diff --git a/.claude/settings.local.json b/.claude/settings.local.json
index 3afad14..8a5ac49 100644
--- a/.claude/settings.local.json
+++ b/.claude/settings.local.json
@@ -64,7 +64,8 @@
       "Bash(git status *)",
       "Bash(git revert *)",
       "Bash(git commit -m 'Fix price display issues and layout in design review page *)",
-      "Bash(git commit -m 'Fix: Calculate effectivePrice in design API endpoints *)"
+      "Bash(git commit -m 'Fix: Calculate effectivePrice in design API endpoints *)",
+      "Bash(git commit -m 'Fix: Handle designJson as string in checkout API *)"
     ]
   }
 }
diff --git a/src/app/api/checkout/route.ts b/src/app/api/checkout/route.ts
index 5b811cf..f3b845e 100644
--- a/src/app/api/checkout/route.ts
+++ b/src/app/api/checkout/route.ts
@@ -78,8 +78,8 @@ export async function POST(req: Request) {
 
     const activePromotions = await fetchActivePromotions();
     const pricedItems = items.map((i) => {
-      const design = i.designJson as { front: unknown[]; back: unknown[] };
-      const isPersonalised = design.front.length > 0 || design.back.length > 0;
+      const design = typeof i.designJson === 'string' ? JSON.parse(i.designJson) : i.designJson;
+      const isPersonalised = (design.front?.length ?? 0) > 0 || (design.back?.length ?? 0) > 0;
       return {
         ...i,
         unitPrice: getEffectivePrice(productsById.get(i.productId)!, activePromotions, new Date(), isPersonalised).price,