diff --git a/userscript/manga-bookmark.user.js b/userscript/manga-bookmark.user.js
index ee2d1d8..e5bf6a3 100644
--- a/userscript/manga-bookmark.user.js
+++ b/userscript/manga-bookmark.user.js
@@ -265,6 +265,14 @@
     return Object.assign({ Authorization: "Bearer " + API_TOKEN }, extra || {});
   }
 
+  // The retry queue treats a 400 differently from a dropped connection, so the
+  // status has to survive the throw. A network failure leaves it undefined.
+  function httpError(what, status) {
+    const e = new Error(what + " " + status);
+    e.status = status;
+    return e;
+  }
+
   async function apiGet() {
     const res = await fetch(API_BASE + "/bookmarks", { headers: authHeaders() });
     if (!res.ok) throw new Error("GET /bookmarks " + res.status);
@@ -283,7 +291,7 @@
       headers: authHeaders({ "Content-Type": "application/json" }),
       body: JSON.stringify(body),
     });
-    if (!res.ok) throw new Error("PUT /bookmarks " + res.status);
+    if (!res.ok) throw httpError("PUT /bookmarks", res.status);
     return res.json();
   }
 
@@ -292,7 +300,8 @@
       method: "DELETE",
       headers: authHeaders(),
     });
-    if (!res.ok) throw new Error("DELETE /bookmarks " + res.status);
+    // 404 means the row is already gone, which is what the caller wanted.
+    if (!res.ok && res.status !== 404) throw httpError("DELETE /bookmarks", res.status);
   }
 
   // ============================================================
@@ -338,6 +347,99 @@
     return b.status || "reading";
   }
 
+  // ============================================================
+  // Retry queue
+  //
+  // A failed write is not rolled back and not lost: the key is parked here and
+  // replayed when the backend is next reachable. Entries carry no payload — the
+  // body is read from state.byKey at send time, because state.list *is* the
+  // desired state and is already persisted in mangabm:cache. One entry per key,
+  // so two writes to the same series cannot replay out of order, an
+  // archive-then-unarchive collapses to whatever the cache now says, and a
+  // queued DELETE replaces a queued PUT rather than racing it.
+  // ============================================================
+
+  const QUEUE_KEY = "mangabm:queue";
+  const QUEUE_MAX = 200; // ~12 KB; realistically bounded by the bookmark count
+  const QUEUE_MAX_ATTEMPTS = 10;
+
+  let authFailed = false; // a 401 was seen; retrying cannot help until the token changes
+
+  function loadQueue() {
+    try {
+      const raw = localStorage.getItem(QUEUE_KEY);
+      return raw ? JSON.parse(raw) : [];
+    } catch (e) {
+      return [];
+    }
+  }
+
+  function saveQueue(q) {
+    try {
+      localStorage.setItem(QUEUE_KEY, JSON.stringify(q));
+    } catch (e) {
+      /* quota / private mode — ignore */
+    }
+  }
+
+  const queue = loadQueue();
+
+  function queueCount() {
+    return queue.length;
+  }
+
+  function queueGet(key) {
+    return queue.find((e) => e.key === key) || null;
+  }
+
+  function titleFor(key) {
+    const b = state.byKey[key];
+    return (b && (b.title || b.series_id)) || key;
+  }
+
+  // Oldest goes first when the cap is hit: the newest entry is the action the
+  // user just took, and dropping that is the bug this queue exists to fix.
+  function queuePush(entry) {
+    if (queue.length >= QUEUE_MAX) {
+      const evicted = queue.shift();
+      toast("Sync queue full — dropped " + titleFor(evicted.key), true);
+    }
+    queue.push(entry);
+    return entry;
+  }
+
+  function queueDrop(key) {
+    const i = queue.findIndex((e) => e.key === key);
+    if (i >= 0) {
+      queue.splice(i, 1);
+      saveQueue(queue);
+    }
+  }
+
+  // Records a failed write and classifies why it failed. A 400 is a payload the
+  // server will never accept, so it is dropped now instead of being retried ten
+  // times; a 401 is the wrong token, so the entry is kept untouched and the
+  // drain gives up until the token changes.
+  function queueEnqueue(key, op, sendStatus, err) {
+    const status = err && err.status;
+    if (status === 400) {
+      queueDrop(key);
+      toast("Couldn't sync " + titleFor(key) + " — change lost", true);
+      return;
+    }
+    const e = queueGet(key) || queuePush({ key: key, op: op, sendStatus: false, attempts: 0 });
+    e.op = op; // a delete replaces a put, and a put replaces a delete
+    e.sendStatus = e.sendStatus || sendStatus; // sticky: an archive intent is never dropped
+    if (status === 401) {
+      authFailed = true;
+    } else if (++e.attempts >= QUEUE_MAX_ATTEMPTS) {
+      queueDrop(key);
+      toast("Gave up syncing " + titleFor(key), true);
+      return;
+    }
+    saveQueue(queue);
+  }
+
   // ============================================================
   // Mutations (optimistic: update UI/cache first, then sync)
   // ============================================================