diff --git a/frontend/src/lib/utils/blockConflictMerge.spec.ts b/frontend/src/lib/utils/blockConflictMerge.spec.ts
new file mode 100644
index 00000000..e9bd1aad
--- /dev/null
+++ b/frontend/src/lib/utils/blockConflictMerge.spec.ts
@@ -0,0 +1,103 @@
+import { describe, it, expect } from 'vitest';
+import { mergeBlockOnConflict } from './blockConflictMerge';
+import type { PersonMention, TranscriptionBlockData } from '$lib/types';
+
+const baseBlock: TranscriptionBlockData = {
+	id: 'b1',
+	annotationId: 'a1',
+	documentId: 'd1',
+	text: 'old text from server',
+	label: null,
+	sortOrder: 0,
+	version: 7,
+	source: 'MANUAL',
+	reviewed: false,
+	mentionedPersons: []
+};
+
+describe('mergeBlockOnConflict', () => {
+	it('keeps the local unsaved text — never overwritten by server text (B12b)', () => {
+		const merged = mergeBlockOnConflict({
+			serverBlock: { ...baseBlock, text: 'server-side text' },
+			localText: 'transcriber unsaved input',
+			localMentions: []
+		});
+		expect(merged.text).toBe('transcriber unsaved input');
+	});
+
+	it('takes server-side displayName for personIds present on both sides (rename win)', () => {
+		const localMentions: PersonMention[] = [
+			{ personId: 'p-aug', displayName: 'Auguste Raddatz' } // stale: server renamed her
+		];
+		const serverMentions: PersonMention[] = [
+			{ personId: 'p-aug', displayName: 'Augusta Raddatz' } // post-rename
+		];
+		const merged = mergeBlockOnConflict({
+			serverBlock: { ...baseBlock, mentionedPersons: serverMentions },
+			localText: '@Augusta Raddatz',
+			localMentions
+		});
+		expect(merged.mentionedPersons).toEqual([
+			{ personId: 'p-aug', displayName: 'Augusta Raddatz' }
+		]);
+	});
+
+	it('keeps local-only mentions added since last save', () => {
+		const localMentions: PersonMention[] = [
+			{ personId: 'p-anna', displayName: 'Anna Schmidt' } // typed since last save
+		];
+		const merged = mergeBlockOnConflict({
+			serverBlock: { ...baseBlock, mentionedPersons: [] },
+			localText: '@Anna Schmidt',
+			localMentions
+		});
+		expect(merged.mentionedPersons).toContainEqual({
+			personId: 'p-anna',
+			displayName: 'Anna Schmidt'
+		});
+	});
+
+	it('returns a union of personIds when local and server diverge', () => {
+		const localMentions: PersonMention[] = [{ personId: 'p-anna', displayName: 'Anna Schmidt' }];
+		const serverMentions: PersonMention[] = [{ personId: 'p-aug', displayName: 'Augusta Raddatz' }];
+		const merged = mergeBlockOnConflict({
+			serverBlock: { ...baseBlock, mentionedPersons: serverMentions },
+			localText: '@Augusta Raddatz und @Anna Schmidt',
+			localMentions
+		});
+		expect(merged.mentionedPersons).toHaveLength(2);
+		expect(merged.mentionedPersons).toContainEqual({
+			personId: 'p-aug',
+			displayName: 'Augusta Raddatz'
+		});
+		expect(merged.mentionedPersons).toContainEqual({
+			personId: 'p-anna',
+			displayName: 'Anna Schmidt'
+		});
+	});
+
+	it('carries server version forward so the next save sends the latest revision', () => {
+		const merged = mergeBlockOnConflict({
+			serverBlock: { ...baseBlock, version: 42 },
+			localText: 'x',
+			localMentions: []
+		});
+		expect(merged.version).toBe(42);
+	});
+
+	it('carries other server fields (sortOrder, reviewed, updatedAt) forward', () => {
+		const merged = mergeBlockOnConflict({
+			serverBlock: {
+				...baseBlock,
+				sortOrder: 9,
+				reviewed: true,
+				updatedAt: '2026-04-29T10:00:00Z'
+			},
+			localText: 'x',
+			localMentions: []
+		});
+		expect(merged.sortOrder).toBe(9);
+		expect(merged.reviewed).toBe(true);
+		expect(merged.updatedAt).toBe('2026-04-29T10:00:00Z');
+	});
+});
diff --git a/frontend/src/lib/utils/blockConflictMerge.ts b/frontend/src/lib/utils/blockConflictMerge.ts
new file mode 100644
index 00000000..af37f9a3
--- /dev/null
+++ b/frontend/src/lib/utils/blockConflictMerge.ts
@@ -0,0 +1,31 @@
+import type { PersonMention, TranscriptionBlockData } from '$lib/types';
+
+type MergeArgs = {
+	serverBlock: TranscriptionBlockData;
+	localText: string;
+	localMentions: PersonMention[];
+};
+
+/**
+ * Resolves a 409-Conflict from the server by combining the latest server
+ * snapshot with the transcriber's unsaved local edits (B12b).
+ *
+ * Rules:
+ * - The transcriber's typed text always wins — never overwrite their input.
+ * - Server is the source of truth for the displayName of any person it
+ *   knows about; renames that just landed on the server replace stale local
+ *   names by personId.
+ * - Local-only mentions added since the last save are preserved.
+ * - All non-mention fields (version, sortOrder, reviewed, updatedAt, ...)
+ *   come from the server snapshot so the next save sends the current
+ *   revision and matches the latest persisted state.
+ */
+export function mergeBlockOnConflict(args: MergeArgs): TranscriptionBlockData {
+	const serverIds = new Set(args.serverBlock.mentionedPersons.map((m) => m.personId));
+	const localOnly = args.localMentions.filter((m) => !serverIds.has(m.personId));
+	return {
+		...args.serverBlock,
+		text: args.localText,
+		mentionedPersons: [...args.serverBlock.mentionedPersons, ...localOnly]
+	};
+}
diff --git a/frontend/src/routes/documents/[id]/+page.svelte b/frontend/src/routes/documents/[id]/+page.svelte
index 1ea3e10d..b07bad86 100644
--- a/frontend/src/routes/documents/[id]/+page.svelte
+++ b/frontend/src/routes/documents/[id]/+page.svelte
@@ -98,6 +98,21 @@ async function saveBlock(
 		headers: { 'Content-Type': 'application/json' },
 		body: JSON.stringify({ text, mentionedPersons })
 	});
+	if (res.status === 409) {
+		// Rename-mid-edit (B12b): refetch latest, merge so transcriber input survives.
+		const { mergeBlockOnConflict } = await import('$lib/utils/blockConflictMerge');
+		const fresh = await fetch(`/api/documents/${doc.id}/transcription-blocks/${blockId}`);
+		if (fresh.ok) {
+			const serverBlock = await fresh.json();
+			const merged = mergeBlockOnConflict({
+				serverBlock,
+				localText: text,
+				localMentions: mentionedPersons
+			});
+			transcriptionBlocks = transcriptionBlocks.map((b) => (b.id === blockId ? merged : b));
+		}
+		throw new Error('Conflict resolved — please save again');
+	}
 	if (!res.ok) throw new Error('Save failed');
 	const updated = await res.json();
 	transcriptionBlocks = transcriptionBlocks.map((b) => (b.id === blockId ? updated : b));