diff --git a/src/lib/offline/queue.test.ts b/src/lib/offline/queue.test.ts
index f5e6d0f..550a0c3 100644
--- a/src/lib/offline/queue.test.ts
+++ b/src/lib/offline/queue.test.ts
@@ -2,7 +2,15 @@
 // src/lib/offline/queue.test.ts
 import { describe, it, expect, beforeEach } from "vitest";
 import "fake-indexeddb/auto";
-import { enqueueAction, dequeueAction, getQueuedActions, markSynced, markConflict } from "./queue";
+import {
+  enqueueAction,
+  dequeueAction,
+  getQueuedActions,
+  markSynced,
+  markConflict,
+  markInFlight,
+  markFailed,
+} from "./queue";
 
 beforeEach(async () => {
   // Clear the queue between tests
@@ -61,4 +69,45 @@ describe("offline queue", () => {
     expect(all[0].status).toBe("conflict");
     expect(all[0].error).toBe("stale updated_at");
   });
+
+  it("markInFlight transitions the action to in-flight and records an attempt", async () => {
+    const a = await enqueueAction("markOrderReady", { orderId: "abc" });
+    const before = await getQueuedActions();
+    expect(before[0].attempts).toBe(0);
+    expect(before[0].status).toBe("pending");
+    expect(before[0].lastAttemptAt).toBeNull();
+
+    const beforeMs = Date.now();
+    await markInFlight(a.id);
+    const afterMs = Date.now();
+
+    const all = await getQueuedActions();
+    expect(all).toHaveLength(1);
+    expect(all[0].status).toBe("in-flight");
+    expect(all[0].attempts).toBe(1);
+    expect(typeof all[0].lastAttemptAt).toBe("number");
+    expect(all[0].lastAttemptAt).not.toBeNull();
+    expect(all[0].lastAttemptAt!).toBeGreaterThanOrEqual(beforeMs);
+    expect(all[0].lastAttemptAt!).toBeLessThanOrEqual(afterMs);
+  });
+
+  it("markFailed transitions the action to failed and increments attempts", async () => {
+    const a = await enqueueAction("markOrderReady", { orderId: "abc" });
+    // Pretend it has already been tried once
+    await markInFlight(a.id);
+
+    const beforeMs = Date.now();
+    await markFailed(a.id, "server returned 500");
+    const afterMs = Date.now();
+
+    const all = await getQueuedActions();
+    expect(all).toHaveLength(1);
+    expect(all[0].status).toBe("failed");
+    expect(all[0].attempts).toBe(2); // markInFlight set it to 1, markFailed adds 1
+    expect(all[0].error).toBe("server returned 500");
+    expect(typeof all[0].lastAttemptAt).toBe("number");
+    expect(all[0].lastAttemptAt).not.toBeNull();
+    expect(all[0].lastAttemptAt!).toBeGreaterThanOrEqual(beforeMs);
+    expect(all[0].lastAttemptAt!).toBeLessThanOrEqual(afterMs);
+  });
 });
diff --git a/src/lib/offline/queue.ts b/src/lib/offline/queue.ts
index f4d3b6d..1886754 100644
--- a/src/lib/offline/queue.ts
+++ b/src/lib/offline/queue.ts
@@ -13,8 +13,15 @@ export async function enqueueAction(
 ): Promise<QueuedAction> {
   const db = await getOfflineDB();
   const id = options.id ?? uuidv4();
-  const existing = await db.get("queue", id);
-  if (existing) return existing;
+  // Atomic read-then-write inside a single readwrite transaction so two
+  // concurrent calls with the same explicit `id` can't both pass the
+  // existence check and write twice.
+  const tx = db.transaction("queue", "readwrite");
+  const existing = await tx.store.get(id);
+  if (existing) {
+    await tx.done;
+    return existing;
+  }
   const action: QueuedAction = {
     id,
     actionName,
@@ -24,7 +31,8 @@ export async function enqueueAction(
     lastAttemptAt: null,
     status: "pending",
   };
-  await db.put("queue", action);
+  await tx.store.put(action);
+  await tx.done;
   return action;
 }
 
@@ -49,12 +57,17 @@ export async function dequeueAction(id: string): Promise<QueuedAction | null> {
 
 export async function markInFlight(id: string): Promise<void> {
   const db = await getOfflineDB();
-  const action = await db.get("queue", id);
-  if (!action) return;
+  const tx = db.transaction("queue", "readwrite");
+  const action = await tx.store.get(id);
+  if (!action) {
+    await tx.done;
+    return;
+  }
   action.status = "in-flight";
   action.lastAttemptAt = Date.now();
   action.attempts += 1;
-  await db.put("queue", action);
+  await tx.store.put(action);
+  await tx.done;
 }
 
 export async function markSynced(id: string): Promise<void> {
@@ -64,20 +77,30 @@ export async function markSynced(id: string): Promise<void> {
 
