diff --git a/lib/shared/dm-ops/add-members-spec.js b/lib/shared/dm-ops/add-members-spec.js
--- a/lib/shared/dm-ops/add-members-spec.js
+++ b/lib/shared/dm-ops/add-members-spec.js
@@ -11,6 +11,7 @@
   DMOperationSpec,
   ProcessDMOperationUtilities,
 } from './dm-op-spec.js';
+import { createUpdateUnreadCountUpdate } from './dm-op-utils.js';
 import type { DMAddMembersOperation } from '../../types/dm-ops.js';
 import { messageTypes } from '../../types/message-types-enum.js';
 import {
@@ -79,6 +80,17 @@
         unread: true,
       },
     );
+    const repliesCountUpdate = createUpdateUnreadCountUpdate(newThread, [
+      addMembersMessage,
+    ]);
+    if (
+      repliesCountUpdate &&
+      repliesCountUpdate.type === updateTypes.UPDATE_THREAD
+    ) {
+      updateInfos.push(repliesCountUpdate);
+      resultThreadInfo.repliesCount =
+        repliesCountUpdate.threadInfo.repliesCount;
+    }
   } else {
     const currentThreadInfoOptional =
       utilities.threadInfos[existingThreadDetails.threadID];
@@ -117,8 +129,12 @@
       members: [...currentThreadInfo.members, ...newMembers],
     };
     resultThreadInfo = newThreadInfo;
+    const updateWithRepliesCount = createUpdateUnreadCountUpdate(
+      newThreadInfo,
+      [addMembersMessage],
+    );
     updateInfos.push(
-      {
+      updateWithRepliesCount ?? {
         type: updateTypes.UPDATE_THREAD,
         id: uuid.v4(),
         time,
diff --git a/lib/shared/dm-ops/change-thread-settings-spec.js b/lib/shared/dm-ops/change-thread-settings-spec.js
--- a/lib/shared/dm-ops/change-thread-settings-spec.js
+++ b/lib/shared/dm-ops/change-thread-settings-spec.js
@@ -7,6 +7,7 @@
   DMOperationSpec,
   ProcessDMOperationUtilities,
 } from './dm-op-spec.js';
+import { createUpdateUnreadCountUpdate } from './dm-op-utils.js';
 import type { DMChangeThreadSettingsOperation } from '../../types/dm-ops.js';
 import { messageTypes } from '../../types/message-types-enum.js';
 import type { RawMessageInfo } from '../../types/message-types.js';
@@ -126,7 +127,13 @@
         });
       }
 
-      if (values(changedFields).length > 0) {
+      const repliesCountUpdate = createUpdateUnreadCountUpdate(
+        threadInfoToUpdate,
+        rawMessageInfos,
+      );
+      if (repliesCountUpdate) {
+        updateInfos.push(repliesCountUpdate);
+      } else if (values(changedFields).length > 0) {
         updateInfos.push({
           type: updateTypes.UPDATE_THREAD,
           id: uuid.v4(),
diff --git a/lib/shared/dm-ops/create-sidebar-spec.js b/lib/shared/dm-ops/create-sidebar-spec.js
--- a/lib/shared/dm-ops/create-sidebar-spec.js
+++ b/lib/shared/dm-ops/create-sidebar-spec.js
@@ -48,6 +48,7 @@
           creatorID,
           sourceMessageID,
           containingThreadID: parentThreadID,
+          repliesCount: 1,
         },
         viewerID,
       );
diff --git a/lib/shared/dm-ops/dm-op-utils.js b/lib/shared/dm-ops/dm-op-utils.js
--- a/lib/shared/dm-ops/dm-op-utils.js
+++ b/lib/shared/dm-ops/dm-op-utils.js
@@ -4,13 +4,21 @@
 
