diff --git a/lib/reducers/message-reducer.js b/lib/reducers/message-reducer.js
--- a/lib/reducers/message-reducer.js
+++ b/lib/reducers/message-reducer.js
@@ -76,7 +76,6 @@
   type ReplaceMessageOperation,
   type MessageStoreOperation,
   type MessageTruncationStatus,
-  type ThreadMessageInfo,
   type MessageTruncationStatuses,
   messageTruncationStatus,
   messageTypes,
@@ -1521,23 +1520,31 @@
     const messageIDs =
       messageStore.threads[messageInfo.threadID]?.messageIDs ?? [];
     const now = Date.now();
-    const threadState: ThreadMessageInfo = messageStore.threads[threadID]
-      ? {
-          ...messageStore.threads[threadID],
-          messageIDs: [localID, ...messageIDs],
-        }
-      : {
-          messageIDs: [localID],
-          startReached: true,
-          lastNavigatedTo: now,
-          lastPruned: now,
-        };
+    const updatedThreads = {
+      [threadID]: messageStore.threads[threadID]
+        ? {
+            ...messageStore.threads[threadID],
+            messageIDs: [localID, ...messageIDs],
+          }
+        : {
+            messageIDs: [localID],
+            startReached: true,
+            lastNavigatedTo: now,
+            lastPruned: now,
+          },
+    };
 
     const messageStoreOperations = [
       {
         type: 'replace',
         payload: { id: localID, messageInfo },
       },
+      {
+        type: 'replace_threads',
+        payload: {
+          threads: { ...updatedThreads },
+        },
+      },
     ];
 
     const processedMessageStore = processMessageStoreOperations(
@@ -1545,16 +1552,24 @@
       messageStoreOperations,
     );
 
+    const newMessageStore = {
+      ...messageStore,
+      threads: {
+        ...messageStore.threads,
+        ...updatedThreads,
+      },
+      messages: processedMessageStore.messages,
+    };
+
+    assertMessageStoreThreadsAreEqual(
+      processedMessageStore,
+      newMessageStore,
+      action.type,
+    );
+
     return {
       messageStoreOperations,
-      messageStore: {
-        ...messageStore,
-        threads: {
-          ...messageStore.threads,
-          [threadID]: threadState,
-        },
-        messages: processedMessageStore.messages,
-      },
+      messageStore: newMessageStore,
     };
   } else if (action.type === processServerRequestsActionType) {
     const {