 export async function markConflict(id: string, error: string): Promise<void> {
   const db = await getOfflineDB();
-  const action = await db.get("queue", id);
-  if (!action) return;
+  const tx = db.transaction("queue", "readwrite");
+  const action = await tx.store.get(id);
+  if (!action) {
+    await tx.done;
+    return;
+  }
   action.status = "conflict";
   action.error = error;
-  await db.put("queue", action);
+  await tx.store.put(action);
+  await tx.done;
 }
 
 export async function markFailed(id: string, error: string): Promise<void> {
   const db = await getOfflineDB();
-  const action = await db.get("queue", id);
-  if (!action) return;
+  const tx = db.transaction("queue", "readwrite");
+  const action = await tx.store.get(id);
+  if (!action) {
+    await tx.done;
+    return;
+  }
   action.status = "failed";
   action.error = error;
   action.attempts += 1;
   action.lastAttemptAt = Date.now();
-  await db.put("queue", action);
+  await tx.store.put(action);
+  await tx.done;
 }
diff --git a/src/lib/offline/sync.test.ts b/src/lib/offline/sync.test.ts
index 7a4489b..ce233dc 100644
--- a/src/lib/offline/sync.test.ts
+++ b/src/lib/offline/sync.test.ts
@@ -61,4 +61,100 @@ describe("offline sync — replay", () => {
     expect(dispatcher).not.toHaveBeenCalled();
     expect(result.synced).toBe(0);
   });
+
+  it("syncPending is a no-op for an empty queue and does not call the dispatcher", async () => {
+    const dispatcher = vi.fn();
+    const result = await syncPending(dispatcher, { online: true, sleep: () => Promise.resolve() });
+    expect(dispatcher).not.toHaveBeenCalled();
+    expect(result).toEqual({ synced: 0, conflicts: 0, failed: 0 });
+  });
+
+  it("syncPending moves an action to failed once max attempts is reached", async () => {
+    // Seed an action that's already at the max-attempt threshold
+    await enqueueAction("markOrderReady", { orderId: "abc" }, { id: "max-attempts-id" });
+    const { getOfflineDB } = await import("./db");
+    const db = await getOfflineDB();
+    const tx = db.transaction("queue", "readwrite");
+    const a = await tx.store.get("max-attempts-id");
+    if (!a) throw new Error("seed not found");
+    a.attempts = MAX_ATTEMPTS; // 5
+    await tx.store.put(a);
+    await tx.done;
+
+    const dispatcher = vi.fn();
+    const result = await syncPending(dispatcher, { online: true, sleep: () => Promise.resolve() });
+    expect(dispatcher).not.toHaveBeenCalled();
+    expect(result.failed).toBe(1);
+    expect(result.synced).toBe(0);
+    expect(result.conflicts).toBe(0);
+
+    const { getQueuedActions } = await import("./queue");
+    const all = await getQueuedActions();
+    expect(all).toHaveLength(1);
+    expect(all[0].status).toBe("failed");
+    expect(all[0].error).toBe("max attempts reached");
+  });
+
+  it("syncPending skips actions whose backoff window has not yet elapsed", async () => {
+    await enqueueAction("markOrderReady", { orderId: "abc" }, { id: "backoff-id" });
+    // Pretend a previous attempt happened 100ms ago. With attempts=0, the
+    // backoff schedule says we must wait 1000ms — so we should skip.
+    const fixedNow = 1_700_000_000_000;
+    const { getOfflineDB } = await import("./db");
+    const db = await getOfflineDB();
+    const tx = db.transaction("queue", "readwrite");
+    const a = await tx.store.get("backoff-id");
+    if (!a) throw new Error("seed not found");
+    a.attempts = 0;
+    a.lastAttemptAt = fixedNow - 100;
+    await tx.store.put(a);
+    await tx.done;
+
+    const dispatcher = vi.fn();
+    const result = await syncPending(dispatcher, {
+      online: true,
+      sleep: () => Promise.resolve(),
+      now: () => fixedNow,
+    });
+    expect(dispatcher).not.toHaveBeenCalled();
+    expect(result.synced).toBe(0);
+    expect(result.conflicts).toBe(0);
+    expect(result.failed).toBe(0);
+
+    // Action is still pending (not advanced to in-flight or failed)
+    const { getQueuedActions } = await import("./queue");
+    const all = await getQueuedActions();
+    expect(all).toHaveLength(1);
+    expect(all[0].status).toBe("pending");
+  });
+
+  it("syncPending marks the action as failed when the dispatcher throws", async () => {
+    const a = await enqueueAction("markOrderReady", { orderId: "abc" });
+    const dispatcher = vi.fn().mockRejectedValue(new Error("network down"));
+    const result = await syncPending(dispatcher, { online: true, sleep: () => Promise.resolve() });
+    expect(dispatcher).toHaveBeenCalledWith("markOrderReady", { orderId: "abc" }, a.id);
+    expect(result.failed).toBe(1);
+    expect(result.synced).toBe(0);
+    expect(result.conflicts).toBe(0);
+
+    const { getQueuedActions } = await import("./queue");
+    const all = await getQueuedActions();
+    expect(all).toHaveLength(1);
+    expect(all[0].status).toBe("failed");
+    expect(all[0].error).toBe("network down");
+  });
+
+  it("syncPending leaves queued actions untouched when offline even if the queue is non-empty", async () => {
+    await enqueueAction("markOrderReady", { orderId: "abc" });
+    await enqueueAction("markOrderReady", { orderId: "def" });
+    const dispatcher = vi.fn();
+    const result = await syncPending(dispatcher, { online: false, sleep: () => Promise.resolve() });
+    expect(dispatcher).not.toHaveBeenCalled();
+    expect(result).toEqual({ synced: 0, conflicts: 0, failed: 0 });
+
+    const { getQueuedActions } = await import("./queue");
+    const all = await getQueuedActions();
+    expect(all).toHaveLength(2);
+    expect(all.every((x) => x.status === "pending")).toBe(true);
+  });
 });
diff --git a/src/lib/offline/sync.ts b/src/lib/offline/sync.ts
index 387e422..65c65ed 100644
--- a/src/lib/offline/sync.ts
+++ b/src/lib/offline/sync.ts
@@ -27,47 +27,67 @@ export interface SyncResult {
   failed: number;
 }
 
