diff --git a/lib/utils/thread-ops-utils.js b/lib/utils/thread-ops-utils.js
--- a/lib/utils/thread-ops-utils.js
+++ b/lib/utils/thread-ops-utils.js
@@ -1,10 +1,20 @@
 // @flow
 
-import type { MinimallyEncodedRawThreadInfo } from '../types/minimally-encoded-thread-permissions-types.js';
-import { decodeMinimallyEncodedRawThreadInfo } from '../types/minimally-encoded-thread-permissions-types.js';
+import invariant from 'invariant';
+
+import { minimallyEncodedMemberInfoValidator } from '../permissions/minimally-encoded-thread-permissions-validators.js';
+import type {
+  MinimallyEncodedMemberInfo,
+  MinimallyEncodedRawThreadInfo,
+} from '../types/minimally-encoded-thread-permissions-types.js';
+import {
+  decodeMinimallyEncodedRawThreadInfo,
+  minimallyEncodeMemberInfo,
+} from '../types/minimally-encoded-thread-permissions-types.js';
 import { assertThreadType } from '../types/thread-types-enum.js';
 import {
   type ClientDBThreadInfo,
+  legacyMemberInfoValidator,
   type LegacyRawThreadInfo,
   type RawThreadInfo,
 } from '../types/thread-types.js';
@@ -26,6 +36,22 @@
 function convertClientDBThreadInfoToRawThreadInfo(
   clientDBThreadInfo: ClientDBThreadInfo,
 ): MinimallyEncodedRawThreadInfo {
+  // 1. Validate and potentially minimally encode `rawMembers`.
+  const rawMembers = JSON.parse(clientDBThreadInfo.members);
+  rawMembers.forEach(rawMember =>
+    invariant(
+      minimallyEncodedMemberInfoValidator.is(rawMember) ||
+        legacyMemberInfoValidator.is(rawMember),
+      'rawMember must be correctly formed [MinimallyEncoded/Legacy]MemberInfo',
+    ),
+  );
+  const minimallyEncodedMembers: $ReadOnlyArray<MinimallyEncodedMemberInfo> =
+    rawMembers.map(rawMember =>
+      rawMember.minimallyEncoded
+        ? rawMember
+        : minimallyEncodeMemberInfo(rawMember),
+    );
+
   let rawThreadInfo: MinimallyEncodedRawThreadInfo = {
     minimallyEncoded: true,
     id: clientDBThreadInfo.id,
@@ -37,7 +63,7 @@
     parentThreadID: clientDBThreadInfo.parentThreadID,
     containingThreadID: clientDBThreadInfo.containingThreadID,
     community: clientDBThreadInfo.community,
-    members: JSON.parse(clientDBThreadInfo.members),
+    members: minimallyEncodedMembers,
     roles: JSON.parse(clientDBThreadInfo.roles),
     currentUser: JSON.parse(clientDBThreadInfo.currentUser),
     repliesCount: clientDBThreadInfo.repliesCount,