diff --git a/keyserver/src/shared/state-sync/state-sync-spec.js b/keyserver/src/shared/state-sync/state-sync-spec.js
--- a/keyserver/src/shared/state-sync/state-sync-spec.js
+++ b/keyserver/src/shared/state-sync/state-sync-spec.js
@@ -10,7 +10,7 @@
   FullSocketSyncPayload,
   Info,
   Inconsistencies,
-> = {
+> = $ReadOnly<{
   +fetch: (viewer: Viewer, ids?: $ReadOnlySet<string>) => Promise<Infos>,
   +fetchFullSocketSyncPayload: (
     viewer: Viewer,
@@ -23,4 +23,4 @@
   +getServerInfosHash: (infos: Infos) => Promise<number>,
   +getServerInfoHash: (info: Info) => Promise<number>,
   ...StateSyncSpec<Infos, Info, Inconsistencies>,
-};
+}>;
diff --git a/keyserver/src/updaters/relationship-updaters.js b/keyserver/src/updaters/relationship-updaters.js
--- a/keyserver/src/updaters/relationship-updaters.js
+++ b/keyserver/src/updaters/relationship-updaters.js
@@ -317,7 +317,7 @@
 }
 
 async function updateUndirectedRelationships(
-  changeset: UndirectedRelationshipRow[],
+  changeset: $ReadOnlyArray<UndirectedRelationshipRow>,
   greatest: boolean = true,
 ) {
   if (!changeset.length) {
@@ -340,7 +340,7 @@
 }
 
 async function updateChangedUndirectedRelationships(
-  changeset: UndirectedRelationshipRow[],
+  changeset: $ReadOnlyArray<UndirectedRelationshipRow>,
 ): Promise<UpdateData[]> {
   if (changeset.length === 0) {
     return [];
diff --git a/keyserver/src/utils/relationship-changeset.js b/keyserver/src/utils/relationship-changeset.js
--- a/keyserver/src/utils/relationship-changeset.js
+++ b/keyserver/src/utils/relationship-changeset.js
@@ -75,7 +75,7 @@
     }
   }
 
-  _getRows(): UndirectedRelationshipRow[] {
+  _getRows(): $ReadOnlyArray<UndirectedRelationshipRow> {
     const rows = [];
     for (const [key, status] of this.relationships) {
       if (status === 'existing') {
@@ -87,7 +87,7 @@
     return rows;
   }
 
-  getRows(): UndirectedRelationshipRow[] {
+  getRows(): $ReadOnlyArray<UndirectedRelationshipRow> {
     this.finalized = true;
     return this._getRows();
   }
diff --git a/landing/competitor-data.js b/landing/competitor-data.js
--- a/landing/competitor-data.js
+++ b/landing/competitor-data.js
@@ -27,7 +27,7 @@
   +featureComparison: $ReadOnlyArray<FeatureComparison>,
 };
 
-const competitorData: { [key: string]: Competitor } = Object.freeze({
+const competitorData: { +[key: string]: Competitor } = Object.freeze({
   general: {
     id: 'general',
     name: 'General',
diff --git a/lib/types/relationship-types.js b/lib/types/relationship-types.js
--- a/lib/types/relationship-types.js
+++ b/lib/types/relationship-types.js
@@ -123,9 +123,9 @@
 };
 
 export type RelationshipErrors = Partial<{
-  invalid_user: string[],
-  already_friends: string[],
-  user_blocked: string[],
+  +invalid_user: string[],
+  +already_friends: string[],
+  +user_blocked: string[],
 }>;
 
 export type UserRelationships = {