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
@@ -100,8 +100,11 @@
   processUpdatesActionType,
 } from '../types/update-types.js';
 import { setNewSessionActionType } from '../utils/action-utils.js';
+import { isDev } from '../utils/dev-utils.js';
 import { translateClientDBMessageInfosToRawMessageInfos } from '../utils/message-ops-utils.js';
+import { assertObjectsAreEqual } from '../utils/objects.js';
 
+const PROCESSED_MSG_STORE_INVARIANTS_DISABLED = !isDev;
 const _mapValuesWithKeys = _mapValues.convert({ cap: false });
 
 // Input must already be ordered!
@@ -133,6 +136,22 @@
   );
 }
 
+function assertMessageStoreThreadsAreEqual(
+  processedMessageStore: MessageStore,
+  expectedMessageStore: MessageStore,
+  location: string,
+) {
+  if (PROCESSED_MSG_STORE_INVARIANTS_DISABLED) {
+    return;
+  }
+
+  assertObjectsAreEqual(
+    processedMessageStore.threads,
+    expectedMessageStore.threads,
+    `MessageStore.threads - ${location}`,
+  );
+}
+
 const newThread = () => ({
   messageIDs: [],
   startReached: false,