diff --git a/frontend/src/lib/utils/mentionSerializer.spec.ts b/frontend/src/lib/utils/mentionSerializer.spec.ts
new file mode 100644
index 00000000..19b4c4a2
--- /dev/null
+++ b/frontend/src/lib/utils/mentionSerializer.spec.ts
@@ -0,0 +1,138 @@
+import { describe, it, expect } from 'vitest';
+import { deserialize, serialize } from './mentionSerializer';
+import type { PersonMention } from '$lib/types';
+
+// ─── deserialize ─────────────────────────────────────────────────────────────
+
+describe('deserialize', () => {
+	it('returns a valid Tiptap doc for an empty string', () => {
+		const doc = deserialize('', []);
+		expect(doc.type).toBe('doc');
+		expect(doc.content).toHaveLength(1);
+		expect(doc.content![0].type).toBe('paragraph');
+	});
+
+	it('returns plain text node for text with no mentions', () => {
+		const doc = deserialize('Hallo Welt', []);
+		const para = doc.content![0];
+		expect(para.content).toHaveLength(1);
+		expect(para.content![0]).toEqual({ type: 'text', text: 'Hallo Welt' });
+	});
+
+	it('places a mention node at the correct position in the paragraph', () => {
+		const sidecar: PersonMention[] = [{ personId: 'uuid-x', displayName: 'Clara' }];
+		const doc = deserialize('Hallo @Clara Welt', sidecar);
+		const nodes = doc.content![0].content!;
+		expect(nodes).toHaveLength(3);
+		expect(nodes[0]).toEqual({ type: 'text', text: 'Hallo ' });
+		expect(nodes[1]).toMatchObject({
+			type: 'mention',
+			attrs: { displayName: 'Clara', personId: 'uuid-x' }
+		});
+		expect(nodes[2]).toEqual({ type: 'text', text: ' Welt' });
+	});
+
+	it('prefers the longer displayName when two sidecar entries share a prefix', () => {
+		const sidecar: PersonMention[] = [
+			{ personId: 'uuid-short', displayName: 'Auguste' },
+			{ personId: 'uuid-long', displayName: 'Auguste Raddatz' }
+		];
+		const doc = deserialize('@Auguste Raddatz', sidecar);
+		const nodes = doc.content![0].content!;
+		expect(nodes).toHaveLength(1);
+		expect(nodes[0]).toMatchObject({
+			type: 'mention',
+			attrs: { personId: 'uuid-long', displayName: 'Auguste Raddatz' }
+		});
+	});
+
+	it('splits multiple paragraphs on newline', () => {
+		const doc = deserialize('Zeile 1\nZeile 2', []);
+		expect(doc.content).toHaveLength(2);
+		expect(doc.content![0].content![0]).toEqual({ type: 'text', text: 'Zeile 1' });
+		expect(doc.content![1].content![0]).toEqual({ type: 'text', text: 'Zeile 2' });
+	});
+});
+
+// ─── serialize ───────────────────────────────────────────────────────────────
+
+describe('serialize', () => {
+	it('serializes plain text unchanged', () => {
+		const doc = deserialize('Hallo Welt', []);
+		const { text, mentionedPersons } = serialize(doc);
+		expect(text).toBe('Hallo Welt');
+		expect(mentionedPersons).toEqual([]);
+	});
+
+	it('serializes mention node back to @displayName', () => {
+		const sidecar: PersonMention[] = [{ personId: 'uuid-x', displayName: 'Clara' }];
+		const doc = deserialize('Hallo @Clara Welt', sidecar);
+		const { text, mentionedPersons } = serialize(doc);
+		expect(text).toBe('Hallo @Clara Welt');
+		expect(mentionedPersons).toEqual([{ personId: 'uuid-x', displayName: 'Clara' }]);
+	});
+
+	it('joins multi-paragraph doc back with newlines', () => {
+		const doc = deserialize('Zeile 1\nZeile 2', []);
+		const { text } = serialize(doc);
+		expect(text).toBe('Zeile 1\nZeile 2');
+	});
+
+	it('de-duplicates repeated mentions in the sidecar', () => {
+		const sidecar: PersonMention[] = [{ personId: 'uuid-x', displayName: 'Clara' }];
+		const doc = deserialize('@Clara und @Clara', sidecar);
+		const { mentionedPersons } = serialize(doc);
+		expect(mentionedPersons).toHaveLength(1);
+		expect(mentionedPersons[0].personId).toBe('uuid-x');
+	});
+});
+
+// ─── Round-trip invariant ─────────────────────────────────────────────────────
+
+describe('round-trip invariant', () => {
+	it('text is preserved exactly through deserialize → serialize', () => {
+		const cases = [
+			['', []],
+			['Hallo Welt', []],
+			['@Clara schreibt', [{ personId: 'uuid-x', displayName: 'Clara' }]],
+			['Zeile 1\nZeile 2', []],
+			['Sehr geehrte @Auguste Raddatz,', [{ personId: 'uuid-aug', displayName: 'Auguste Raddatz' }]]
+		] as const;
+
+		for (const [text, sidecar] of cases) {
+			const doc = deserialize(text, sidecar as PersonMention[]);
+			const { text: out } = serialize(doc);
+			expect(out, `round-trip failed for: "${text}"`).toBe(text);
+		}
+	});
+});
+
+// ─── Backward compatibility (AC-6) ────────────────────────────────────────────
+
+describe('backward compatibility', () => {
+	it('old-format full-name sidecar entry still round-trips correctly', () => {
+		// Before this issue, displayName stored the person's full DB name.
