diff --git a/frontend/src/lib/utils/mention.spec.ts b/frontend/src/lib/utils/mention.spec.ts
index 47a84645..0323bf20 100644
--- a/frontend/src/lib/utils/mention.spec.ts
+++ b/frontend/src/lib/utils/mention.spec.ts
@@ -232,9 +232,11 @@ describe('renderTranscriptionBody', () => {
 	});
 
 	it('processes longer displayNames first to avoid prefix shadowing', () => {
-		const augusteShort: PersonMention = { personId: 'p-short', displayName: 'Auguste' };
+		const SHORT_ID = '11111111-1111-4111-8111-111111111111';
+		const LONG_ID = '22222222-2222-4222-8222-222222222222';
+		const augusteShort: PersonMention = { personId: SHORT_ID, displayName: 'Auguste' };
 		const augusteLong: PersonMention = {
-			personId: 'p-long',
+			personId: LONG_ID,
 			displayName: 'Auguste Raddatz'
 		};
 		// Sidecar order is short-first; longer match must still win for the long text
@@ -242,8 +244,8 @@ describe('renderTranscriptionBody', () => {
 			augusteShort,
 			augusteLong
 		]);
-		expect(result).toContain('href="/persons/p-long"');
-		expect(result).toContain('href="/persons/p-short"');
+		expect(result).toContain(`href="/persons/${LONG_ID}"`);
+		expect(result).toContain(`href="/persons/${SHORT_ID}"`);
 		// The "Raddatz" suffix must not leak inside the short-name anchor
 		expect(result).not.toMatch(/>Auguste<\/a> Raddatz/);
 	});
@@ -257,18 +259,20 @@ describe('renderTranscriptionBody', () => {
 
 	it('first-sidecar-wins when two entries share the same displayName', () => {
 		// Two persons named "Hans" — first sidecar entry wins for all occurrences.
-		const hansFirst: PersonMention = { personId: 'p-first', displayName: 'Hans' };
-		const hansSecond: PersonMention = { personId: 'p-second', displayName: 'Hans' };
+		const FIRST_ID = '33333333-3333-4333-8333-333333333333';
+		const SECOND_ID = '44444444-4444-4444-8444-444444444444';
+		const hansFirst: PersonMention = { personId: FIRST_ID, displayName: 'Hans' };
+		const hansSecond: PersonMention = { personId: SECOND_ID, displayName: 'Hans' };
 		const result = renderTranscriptionBody('@Hans und @Hans', [hansFirst, hansSecond]);
-		expect(result).toContain('href="/persons/p-first"');
-		expect(result).not.toContain('href="/persons/p-second"');
+		expect(result).toContain(`href="/persons/${FIRST_ID}"`);
+		expect(result).not.toContain(`href="/persons/${SECOND_ID}"`);
 		const anchorCount = (result.match(/<a /g) ?? []).length;
 		expect(anchorCount).toBe(2);
 	});
 
 	it('escapes HTML in displayName to prevent stored XSS', () => {
 		const xss: PersonMention = {
-			personId: 'p-xss',
+			personId: '55555555-5555-4555-8555-555555555555',
 			displayName: '<script>alert(1)</script>'
 		};
 		const result = renderTranscriptionBody('Hi @<script>alert(1)</script> there', [xss]);
@@ -292,7 +296,7 @@ describe('renderTranscriptionBody', () => {
 
 	it('escapes quotes in displayName so they cannot break the href attribute', () => {
 		const tricky: PersonMention = {
-			personId: 'p-quote',
+			personId: '66666666-6666-4666-8666-666666666666',
 			displayName: 'O"Brien'
 		};
 		const result = renderTranscriptionBody('@O"Brien', [tricky]);
@@ -307,4 +311,40 @@ describe('renderTranscriptionBody', () => {
 		const result = renderTranscriptionBody('Plain old transcription text.', []);
 		expect(result).toBe('Plain old transcription text.');
 	});
+
+	it('skips substitution when personId is not a UUID (defense in depth)', () => {
+		// Nora #5551: if personId ever flowed in from a less-sanitised source
+		// (a future "external person" or a bad sidecar), the renderer must not
+		// emit a clickable link. The escaped text remains as plain content.
+		const evil: PersonMention = {
+			personId: 'javascript:alert(1)',
+			displayName: 'Evil Link'
+		};
+		const result = renderTranscriptionBody('Hi @Evil Link!', [evil]);
+		expect(result).not.toContain('<a ');
+		expect(result).not.toContain('javascript:');
+		// The @-trigger and displayName are preserved as plain text
+		expect(result).toContain('@Evil Link');
+	});
+
+	it('skips substitution when personId is an absolute URL', () => {
+		const evil: PersonMention = {
+			personId: 'https://evil.example/persons/abc',
+			displayName: 'Phisher'
+		};
+		const result = renderTranscriptionBody('Hi @Phisher', [evil]);
+		expect(result).not.toContain('<a ');
+		expect(result).not.toContain('https://evil.example');
+	});
+
+	it('still substitutes when personId is a well-formed UUID', () => {
+		// Sanity check that the validation does not over-reject valid IDs.
+		const valid: PersonMention = {
+			personId: '550e8400-e29b-41d4-a716-446655440000',
+			displayName: 'Auguste Raddatz'
+		};
+		const result = renderTranscriptionBody('Brief an @Auguste Raddatz', [valid]);
+		expect(result).toContain('<a ');
+		expect(result).toContain('href="/persons/550e8400-e29b-41d4-a716-446655440000"');
+	});
 });
diff --git a/frontend/src/lib/utils/mention.ts b/frontend/src/lib/utils/mention.ts
index 200a0b58..16203459 100644
--- a/frontend/src/lib/utils/mention.ts
+++ b/frontend/src/lib/utils/mention.ts
@@ -77,6 +77,18 @@ function escapeRegExp(str: string): string {
 	return str.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
 }
 
+/**
+ * Strict UUID v1–v5 check. Used as a defensive boundary on PersonMention.personId
+ * before substituting it into an `href` — even though the backend currently only
+ * emits UUIDs, a future "external person" feature must not accidentally turn this
+ * helper into an open-redirect surface (CWE-601).
+ */
+const UUID_RE = /^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i;
+
+function isUuid(value: string): boolean {
+	return UUID_RE.test(value);
+}
+
 /**
  * Renders a transcription block's text segment as safe HTML for read mode.
  *
@@ -100,6 +112,10 @@ export function renderTranscriptionBody(text: string, mentionedPersons: PersonMe
 	const unique: PersonMention[] = [];
 	for (const mention of mentionedPersons) {
 		if (seen.has(mention.displayName)) continue;
+		// Defense in depth: refuse to render an anchor for a non-UUID personId.
+		// The escaped block text falls through unchanged, so the @-trigger is
+		// preserved as plain content — no silent data loss, no clickable link.
+		if (!isUuid(mention.personId)) continue;
 		seen.add(mention.displayName);
 		unique.push(mention);
 	}