diff --git a/lambda/blf-sync/blf-sync-handler.ts b/lambda/blf-sync/blf-sync-handler.ts
index d03a66e..39818eb 100644
--- a/lambda/blf-sync/blf-sync-handler.ts
+++ b/lambda/blf-sync/blf-sync-handler.ts
@@ -185,5 +185,8 @@ export async function handler(event: BlfSyncEvent = {}) {
     failed,
   };
   console.log(JSON.stringify(summary));
+  if (failed > 0) {
+    throw new Error(`blf sync patch failed for ${failed} user(s)`);
+  }
   return summary;
 }
diff --git a/lambda/blf-sync/blf-sync.test.ts b/lambda/blf-sync/blf-sync.test.ts
index f564a0c..11764bb 100644
--- a/lambda/blf-sync/blf-sync.test.ts
+++ b/lambda/blf-sync/blf-sync.test.ts
@@ -135,6 +135,34 @@ test("merge is a no-op when XML already matches", () => {
   assert.equal(blfsEqual(first.xml, second.xml), true);
 });
 
+test("merge keeps assigned shared-parking IDs so later runs are a no-op", () => {
+  const first = mergeDepartmentBlfs({
+    currentXml: "<PhoneDevice><BLFS/></PhoneDevice>",
+    selfExtension: "116",
+    colleagues,
+    contract: SLOT_CONTRACT["t54w-door-unlock-with-sp"],
+  });
+  const assigned = serializeBlfs(
+    parseBlfs(first.xml).map((entry) =>
+      entry.blfType === "SharedParking" || entry.blfType === "Line"
+        ? { ...entry, id: String(900 + entry.blfNo) }
+        : entry
+    )
+  );
+  const second = mergeDepartmentBlfs({
+    currentXml: assigned,
+    selfExtension: "116",
+    colleagues,
+    contract: SLOT_CONTRACT["t54w-door-unlock-with-sp"],
+  });
+  assert.equal(second.changed, false);
+  const parking = parseBlfs(second.xml).filter((e) => e.blfNo >= 3 && e.blfNo <= 5);
+  assert.deepEqual(
+    parking.map((e) => e.id),
+    ["903", "904", "905"]
+  );
+});
+
 test("serializeBlfs normalizes empty lists", () => {
   assert.equal(serializeBlfs([]), "<PhoneDevice><BLFS></BLFS></PhoneDevice>");
   assert.equal(blfsEqual("<PhoneDevice><BLFS/></PhoneDevice>", serializeBlfs([])), true);
diff --git a/lambda/blf-sync/merge.ts b/lambda/blf-sync/merge.ts
index d8d1036..a2d2ee8 100644
--- a/lambda/blf-sync/merge.ts
+++ b/lambda/blf-sync/merge.ts
@@ -68,13 +68,21 @@ export function mergeDepartmentBlfs(input: {
     value: "",
   };
 
-  const parking: BlfEntry[] = input.contract.sharedParking.map((slot) => ({
-    id: "-1",
-    blfNo: slot.blfNo,
-    blfType: "SharedParking",
-    blfTypeId: "3",
-    value: slot.value,
-  }));
+  const parking: BlfEntry[] = input.contract.sharedParking.map((slot) => {
+    const existing = current.find(
+      (entry) =>
+        entry.blfNo === slot.blfNo &&
+        entry.blfType === "SharedParking" &&
+        entry.value === slot.value
+    );
+    return {
+      id: existing?.id ?? "-1",
+      blfNo: slot.blfNo,
+      blfType: "SharedParking",
+      blfTypeId: existing?.blfTypeId ?? "3",
+      value: slot.value,
+    };
+  });
 
   const managed: BlfEntry[] = placedColleagues.map((colleague, index) => ({
     id: String(colleague.id),