(`https://app/documents/doc-1?commentId=${COMMENT_ID}`);
+		const opts = buildOpts();
+
+		await scrollToCommentFromQuery(url, opts);
+
+		expect(opts.setActiveAnnotationId).not.toHaveBeenCalled();
+		expect(opts.getElement).not.toHaveBeenCalled();
+	});
+
+	it('scrolls to the comment element and focuses it when both params are present', async () => {
+		const url = new URL(
+			`https://app/documents/doc-1?commentId=${COMMENT_ID}&annotationId=${ANNOTATION_ID}`
+		);
+		const el = fakeElement();
+		const opts = buildOpts({ getElement: vi.fn().mockReturnValue(el) });
+
+		await scrollToCommentFromQuery(url, opts);
+
+		expect(opts.getElement).toHaveBeenCalledWith(`comment-${COMMENT_ID}`);
+		expect(el.scrollIntoView).toHaveBeenCalledWith({ behavior: 'smooth', block: 'center' });
+		expect(el.focus).toHaveBeenCalledWith({ preventScroll: true });
+	});
+
+	it('triggers the annotation flash after scrolling', async () => {
+		const url = new URL(
+			`https://app/documents/doc-1?commentId=${COMMENT_ID}&annotationId=${ANNOTATION_ID}`
+		);
+		const el = fakeElement();
+		const opts = buildOpts({ getElement: vi.fn().mockReturnValue(el) });
+
+		await scrollToCommentFromQuery(url, opts);
+
+		expect(opts.flashAnnotation).toHaveBeenCalledWith(ANNOTATION_ID);
+	});
+
+	it('enters transcribe mode and awaits loadBlocks when transcribe mode is off', async () => {
+		const url = new URL(
+			`https://app/documents/doc-1?commentId=${COMMENT_ID}&annotationId=${ANNOTATION_ID}`
+		);
+		const opts = buildOpts({ transcribeMode: false });
+
+		await scrollToCommentFromQuery(url, opts);
+
+		expect(opts.setTranscribeMode).toHaveBeenCalledWith(true);
+		expect(opts.loadBlocks).toHaveBeenCalled();
+	});
+
+	it('is a graceful no-op when the target element is not in the DOM', async () => {
+		const url = new URL(
+			`https://app/documents/doc-1?commentId=${COMMENT_ID}&annotationId=${ANNOTATION_ID}`
+		);
+		const opts = buildOpts({ getElement: vi.fn().mockReturnValue(null) });
+
+		// Must not throw. Flash should not fire — nothing to highlight.
+		await expect(scrollToCommentFromQuery(url, opts)).resolves.toBeUndefined();
+
+		expect(opts.flashAnnotation).not.toHaveBeenCalled();
+	});
+
+	it('uses behavior "instant" when prefers-reduced-motion is set', async () => {
+		const url = new URL(
+			`https://app/documents/doc-1?commentId=${COMMENT_ID}&annotationId=${ANNOTATION_ID}`
+		);
+		const el = fakeElement();
+		const opts = buildOpts({
+			prefersReducedMotion: true,
+			getElement: vi.fn().mockReturnValue(el)
+		});
+
+		await scrollToCommentFromQuery(url, opts);
+
+		expect(el.scrollIntoView).toHaveBeenCalledWith({ behavior: 'instant', block: 'center' });
+	});
+
+	it('strips both commentId and annotationId from the URL after handling', async () => {
+		const url = new URL(
+			`https://app/documents/doc-1?commentId=${COMMENT_ID}&annotationId=${ANNOTATION_ID}`
+		);
+		const opts = buildOpts();
+
+		await scrollToCommentFromQuery(url, opts);
+
+		expect(opts.onStripUrl).toHaveBeenCalled();
+	});
+});
diff --git a/frontend/src/lib/utils/deepLinkScroll.ts b/frontend/src/lib/utils/deepLinkScroll.ts
new file mode 100644
index 00000000..cc0d49c9
--- /dev/null
+++ b/frontend/src/lib/utils/deepLinkScroll.ts
@@ -0,0 +1,40 @@
+export type DeepLinkScrollOptions = {
+	transcribeMode: boolean;
+	setTranscribeMode: (value: boolean) => void;
+	loadBlocks: () => Promise<void>;
+	setActiveAnnotationId: (id: string) => void;
+	flashAnnotation: (annotationId: string) => void;
+	prefersReducedMotion: boolean;
+	afterTick: () => Promise<void>;
+	getElement: (id: string) => HTMLElement | null;
+	onStripUrl: () => void;
+};
+
+export async function scrollToCommentFromQuery(
+	url: URL,
+	opts: DeepLinkScrollOptions
+): Promise<void> {
+	const commentId = url.searchParams.get('commentId');
+	if (!commentId) return;
+
+	const annotationId = url.searchParams.get('annotationId');
+	if (!annotationId) return;
+
+	if (!opts.transcribeMode) {
+		opts.setTranscribeMode(true);
+		await opts.loadBlocks();
+	}
+
+	opts.setActiveAnnotationId(annotationId);
+	await opts.afterTick();
+
+	const el = opts.getElement(`comment-${commentId}`);
+	if (el) {
+		const behavior: ScrollBehavior = opts.prefersReducedMotion ? 'instant' : 'smooth';
+		el.scrollIntoView({ behavior, block: 'center' });
+		el.focus({ preventScroll: true });
+		opts.flashAnnotation(annotationId);
+	}
+
+	opts.onStripUrl();
+}