diff --git a/lib/shared/dm-ops/process-dm-ops.js b/lib/shared/dm-ops/process-dm-ops.js
--- a/lib/shared/dm-ops/process-dm-ops.js
+++ b/lib/shared/dm-ops/process-dm-ops.js
@@ -15,8 +15,13 @@
   processDMOpsActionType,
   queueDMOpsActionType,
 } from '../../types/dm-ops.js';
+import type { RawThreadInfo } from '../../types/minimally-encoded-thread-permissions-types.js';
+import { threadTypes } from '../../types/thread-types-enum.js';
+import type { LegacyRawThreadInfo } from '../../types/thread-types.js';
 import { updateTypes } from '../../types/update-types-enum.js';
 import { useSelector } from '../../utils/redux-utils.js';
+import { messageSpecs } from '../messages/message-specs.js';
+import { updateSpecs } from '../updates/update-specs.js';
 
 function useProcessDMOperation(): (
   dmOp: DMOperation,
@@ -72,7 +77,43 @@
       const messagesByThreadID = _groupBy(message => message.threadID)(
         allNewMessageInfos,
       );
+
+      const updatedThickSidebars = updateInfos
+        .map(update => updateSpecs[update.type].getUpdatedThreadInfo?.(update))
+        .filter(thread => thread?.type === threadTypes.THICK_SIDEBAR);
+      const updatedThreadInfosByThreadID: {
+        [string]: RawThreadInfo | LegacyRawThreadInfo,
+      } = {};
+      for (const threadInfo of updatedThickSidebars) {
+        if (threadInfo?.id) {
+          // We can have multiple thread infos with the same ID, and we want
+          // to keep the last one.
+          updatedThreadInfosByThreadID[threadInfo.id] = threadInfo;
+        }
+      }
+
       for (const threadID in messagesByThreadID) {
+        const repliesCountIncreasingMessages = messagesByThreadID[
+          threadID
+        ].filter(message => messageSpecs[message.type].includedInRepliesCount);
+        if (repliesCountIncreasingMessages.length > 0) {
+          const threadInfo =
+            updatedThreadInfosByThreadID[threadID] ?? threadInfos[threadID];
+          const repliesCountIncreaseTime = Math.max(
+            repliesCountIncreasingMessages.map(message => message.time),
+          );
+          updateInfos.push({
+            type: updateTypes.UPDATE_THREAD,
+            id: uuid.v4(),
+            time: repliesCountIncreaseTime,
+            threadInfo: {
+              ...threadInfo,
+              repliesCount:
+                threadInfo.repliesCount + repliesCountIncreasingMessages.length,
+            },
+          });
+        }
+
         const messagesFromOtherPeers = messagesByThreadID[threadID].filter(
           message => message.creatorID !== viewerID,
         );
@@ -104,7 +145,7 @@
         metadata,
       );
     },
-    [viewerID, utilities, dispatchWithMessageSource],
+    [viewerID, utilities, dispatchWithMessageSource, threadInfos],
   );
 }