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
@@ -2,14 +2,21 @@
 
 import invariant from 'invariant';
 
-import { parseThreadPermissionString } from './prefixes.js';
-import type {
-  ThreadPermission,
-  ThreadPermissionInfo,
-  ThreadPermissionsInfo,
-  ThreadRolePermissionsBlob,
+import {
+  parseThreadPermissionString,
+  constructThreadPermissionString,
+} from './prefixes.js';
+import {
+  type ThreadPermission,
+  type ThreadPermissionInfo,
+  type ThreadPermissionsInfo,
+  type ThreadRolePermissionsBlob,
+  threadPermissions,
+  assertThreadPermission,
+  assertThreadPermissionPropagationPrefix,
+  assertThreadPermissionFilterPrefix,
+  assertThreadPermissionMembershipPrefix,
 } from '../types/thread-permission-types.js';
-import { threadPermissions } from '../types/thread-permission-types.js';
 import { entries, invertObjectToMap } from '../utils/objects.js';
 import type { TRegex } from '../utils/validation-utils.js';
 import { tRegex } from '../utils/validation-utils.js';
@@ -183,37 +190,43 @@
 const tHexEncodedRolePermission: TRegex = tRegex(/^[0-9a-fA-F]{3,}$/);
 const decodeRolePermissionBitmask = (bitmask: string): string => {
   const bitmaskInt = BigInt(`0x${bitmask}`);
-  const basePermission = (bitmaskInt >> BigInt(4)) & BigInt(63);
-  const propagationPrefix = (bitmaskInt >> BigInt(2)) & BigInt(3);
-  const filterPrefix = bitmaskInt & BigInt(3);
-  const membershipPrefix = (bitmaskInt >> BigInt(10)) & BigInt(1);
-
-  const basePermissionString =
-    inverseBaseRolePermissionEncoding.get(basePermission);
-  const propagationPrefixString =
-    inversePropagationPrefixes.get(propagationPrefix) ?? '';
-  const filterPrefixString = inverseFilterPrefixes.get(filterPrefix) ?? '';
-  const membershipPrefixString =
-    inverseMembershipPrefixes.get(membershipPrefix) ?? '';
 
+  const basePermissionBits = (bitmaskInt >> BigInt(4)) & BigInt(63);
+  const permissionString =
+    inverseBaseRolePermissionEncoding.get(basePermissionBits);
   invariant(
-    basePermissionString !== null &&
-      basePermissionString !== undefined &&
-      propagationPrefixString !== null &&
-      propagationPrefixString !== undefined &&
-      filterPrefixString !== null &&
-      filterPrefixString !== undefined &&
-      membershipPrefixString !== null &&
-      membershipPrefixString !== undefined,
+    permissionString !== null && permissionString !== undefined,
     'invalid bitmask',
   );
+  const permission = assertThreadPermission(permissionString);
 
-  return (
-    propagationPrefixString +
-    filterPrefixString +
-    membershipPrefixString +
-    basePermissionString
+  const propagationPrefixBits = (bitmaskInt >> BigInt(2)) & BigInt(3);
+  const propagationPrefixString = inversePropagationPrefixes.get(
+    propagationPrefixBits,
   );
+  const propagationPrefix = propagationPrefixString
+    ? assertThreadPermissionPropagationPrefix(propagationPrefixString)
+    : undefined;
+
+  const filterPrefixBits = bitmaskInt & BigInt(3);
+  const filterPrefixString = inverseFilterPrefixes.get(filterPrefixBits);
+  const filterPrefix = filterPrefixString
+    ? assertThreadPermissionFilterPrefix(filterPrefixString)
+    : undefined;
+
+  const membershipPrefixBits = (bitmaskInt >> BigInt(10)) & BigInt(1);
+  const membershipPrefixString =
+    inverseMembershipPrefixes.get(membershipPrefixBits);
+  const membershipPrefix = membershipPrefixString
+    ? assertThreadPermissionMembershipPrefix(membershipPrefixString)
+    : undefined;
+
+  return constructThreadPermissionString({
+    permission,
+    propagationPrefix,
+    filterPrefix,
+    membershipPrefix,
+  });
 };
 
 const threadRolePermissionsBlobToBitmaskArray = (
diff --git a/lib/permissions/prefixes.js b/lib/permissions/prefixes.js
--- a/lib/permissions/prefixes.js
+++ b/lib/permissions/prefixes.js
@@ -5,7 +5,7 @@
   type ThreadPermissionFilterPrefix,
   type ThreadPermissionPropagationPrefix,
   type ThreadPermissionMembershipPrefix,
-  assertThreadPermissions,
+  assertThreadPermission,
   threadPermissionFilterPrefixes,
   threadPermissionPropagationPrefixes,
   threadPermissionMembershipPrefixes,
@@ -60,7 +60,7 @@
     break;
   }
 
-  const permission = assertThreadPermissions(remainingString);
+  const permission = assertThreadPermission(remainingString);
 
   return { permission, propagationPrefix, filterPrefix, membershipPrefix };
 }
diff --git a/lib/types/thread-permission-types.js b/lib/types/thread-permission-types.js
--- a/lib/types/thread-permission-types.js
+++ b/lib/types/thread-permission-types.js
@@ -49,36 +49,36 @@
 });
 export type ThreadPermission = $Values<typeof threadPermissions>;
 