 import type { AuxUserStore } from '../../types/aux-user-types.js';
 import type { DMOperation } from '../../types/dm-ops.js';
+import { messageTypes } from '../../types/message-types-enum.js';
+import type { RawMessageInfo } from '../../types/message-types.js';
+import type { RawThreadInfo } from '../../types/minimally-encoded-thread-permissions-types.js';
 import {
   outboundP2PMessageStatuses,
   type OutboundP2PMessage,
 } from '../../types/sqlite-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 type { ClientUpdateInfo } from '../../types/update-types.js';
 import type { CurrentUserInfo } from '../../types/user-types.js';
 import { getContentSigningKey } from '../../utils/crypto-utils.js';
 import { values } from '../../utils/objects.js';
+import { messageSpecs } from '../messages/message-specs.js';
 
 function generateMessagesToPeers(
   message: DMOperation,
@@ -66,4 +74,35 @@
   );
 }
 
-export { createMessagesToPeersFromDMOp };
+function createUpdateUnreadCountUpdate(
+  threadInfo: RawThreadInfo | LegacyRawThreadInfo,
+  newMessages: $ReadOnlyArray<RawMessageInfo>,
+): ?ClientUpdateInfo {
+  if (threadInfo.type !== threadTypes.THICK_SIDEBAR) {
+    return null;
+  }
+  const includedMessageTypes = new Set(
+    Object.keys(messageTypes)
+      .map(key => messageTypes[key])
+      .filter(type => messageSpecs[type].includedInRepliesCount),
+  );
+  const filteredMessages = newMessages.filter(message =>
+    includedMessageTypes.has(message.type),
+  );
+  const countIncrease = filteredMessages.length;
+  if (countIncrease === 0) {
+    return null;
+  }
+  const time = Math.max(...filteredMessages.map(message => message.time));
+  return {
+    type: updateTypes.UPDATE_THREAD,
+    id: uuid.v4(),
+    time,
+    threadInfo: {
+      ...threadInfo,
+      repliesCount: threadInfo.repliesCount + countIncrease,
+    },
+  };
+}
+
+export { createMessagesToPeersFromDMOp, createUpdateUnreadCountUpdate };
diff --git a/lib/shared/dm-ops/join-thread-spec.js b/lib/shared/dm-ops/join-thread-spec.js
--- a/lib/shared/dm-ops/join-thread-spec.js
+++ b/lib/shared/dm-ops/join-thread-spec.js
@@ -11,6 +11,7 @@
   DMOperationSpec,
   ProcessDMOperationUtilities,
 } from './dm-op-spec.js';
+import { createUpdateUnreadCountUpdate } from './dm-op-utils.js';
 import type { DMJoinThreadOperation } from '../../types/dm-ops.js';
 import { messageTypes } from '../../types/message-types-enum.js';
 import {
@@ -64,18 +65,19 @@
     const updateInfos: Array<ClientUpdateInfo> = [];
     const rawMessageInfos: Array<RawMessageInfo> = [];
     if (viewerID === editorID) {
+      const newThreadInfo = createThickRawThreadInfo(
+        {
+          ...existingThreadDetails,
+          allMemberIDs: [...existingThreadDetails.allMemberIDs, editorID],
+        },
+        viewerID,
+      );
       updateInfos.push(
         {
           type: updateTypes.JOIN_THREAD,
           id: uuid.v4(),
           time,
-          threadInfo: createThickRawThreadInfo(
-            {
-              ...existingThreadDetails,
-              allMemberIDs: [...existingThreadDetails.allMemberIDs, editorID],
-            },
-            viewerID,
-          ),
+          threadInfo: newThreadInfo,
           rawMessageInfos: [joinThreadMessage],
