diff --git a/npiai/tools/web/scraper/app.py b/npiai/tools/web/scraper/app.py
index 91a7b6a..5372dc9 100644
--- a/npiai/tools/web/scraper/app.py
+++ b/npiai/tools/web/scraper/app.py
@@ -133,6 +133,7 @@ async def summarize_stream(
         batch_index = 0
 
         results_queue: asyncio.Queue[SummaryChunk] = asyncio.Queue()
+        lock = asyncio.Lock()
 
         skip_item_hashes_set = set(skip_item_hashes) if skip_item_hashes else None
 
@@ -142,14 +143,17 @@ async def run_batch():
             if limit != -1 and remaining <= 0:
                 return
 
-            current_index = batch_index
-            batch_index += 1
+            async with lock:
+                current_index = batch_index
+                batch_index += 1
 
-            # calculate the number of items to summarize in the current batch
-            requested_count = min(self._batch_size, remaining) if limit != -1 else -1
-            # reduce the remaining count by the number of items in the current batch
-            # so that the other tasks will not exceed the limit
-            remaining -= requested_count
+                # calculate the number of items to summarize in the current batch
+                requested_count = (
+                    min(self._batch_size, remaining) if limit != -1 else -1
+                )
+                # reduce the remaining count by the number of items in the current batch
+                # so that the other tasks will not exceed the limit
+                remaining -= requested_count
 
             parsed_result = await self._convert(
                 ancestor_selector=ancestor_selector,
@@ -179,12 +183,13 @@ async def run_batch():
                 await ctx.send_debug_message(f"[{self.name}] No items summarized")
                 return
 
-            items_slice = items[:requested_count] if limit != -1 else items
-            summarized_count = len(items_slice)
-            count += summarized_count
-            # correct the remaining count in case summary returned fewer items than requested
-            if summarized_count < requested_count:
-                remaining += requested_count - summarized_count
+            async with lock:
+                items_slice = items[:requested_count] if limit != -1 else items
+                summarized_count = len(items_slice)
+                count += summarized_count
+                # recalculate the remaining count in case summary returned fewer items than requested
+                if summarized_count < requested_count:
+                    remaining += requested_count - summarized_count
 
             await results_queue.put(
                 {