diff --git a/scripts/taskToggleMemory.test.ts b/scripts/taskToggleMemory.test.ts
index e2d559f..cbd9bf2 100644
--- a/scripts/taskToggleMemory.test.ts
+++ b/scripts/taskToggleMemory.test.ts
@@ -2,10 +2,10 @@ import assert from 'node:assert/strict';
 import { readFileSync } from 'node:fs';
 import test from 'node:test';
 
-const viewer = readFileSync(new URL('../src/lib/MarkdownViewer.svelte', import.meta.url), 'utf8');
+const session = readFileSync(new URL('../src/lib/sessions/documentSession.svelte.ts', import.meta.url), 'utf8');
 
 test('preview task toggles transform the active in-memory buffer before saving', () => {
-	const taskToggle = viewer.match(/async function toggleTaskCheckbox[\s\S]*?\n\t}\n\n\n\n\tfunction saveRecentFile/);
+	const taskToggle = session.match(/async function toggleTaskCheckbox[\s\S]*?\n\t}\n\n\treturn/);
 	assert.ok(taskToggle);
 	assert.doesNotMatch(taskToggle[0], /read_file_content/);
 	assert.match(taskToggle[0], /const raw = tab\.rawContent;/);
diff --git a/src/lib/MarkdownViewer.svelte b/src/lib/MarkdownViewer.svelte
index c686379..98172c6 100644
--- a/src/lib/MarkdownViewer.svelte
+++ b/src/lib/MarkdownViewer.svelte
@@ -1469,37 +1469,11 @@ import { createDocumentSession, type LoadMarkdownOptions } from './sessions/docu
     }
 
 	async function toggleTaskCheckbox(checkbox: HTMLInputElement) {
-		const tab = tabManager.activeTab;
-		if (!tab || !tab.path) return;
-		const raw = tab.rawContent;
-
-		// find which task item this is by counting checkboxes in DOM
 		const allBoxes = Array.from(markdownBody?.querySelectorAll('[data-task-checkbox]') || []);
 		const index = allBoxes.indexOf(checkbox);
 		if (index === -1) return;
-
-		// checkbox.checked is still the OLD state (e.preventDefault blocked the toggle)
 		const nowChecked = !checkbox.checked;
-
-		// replace the nth [ ] or [x] in the raw markdown
-		let count = 0;
-		const updated = raw.replace(/^(\s*[-*+] )\[( |x|X)\]/gm, (match, prefix) => {
-			if (count === index) {
-				count++;
-				return `${prefix}[${nowChecked ? 'x' : ' '}]`;
-			}
-			count++;
-			return match;
-		});
-
-		if (updated === raw) return;
-
-		// Update the single in-memory source before saving. saveContent keeps the
-		// tab dirty if its write fails, so a disk error cannot discard this edit.
-		tabManager.updateTabRawContent(tab.id, updated);
-		await saveContent(tab.id);
-
-		// update DOM optimistically
+		if (!(await documentSession.toggleTaskCheckbox(index, nowChecked))) return;
 		checkbox.checked = nowChecked;
 		const li = checkbox.closest('li');
 		if (li) {
diff --git a/src/lib/sessions/documentSession.svelte.ts b/src/lib/sessions/documentSession.svelte.ts
index 45d51ae..fe453ec 100644
--- a/src/lib/sessions/documentSession.svelte.ts
+++ b/src/lib/sessions/documentSession.svelte.ts
@@ -192,5 +192,20 @@ export function createDocumentSession(options: DocumentSessionOptions) {
 		}
 	}
 
-	return { loadMarkdown, saveContent, saveContentAs };
+	async function toggleTaskCheckbox(index: number, nowChecked: boolean) {
+		const tab = tabManager.activeTab;
+		if (!tab || !tab.path) return false;
+		const raw = tab.rawContent;
+		let count = 0;
+		const updated = raw.replace(/^(\s*[-*+] )\[( |x|X)\]/gm, (match, prefix) => {
+			if (count++ === index) return `${prefix}[${nowChecked ? 'x' : ' '}]`;
+			return match;
+		});
+		if (updated === raw) return false;
+		tabManager.updateTabRawContent(tab.id, updated);
+		await saveContent(tab.id);
+		return true;
+	}
+
+	return { loadMarkdown, saveContent, saveContentAs, toggleTaskCheckbox };
 }