           truncationStatus: messageTruncationStatus.EXHAUSTIVE,
           rawEntryInfos: [],
@@ -88,6 +90,12 @@
           unread: true,
         },
       );
+      const repliesCountUpdate = createUpdateUnreadCountUpdate(newThreadInfo, [
+        joinThreadMessage,
+      ]);
+      if (repliesCountUpdate) {
+        updateInfos.push(repliesCountUpdate);
+      }
     } else {
       if (!currentThreadInfoOptional || !currentThreadInfoOptional.thick) {
         // We can't perform this operation now. It should be queued for later.
@@ -120,8 +128,12 @@
         ...currentThreadInfo,
         members: [...currentThreadInfo.members, member],
       };
+      const updateWithRepliesCount = createUpdateUnreadCountUpdate(
+        updatedThreadInfo,
+        [joinThreadMessage],
+      );
       updateInfos.push(
-        {
+        updateWithRepliesCount ?? {
           type: updateTypes.UPDATE_THREAD,
           id: uuid.v4(),
           time,
diff --git a/lib/shared/dm-ops/leave-thread-spec.js b/lib/shared/dm-ops/leave-thread-spec.js
--- a/lib/shared/dm-ops/leave-thread-spec.js
+++ b/lib/shared/dm-ops/leave-thread-spec.js
@@ -6,6 +6,7 @@
   DMOperationSpec,
   ProcessDMOperationUtilities,
 } from './dm-op-spec.js';
+import { createUpdateUnreadCountUpdate } from './dm-op-utils.js';
 import type { DMLeaveThreadOperation } from '../../types/dm-ops.js';
 import { messageTypes } from '../../types/message-types-enum.js';
 import type { ThickRawThreadInfo } from '../../types/minimally-encoded-thread-permissions-types.js';
@@ -59,8 +60,12 @@
         ...threadInfo,
         members: threadInfo.members.filter(member => member.id !== editorID),
       };
+      const updateWithRepliesCount = createUpdateUnreadCountUpdate(
+        updatedThreadInfo,
+        [leaveThreadMessage],
+      );
       updateInfos.push(
-        {
+        updateWithRepliesCount ?? {
           type: updateTypes.UPDATE_THREAD,
           id: uuid.v4(),
           time,
diff --git a/lib/shared/dm-ops/remove-members-spec.js b/lib/shared/dm-ops/remove-members-spec.js
--- a/lib/shared/dm-ops/remove-members-spec.js
+++ b/lib/shared/dm-ops/remove-members-spec.js
@@ -6,6 +6,7 @@
   DMOperationSpec,
   ProcessDMOperationUtilities,
 } from './dm-op-spec.js';
+import { createUpdateUnreadCountUpdate } from './dm-op-utils.js';
 import type { DMRemoveMembersOperation } from '../../types/dm-ops.js';
 import { messageTypes } from '../../types/message-types-enum.js';
 import type { ThickRawThreadInfo } from '../../types/minimally-encoded-thread-permissions-types.js';
@@ -64,8 +65,12 @@
             member => !removedUserIDsSet.has(member.id),
           ),
         };
+        const updateWithRepliesCount = createUpdateUnreadCountUpdate(
+          updatedThreadInfo,
+          [removeMembersMessage],
+        );
         updateInfos.push(
-          {
+          updateWithRepliesCount ?? {
             type: updateTypes.UPDATE_THREAD,
             id: uuid.v4(),
             time,
diff --git a/lib/shared/dm-ops/send-edit-message-spec.js b/lib/shared/dm-ops/send-edit-message-spec.js
--- a/lib/shared/dm-ops/send-edit-message-spec.js
+++ b/lib/shared/dm-ops/send-edit-message-spec.js
@@ -2,7 +2,11 @@
 
 import uuid from 'uuid';
 