-export function assertThreadPermissions(
-  ourThreadPermissions: string,
+export function assertThreadPermission(
+  ourThreadPermission: string,
 ): ThreadPermission {
   invariant(
-    ourThreadPermissions === 'know_of' ||
-      ourThreadPermissions === 'visible' ||
-      ourThreadPermissions === 'voiced' ||
-      ourThreadPermissions === 'edit_entries' ||
-      ourThreadPermissions === 'edit_thread' ||
-      ourThreadPermissions === 'edit_thread_description' ||
-      ourThreadPermissions === 'edit_thread_color' ||
-      ourThreadPermissions === 'delete_thread' ||
-      ourThreadPermissions === 'create_subthreads' ||
-      ourThreadPermissions === 'create_sidebars' ||
-      ourThreadPermissions === 'join_thread' ||
-      ourThreadPermissions === 'edit_permissions' ||
-      ourThreadPermissions === 'add_members' ||
-      ourThreadPermissions === 'remove_members' ||
-      ourThreadPermissions === 'change_role' ||
-      ourThreadPermissions === 'leave_thread' ||
-      ourThreadPermissions === 'react_to_message' ||
-      ourThreadPermissions === 'edit_message' ||
-      ourThreadPermissions === 'edit_thread_avatar' ||
-      ourThreadPermissions === 'manage_pins' ||
-      ourThreadPermissions === 'manage_invite_links' ||
-      ourThreadPermissions === 'voiced_in_announcement_channels' ||
-      ourThreadPermissions === 'manage_farcaster_channel_tags',
+    ourThreadPermission === 'know_of' ||
+      ourThreadPermission === 'visible' ||
+      ourThreadPermission === 'voiced' ||
+      ourThreadPermission === 'edit_entries' ||
+      ourThreadPermission === 'edit_thread' ||
+      ourThreadPermission === 'edit_thread_description' ||
+      ourThreadPermission === 'edit_thread_color' ||
+      ourThreadPermission === 'delete_thread' ||
+      ourThreadPermission === 'create_subthreads' ||
+      ourThreadPermission === 'create_sidebars' ||
+      ourThreadPermission === 'join_thread' ||
+      ourThreadPermission === 'edit_permissions' ||
+      ourThreadPermission === 'add_members' ||
+      ourThreadPermission === 'remove_members' ||
+      ourThreadPermission === 'change_role' ||
+      ourThreadPermission === 'leave_thread' ||
+      ourThreadPermission === 'react_to_message' ||
+      ourThreadPermission === 'edit_message' ||
+      ourThreadPermission === 'edit_thread_avatar' ||
+      ourThreadPermission === 'manage_pins' ||
+      ourThreadPermission === 'manage_invite_links' ||
+      ourThreadPermission === 'voiced_in_announcement_channels' ||
+      ourThreadPermission === 'manage_farcaster_channel_tags',
     'string is not threadPermissions enum',
   );
-  return ourThreadPermissions;
+  return ourThreadPermission;
 }
 
 const threadPermissionValidator = t.enums.of(values(threadPermissions));
@@ -90,6 +90,15 @@
 export type ThreadPermissionPropagationPrefix = $Values<
   typeof threadPermissionPropagationPrefixes,
 >;
+export function assertThreadPermissionPropagationPrefix(
+  ourPrefix: string,
+): ThreadPermissionPropagationPrefix {
+  invariant(
+    ourPrefix === 'descendant_' || ourPrefix === 'child_',
+    'string is not ThreadPermissionPropagationPrefix',
+  );
+  return ourPrefix;
+}
 
 export const threadPermissionFilterPrefixes = Object.freeze({
   // includes only SIDEBAR, COMMUNITY_OPEN_SUBTHREAD,
@@ -104,6 +113,17 @@
 export type ThreadPermissionFilterPrefix = $Values<
   typeof threadPermissionFilterPrefixes,
 >;
+export function assertThreadPermissionFilterPrefix(
+  ourPrefix: string,
+): ThreadPermissionFilterPrefix {
+  invariant(
+    ourPrefix === 'open_' ||
+      ourPrefix === 'toplevel_' ||
+      ourPrefix === 'opentoplevel_',
+    'string is not ThreadPermissionFilterPrefix',
+  );
+  return ourPrefix;
+}
 
 export const threadPermissionMembershipPrefixes = Object.freeze({
   MEMBER: 'member_',
@@ -111,6 +131,15 @@
 export type ThreadPermissionMembershipPrefix = $Values<
   typeof threadPermissionMembershipPrefixes,
 >;
+export function assertThreadPermissionMembershipPrefix(
+  ourPrefix: string,
+): ThreadPermissionMembershipPrefix {
+  invariant(
+    ourPrefix === 'member_',
+    'string is not ThreadPermissionMembershipPrefix',
+  );
+  return ourPrefix;
+}
 
 // These are the set of user-facing permissions that we display as configurable
 // to the user when they are creating a custom role for their given community.