+		// renderTranscriptionBody already handles this — so does the serializer.
+		const oldSidecar: PersonMention[] = [{ personId: 'uuid-aug', displayName: 'Auguste Raddatz' }];
+		const text = 'Brief von @Auguste Raddatz';
+		const doc = deserialize(text, oldSidecar);
+		const { text: out, mentionedPersons } = serialize(doc);
+		expect(out).toBe(text);
+		expect(mentionedPersons).toEqual(oldSidecar);
+	});
+});
+
+// ─── Security ─────────────────────────────────────────────────────────────────
+
+describe('security', () => {
+	it('displayName containing HTML special chars is preserved as a string, not injected', () => {
+		const sidecar: PersonMention[] = [
+			{ personId: 'uuid-x', displayName: '<script>alert(1)</script>' }
+		];
+		const text = '@<script>alert(1)</script>';
+		const doc = deserialize(text, sidecar);
+		const { mentionedPersons } = serialize(doc);
+		// The displayName is stored verbatim — HTML escaping is the renderer's job
+		expect(mentionedPersons[0].displayName).toBe('<script>alert(1)</script>');
+	});
+});
diff --git a/frontend/src/lib/utils/mentionSerializer.ts b/frontend/src/lib/utils/mentionSerializer.ts
new file mode 100644
index 00000000..b6213f25
--- /dev/null
+++ b/frontend/src/lib/utils/mentionSerializer.ts
@@ -0,0 +1,113 @@
+import type { JSONContent } from '@tiptap/core';
+import type { PersonMention } from '$lib/types';
+
+/**
+ * Converts stored block text + sidecar into a Tiptap ProseMirror document.
+ *
+ * The text is split by "\n" into paragraphs. Within each paragraph, sidecar
+ * entries are matched against "@displayName" tokens (longest first) and
+ * converted to mention nodes. Unmatched text becomes plain text nodes.
+ *
+ * Round-trip invariant: serialize(deserialize(text, sidecar)).text === text
+ */
+export function deserialize(text: string, sidecar: PersonMention[]): JSONContent {
+	const lines = text === '' ? [''] : text.split('\n');
+
+	// Sort sidecar by displayName length descending so longer names shadow
+	// shorter prefix matches (same heuristic as renderTranscriptionBody).
+	const sorted = [...sidecar].sort((a, b) => b.displayName.length - a.displayName.length);
+
+	return {
+		type: 'doc',
+		content: lines.map((line) => ({
+			type: 'paragraph',
+			content: parseLine(line, sorted)
+		}))
+	};
+}
+
+function parseLine(text: string, sidecar: PersonMention[]): JSONContent[] {
+	if (text === '') return [];
+
+	if (sidecar.length === 0) {
+		return [{ type: 'text', text }];
+	}
+
+	// Build a list of mention ranges: { start, end, mention }
+	const ranges: Array<{ start: number; end: number; mention: PersonMention }> = [];
+
+	for (const mention of sidecar) {
+		const needle = `@${mention.displayName}`;
+		let idx = 0;
+		while (idx < text.length) {
+			const pos = text.indexOf(needle, idx);
+			if (pos === -1) break;
+			// Check that the range doesn't overlap an already-found range
+			const end = pos + needle.length;
+			const overlaps = ranges.some((r) => r.start < end && r.end > pos);
+			if (!overlaps) {
+				ranges.push({ start: pos, end, mention });
+			}
+			idx = pos + 1;
+		}
+	}
+
+	if (ranges.length === 0) {
+		return [{ type: 'text', text }];
+	}
+
+	// Sort by position
+	ranges.sort((a, b) => a.start - b.start);
+
+	const nodes: JSONContent[] = [];
+	let cursor = 0;
+
+	for (const { start, end, mention } of ranges) {
+		if (start > cursor) {
+			nodes.push({ type: 'text', text: text.slice(cursor, start) });
+		}
+		nodes.push({
+			type: 'mention',
+			attrs: { displayName: mention.displayName, personId: mention.personId }
+		});
+		cursor = end;
+	}
+
+	if (cursor < text.length) {
+		nodes.push({ type: 'text', text: text.slice(cursor) });
+	}
+
+	return nodes;
+}
+
+/**
+ * Converts a Tiptap ProseMirror document back to stored block text + sidecar.
+ *
+ * Paragraphs are joined with "\n". Mention nodes are emitted as "@displayName"
+ * and collected into mentionedPersons (de-duplicated by personId).
+ */
+export function serialize(doc: JSONContent): { text: string; mentionedPersons: PersonMention[] } {
+	const paragraphs = doc.content ?? [];
+	const mentionedPersons: PersonMention[] = [];
+	const seenIds = new Set<string>();
+	const lines: string[] = [];
+
+	for (const para of paragraphs) {
+		let line = '';
+		for (const node of para.content ?? []) {
+			if (node.type === 'text') {
+				line += node.text ?? '';
+			} else if (node.type === 'mention') {
+				const { displayName, personId } = node.attrs ?? {};
+				line += `@${displayName}`;
+				if (!seenIds.has(personId)) {
+					seenIds.add(personId);
+					mentionedPersons.push({ personId, displayName });
+				}
+			}
+		}
+		lines.push(line);
+	}
+
+	return { text: lines.join('\n'), mentionedPersons };
+}