-import type { DMOperationSpec } from './dm-op-spec.js';
+import type {
+  DMOperationSpec,
+  ProcessDMOperationUtilities,
+} from './dm-op-spec.js';
+import { createUpdateUnreadCountUpdate } from './dm-op-utils.js';
 import type { DMSendEditMessageOperation } from '../../types/dm-ops.js';
 import { messageTypes } from '../../types/message-types-enum.js';
 import { updateTypes } from '../../types/update-types-enum.js';
@@ -13,6 +17,7 @@
     processDMOperation: async (
       dmOperation: DMSendEditMessageOperation,
       viewerID: string,
+      utilities: ProcessDMOperationUtilities,
     ) => {
       const { threadID, creatorID, time, messageID, targetMessageID, text } =
         dmOperation;
@@ -36,6 +41,15 @@
           unread: true,
         });
       }
+      const threadInfo = utilities.threadInfos[threadID];
+      if (threadInfo) {
+        const repliesCountUpdate = createUpdateUnreadCountUpdate(threadInfo, [
+          editMessage,
+        ]);
+        if (repliesCountUpdate) {
+          updateInfos.push(repliesCountUpdate);
+        }
+      }
       return {
         rawMessageInfos: [editMessage],
         updateInfos,
diff --git a/lib/shared/dm-ops/send-reaction-message-spec.js b/lib/shared/dm-ops/send-reaction-message-spec.js
--- a/lib/shared/dm-ops/send-reaction-message-spec.js
+++ b/lib/shared/dm-ops/send-reaction-message-spec.js
@@ -2,7 +2,11 @@
 
 import uuid from 'uuid';
 
-import type { DMOperationSpec } from './dm-op-spec.js';
+import type {
+  DMOperationSpec,
+  ProcessDMOperationUtilities,
+} from './dm-op-spec.js';
+import { createUpdateUnreadCountUpdate } from './dm-op-utils.js';
 import type { DMSendReactionMessageOperation } from '../../types/dm-ops.js';
 import { messageTypes } from '../../types/message-types-enum.js';
 import { updateTypes } from '../../types/update-types-enum.js';
@@ -13,6 +17,7 @@
     processDMOperation: async (
       dmOperation: DMSendReactionMessageOperation,
       viewerID: string,
+      utilities: ProcessDMOperationUtilities,
     ) => {
       const {
         threadID,
@@ -44,6 +49,15 @@
           unread: true,
         });
       }
+      const threadInfo = utilities.threadInfos[threadID];
+      if (threadInfo) {
+        const repliesCountUpdate = createUpdateUnreadCountUpdate(threadInfo, [
+          reactionMessage,
+        ]);
+        if (repliesCountUpdate) {
+          updateInfos.push(repliesCountUpdate);
+        }
+      }
       return {
         rawMessageInfos: [reactionMessage],
         updateInfos,
diff --git a/lib/shared/dm-ops/send-text-message-spec.js b/lib/shared/dm-ops/send-text-message-spec.js
--- a/lib/shared/dm-ops/send-text-message-spec.js
+++ b/lib/shared/dm-ops/send-text-message-spec.js
@@ -2,7 +2,11 @@
 
 import uuid from 'uuid';
 
-import type { DMOperationSpec } from './dm-op-spec.js';
+import type {
+  DMOperationSpec,
+  ProcessDMOperationUtilities,
+} from './dm-op-spec.js';
+import { createUpdateUnreadCountUpdate } from './dm-op-utils.js';
 import type { DMSendTextMessageOperation } from '../../types/dm-ops.js';
 import { messageTypes } from '../../types/message-types-enum.js';
 import { updateTypes } from '../../types/update-types-enum.js';
@@ -13,6 +17,7 @@
     processDMOperation: async (
       dmOperation: DMSendTextMessageOperation,
       viewerID: string,
+      utilities: ProcessDMOperationUtilities,
     ) => {
       const { threadID, creatorID, time, messageID, text } = dmOperation;
       const textMessage = {
@@ -33,6 +38,15 @@
           unread: true,
         });
       }
+      const threadInfo = utilities.threadInfos[threadID];
+      if (threadInfo) {
+        const repliesCountUpdate = createUpdateUnreadCountUpdate(threadInfo, [
+          textMessage,
+        ]);
+        if (repliesCountUpdate) {
+          updateInfos.push(repliesCountUpdate);
+        }
+      }
       return {
         rawMessageInfos: [textMessage],
         updateInfos,