diff --git a/lib/permissions/minimally-encoded-thread-permissions-test-data.js b/lib/permissions/minimally-encoded-thread-permissions-test-data.js
--- a/lib/permissions/minimally-encoded-thread-permissions-test-data.js
+++ b/lib/permissions/minimally-encoded-thread-permissions-test-data.js
@@ -295,6 +295,7 @@
 };
 
 const exampleMinimallyEncodedRawThreadInfoA: MinimallyEncodedRawThreadInfo = {
+  minimallyEncoded: true,
   id: '85171',
   type: threadTypes.PERSONAL,
   name: '',
diff --git a/lib/permissions/minimally-encoded-thread-permissions.js b/lib/permissions/minimally-encoded-thread-permissions.js
--- a/lib/permissions/minimally-encoded-thread-permissions.js
+++ b/lib/permissions/minimally-encoded-thread-permissions.js
@@ -24,7 +24,7 @@
   threadCurrentUserInfoValidator,
 } from '../types/thread-types.js';
 import { entries, invertObjectToMap } from '../utils/objects.js';
-import { tID, tRegex, tShape } from '../utils/validation-utils.js';
+import { tBool, tID, tRegex, tShape } from '../utils/validation-utils.js';
 import type { TRegex } from '../utils/validation-utils.js';
 
 // `baseRolePermissionEncoding` maps permission names to indices.
@@ -304,6 +304,7 @@
 });
 
 export type MinimallyEncodedRawThreadInfo = {
+  minimallyEncoded: true,
   ...RawThreadInfo,
   +members: $ReadOnlyArray<MinimallyEncodedMemberInfo>,
   +roles: { +[id: string]: MinimallyEncodedRoleInfo },
@@ -312,6 +313,7 @@
 
 const minimallyEncodedRawThreadInfoValidator: TInterface<MinimallyEncodedRawThreadInfo> =
   tShape<MinimallyEncodedRawThreadInfo>({
+    minimallyEncoded: tBool(true),
     ...rawThreadInfoValidator.meta.props,
     members: t.list(minimallyEncodedMemberInfoValidator),
     roles: t.dict(tID, minimallyEncodedRoleInfoValidator),
@@ -323,6 +325,7 @@
 ): MinimallyEncodedRawThreadInfo => {
   const { members, roles, currentUser, ...rest } = rawThreadInfo;
   return {
+    minimallyEncoded: true,
     ...rest,
     members: members.map(minimallyEncodeMemberInfo),
     roles: _mapValues(minimallyEncodeRoleInfo)(roles),
@@ -333,7 +336,7 @@
 const decodeMinimallyEncodedRawThreadInfo = (
   minimallyEncodedRawThreadInfo: MinimallyEncodedRawThreadInfo,
 ): RawThreadInfo => {
-  const { members, roles, currentUser, ...rest } =
+  const { minimallyEncoded, members, roles, currentUser, ...rest } =
     minimallyEncodedRawThreadInfo;
   return {
     ...rest,