diff --git a/packages/web/src/components/checklist/ChecklistYjsWrapper.tsx b/packages/web/src/components/checklist/ChecklistYjsWrapper.tsx
index 133bcb43..d2ca709b 100644
--- a/packages/web/src/components/checklist/ChecklistYjsWrapper.tsx
+++ b/packages/web/src/components/checklist/ChecklistYjsWrapper.tsx
@@ -11,6 +11,7 @@ import type { ChecklistAnswerInput } from '@corates/shared/sync';
 import { useProjectStore, selectConnectionPhase } from '@/stores/projectStore';
 import { useAuthStore, selectUser } from '@/stores/authStore';
 import { useStudyAnnotations } from '@/primitives/useProject/useStudyAnnotations';
+import { annotationTypeName } from '@/lib/annotationType';
 import { ACCESS_DENIED_ERRORS } from '@/constants/errors.js';
 import {
   CHECKLIST_STATUS,
@@ -46,7 +47,8 @@ interface ChecklistYjsWrapperProps {
 /** The EmbedPDF payload shape the viewer hands back for annotation writes. */
 interface AnnotationData {
   id?: string;
-  type?: string;
+  /** EmbedPDF's numeric subtype; the synced row stores its name. */
+  type?: number | string;
   pageIndex?: number;
   pdfId?: string;
   embedPdfData?: string;
@@ -281,7 +283,7 @@ export function ChecklistYjsWrapper({ projectId, studyId, checklistId }: Checkli
         studyId,
         checklistId,
         pdfId: selectedPdfId,
-        type: annotation.type ?? '',
+        type: annotationTypeName(annotation.type),
         pageIndex: annotation.pageIndex ?? 0,
         // The serialized EmbedPDF payload must carry the row's id.
         embedPdfData: JSON.stringify({ ...annotation, id }),
@@ -298,7 +300,7 @@ export function ChecklistYjsWrapper({ projectId, studyId, checklistId }: Checkli
       void client?.mutate.annotation.update({
         id: annotation.id,
         updates: {
-          type: annotation.type,
+          type: annotationTypeName(annotation.type),
           pageIndex: annotation.pageIndex,
           embedPdfData: JSON.stringify(annotation),
         },
diff --git a/packages/web/src/lib/__tests__/annotationType.test.ts b/packages/web/src/lib/__tests__/annotationType.test.ts
new file mode 100644
index 00000000..6c8c2962
--- /dev/null
+++ b/packages/web/src/lib/__tests__/annotationType.test.ts
@@ -0,0 +1,48 @@
+import { describe, it, expect } from 'vitest';
+import { PdfAnnotationSubtype } from '@embedpdf/models';
+import { annotationTypeName } from '@/lib/annotationType';
+import { syncApp } from '@corates/shared/sync';
+
+describe('annotationTypeName', () => {
+  it('names the numeric subtypes EmbedPDF emits', () => {
+    expect(annotationTypeName(PdfAnnotationSubtype.HIGHLIGHT)).toBe('highlight');
+    expect(annotationTypeName(PdfAnnotationSubtype.INK)).toBe('ink');
+    expect(annotationTypeName(999)).toBe('999');
+  });
+
+  it('passes strings through and falls back to empty', () => {
+    expect(annotationTypeName('highlight')).toBe('highlight');
+    expect(annotationTypeName(undefined)).toBe('');
+  });
+
+  it('produces args the annotation mutators accept', () => {
+    const base = {
+      id: 'a1',
+      studyId: 's1',
+      checklistId: 'c1',
+      pdfId: 'p1',
+      type: annotationTypeName(PdfAnnotationSubtype.HIGHLIGHT),
+      pageIndex: 0,
+      embedPdfData: '{}',
+      createdBy: 'u1',
+      now: 1_700_000_000_000,
+    };
+
+    expect(syncApp.mutators['annotation.add'].args.safeParse(base).success).toBe(true);
+    expect(
+      syncApp.mutators['annotation.update'].args.safeParse({
+        id: base.id,
+        updates: { type: base.type, pageIndex: 0, embedPdfData: '{}' },
+        now: base.now,
+      }).success,
+    ).toBe(true);
+
+    // The pre-fix payload: the raw numeric subtype is what the engine rejected.
+    expect(
+      syncApp.mutators['annotation.add'].args.safeParse({
+        ...base,
+        type: PdfAnnotationSubtype.HIGHLIGHT,
+      }).success,
+    ).toBe(false);
+  });
+});
diff --git a/packages/web/src/lib/annotationType.ts b/packages/web/src/lib/annotationType.ts
new file mode 100644
index 00000000..4112e8ec
--- /dev/null
+++ b/packages/web/src/lib/annotationType.ts
@@ -0,0 +1,13 @@
+import { PdfAnnotationSubtypeName, type PdfAnnotationSubtype } from '@embedpdf/models';
+
+/**
+ * EmbedPDF reports an annotation's subtype as a numeric enum (HIGHLIGHT = 9),
+ * but the synced `annotations.type` column is a string, so an uncoerced value
+ * fails the mutator's args schema and the write is rejected outright.
+ */
+export function annotationTypeName(type: unknown): string {
+  if (typeof type === 'number') {
+    return PdfAnnotationSubtypeName[type as PdfAnnotationSubtype] ?? String(type);
+  }
+  return typeof type === 'string' ? type : '';
+}