+// Re-entrancy guard: concurrent `online` events shouldn't kick off parallel
+// sync passes that race on the same `markInFlight`/`markSynced` records.
+let inFlightSync: Promise<SyncResult> | null = null;
+
 export async function syncPending(dispatcher: Dispatcher, options: SyncOptions): Promise<SyncResult> {
-  const result: SyncResult = { synced: 0, conflicts: 0, failed: 0 };
-  if (!options.online) return result;
+  if (inFlightSync) return inFlightSync;
+  inFlightSync = (async () => {
+    const result: SyncResult = { synced: 0, conflicts: 0, failed: 0 };
+    if (!options.online) return result;
 
-  const actions = await getQueuedActions();
-  for (const action of actions) {
-    if (action.status === "conflict" || action.status === "failed") {
-      // Don't auto-retry conflicts/failures; require manual user intervention
-      continue;
-    }
-    if (action.attempts >= MAX_ATTEMPTS) {
-      await markFailed(action.id, "max attempts reached");
-      result.failed += 1;
-      continue;
-    }
+    const actions = await getQueuedActions();
+    for (const action of actions) {
+      if (action.status === "conflict" || action.status === "failed") {
+        // Don't auto-retry conflicts/failures; require manual user intervention
+        continue;
+      }
+      if (action.attempts >= MAX_ATTEMPTS) {
+        await markFailed(action.id, "max attempts reached");
+        result.failed += 1;
+        continue;
+      }
 
-    const backoff = calculateBackoff(action.attempts);
-    if (action.lastAttemptAt) {
-      const elapsed = (options.now?.() ?? Date.now()) - action.lastAttemptAt;
-      if (elapsed < backoff) continue; // not time yet
-    }
+      const backoff = calculateBackoff(action.attempts);
+      if (action.lastAttemptAt) {
+        const elapsed = (options.now?.() ?? Date.now()) - action.lastAttemptAt;
+        if (elapsed < backoff) continue; // not time yet
+      }
 
-    await markInFlight(action.id);
-    try {
-      const dispatchResult = await dispatcher(action.actionName, action.payload, action.id);
-      if (dispatchResult.ok) {
-        await markSynced(action.id);
-        result.synced += 1;
-      } else if ("conflict" in dispatchResult) {
-        await markConflict(action.id, dispatchResult.conflict);
-        result.conflicts += 1;
-      } else {
-        await markFailed(action.id, dispatchResult.error);
+      await markInFlight(action.id);
+      try {
+        const dispatchResult = await dispatcher(action.actionName, action.payload, action.id);
+        if (dispatchResult.ok) {
+          await markSynced(action.id);
+          result.synced += 1;
+        } else if ("conflict" in dispatchResult) {
+          await markConflict(action.id, dispatchResult.conflict);
+          result.conflicts += 1;
+        } else if ("error" in dispatchResult) {
+          await markFailed(action.id, dispatchResult.error);
+          result.failed += 1;
+        }
+      } catch (err) {
+        // A single poisoned record (IDB closed, quota, etc.) must not abort
+        // the whole sync pass. Swallow the secondary failure with a warn so
+        // it stays observable in dev.
+        const message = err instanceof Error ? err.message : String(err);
+        try {
+          await markFailed(action.id, message);
+        } catch (innerErr) {
+          console.warn("[offline-sync] markFailed threw for", action.id, innerErr);
+        }
         result.failed += 1;
       }
-    } catch (err) {
-      await markFailed(action.id, (err as Error).message);
-      result.failed += 1;
     }
+    return result;
+  })();
+  try {
+    return await inFlightSync;
+  } finally {
+    inFlightSync = null;
   }
-  return result;
 }
 
 /**