diff --git a/backend/src/main/resources/db/migration/V51__backfill_block_comment_annotation_id.sql b/backend/src/main/resources/db/migration/V51__backfill_block_comment_annotation_id.sql
new file mode 100644
index 00000000..f136adb7
--- /dev/null
+++ b/backend/src/main/resources/db/migration/V51__backfill_block_comment_annotation_id.sql
@@ -0,0 +1,24 @@
+-- Backfill annotation_id on block comments and their notifications.
+--
+-- Before the upstream fix, CommentService.postBlockComment did not set
+-- DocumentComment.annotationId, so block comments were stored with
+-- annotation_id = NULL and every notification built from them inherited
+-- that NULL (see NotificationService.notifyMentions/notifyReply).
+--
+-- The frontend deep-link flow needs annotationId in the URL query string
+-- to open the correct annotation panel and scroll to the comment.
+-- Without this backfill, previously issued notifications would still
+-- carry annotation_id = NULL even after the code fix lands.
+
+UPDATE document_comments dc
+SET annotation_id = tb.annotation_id
+FROM transcription_blocks tb
+WHERE dc.block_id = tb.id
+  AND dc.annotation_id IS NULL;
+
+UPDATE notifications n
+SET annotation_id = dc.annotation_id
+FROM document_comments dc
+WHERE n.reference_id = dc.id
+  AND n.annotation_id IS NULL
+  AND dc.annotation_id IS NOT NULL;
diff --git a/backend/src/test/java/org/raddatz/familienarchiv/repository/MigrationIntegrationTest.java b/backend/src/test/java/org/raddatz/familienarchiv/repository/MigrationIntegrationTest.java
index 7709b486..b482a0be 100644
--- a/backend/src/test/java/org/raddatz/familienarchiv/repository/MigrationIntegrationTest.java
+++ b/backend/src/test/java/org/raddatz/familienarchiv/repository/MigrationIntegrationTest.java
@@ -302,6 +302,57 @@ class MigrationIntegrationTest {
         ).isInstanceOf(DataIntegrityViolationException.class);
     }
 
+    // ─── V51: backfill annotation_id on block comments and notifications ─────
+
+    @Test
+    void v51_backfillsAnnotationIdOnBlockCommentsFromTheirBlocks() {
+        UUID docId = createDocument();
+        UUID annotationId = insertAnnotation(docId);
+        UUID blockId = insertBlock(docId, annotationId);
+        UUID commentId = insertBlockCommentWithNullAnnotationId(docId, blockId);
+
+        jdbc.update(V51_BACKFILL_COMMENTS_SQL);
+
+        UUID stored = jdbc.queryForObject(
+                "SELECT annotation_id FROM document_comments WHERE id = ?",
+                UUID.class, commentId);
+        assertThat(stored).isEqualTo(annotationId);
+    }
+
+    @Test
+    void v51_backfillsAnnotationIdOnNotificationsFromTheirReferencedComment() {
+        UUID docId = createDocument();
+        UUID userId = insertUser("recipient-" + UUID.randomUUID() + "@example.com");
+        UUID annotationId = insertAnnotation(docId);
+        UUID blockId = insertBlock(docId, annotationId);
+        UUID commentId = insertBlockCommentWithAnnotationId(docId, blockId, annotationId);
+        UUID notificationId = insertNotificationWithNullAnnotationId(docId, commentId, userId);
+
+        jdbc.update(V51_BACKFILL_NOTIFICATIONS_SQL);
+
+        UUID stored = jdbc.queryForObject(
+                "SELECT annotation_id FROM notifications WHERE id = ?",
+                UUID.class, notificationId);
+        assertThat(stored).isEqualTo(annotationId);
+    }
+
+    private static final String V51_BACKFILL_COMMENTS_SQL = """
+            UPDATE document_comments dc
+            SET annotation_id = tb.annotation_id
+            FROM transcription_blocks tb
+            WHERE dc.block_id = tb.id
+              AND dc.annotation_id IS NULL
+            """;
+
+    private static final String V51_BACKFILL_NOTIFICATIONS_SQL = """
+            UPDATE notifications n
+            SET annotation_id = dc.annotation_id
+            FROM document_comments dc
+            WHERE n.reference_id = dc.id
+              AND n.annotation_id IS NULL
+              AND dc.annotation_id IS NOT NULL
+            """;
+
     // ─── helpers ─────────────────────────────────────────────────────────────
 
     private UUID createPerson(String firstName, String lastName) {
@@ -326,4 +377,63 @@ class MigrationIntegrationTest {
         em.flush();
         return doc.getId();
     }
+
+    private UUID insertAnnotation(UUID docId) {
+        UUID id = UUID.randomUUID();
+        jdbc.update("""
+                INSERT INTO document_annotations
+                    (id, document_id, page_number, x, y, width, height, color)
+                VALUES (?, ?, 1, 0.1, 0.1, 0.3, 0.1, '#00C7B1')
+                """, id, docId);
+        return id;
+    }
+
+    private UUID insertBlock(UUID docId, UUID annotationId) {
+        UUID id = UUID.randomUUID();
+        jdbc.update("""
+                INSERT INTO transcription_blocks
+                    (id, annotation_id, document_id, text, sort_order)
+                VALUES (?, ?, ?, '', 0)
+                """, id, annotationId, docId);
+        return id;
+    }
+
+    private UUID insertUser(String email) {
+        UUID id = UUID.randomUUID();
+        jdbc.update("""
+                INSERT INTO users (id, email, password, enabled, notify_on_reply, notify_on_mention)
+                VALUES (?, ?, 'hash', true, false, false)
+                """, id, email);
+        return id;
+    }
+
+    private UUID insertBlockCommentWithNullAnnotationId(UUID docId, UUID blockId) {
+        UUID id = UUID.randomUUID();
+        jdbc.update("""
+                INSERT INTO document_comments
+                    (id, document_id, block_id, annotation_id, author_name, content)
+                VALUES (?, ?, ?, NULL, 'Tester', 'Hi')
+                """, id, docId, blockId);
+        return id;
+    }
+
+    private UUID insertBlockCommentWithAnnotationId(UUID docId, UUID blockId, UUID annotationId) {
+        UUID id = UUID.randomUUID();
+        jdbc.update("""
+                INSERT INTO document_comments
+                    (id, document_id, block_id, annotation_id, author_name, content)
+                VALUES (?, ?, ?, ?, 'Tester', 'Hi')
+                """, id, docId, blockId, annotationId);
+        return id;
+    }
+
+    private UUID insertNotificationWithNullAnnotationId(UUID docId, UUID commentId, UUID recipientId) {
+        UUID id = UUID.randomUUID();
+        jdbc.update("""
+                INSERT INTO notifications
+                    (id, recipient_id, type, document_id, reference_id, annotation_id, read, actor_name)
+                VALUES (?, ?, 'MENTION', ?, ?, NULL, false, 'Tester')
+                """, id, recipientId, docId, commentId);
+        return id;
+    }
 }