diff --git a/browse/src/config.ts b/browse/src/config.ts
index 04f16643..498c083b 100644
--- a/browse/src/config.ts
+++ b/browse/src/config.ts
@@ -79,7 +79,7 @@ export function resolveConfig(
  */
 export function ensureStateDir(config: BrowseConfig): void {
   try {
-    fs.mkdirSync(config.stateDir, { recursive: true });
+    fs.mkdirSync(config.stateDir, { recursive: true, mode: 0o700 });
   } catch (err: any) {
     if (err.code === 'EACCES') {
       throw new Error(`Cannot create state directory ${config.stateDir}: permission denied`);
diff --git a/browse/src/server.ts b/browse/src/server.ts
index ceff414c..0348489a 100644
--- a/browse/src/server.ts
+++ b/browse/src/server.ts
@@ -398,10 +398,10 @@ function createSession(): SidebarSession {
     lastActiveAt: new Date().toISOString(),
   };
   const sessionDir = path.join(SESSIONS_DIR, id);
-  fs.mkdirSync(sessionDir, { recursive: true });
-  fs.writeFileSync(path.join(sessionDir, 'session.json'), JSON.stringify(session, null, 2));
-  fs.writeFileSync(path.join(sessionDir, 'chat.jsonl'), '');
-  fs.writeFileSync(path.join(SESSIONS_DIR, 'active.json'), JSON.stringify({ id }));
+  fs.mkdirSync(sessionDir, { recursive: true, mode: 0o700 });
+  fs.writeFileSync(path.join(sessionDir, 'session.json'), JSON.stringify(session, null, 2), { mode: 0o600 });
+  fs.writeFileSync(path.join(sessionDir, 'chat.jsonl'), '', { mode: 0o600 });
+  fs.writeFileSync(path.join(SESSIONS_DIR, 'active.json'), JSON.stringify({ id }), { mode: 0o600 });
   chatBuffer = [];
   chatNextId = 0;
   return session;
@@ -411,7 +411,7 @@ function saveSession(): void {
   if (!sidebarSession) return;
   sidebarSession.lastActiveAt = new Date().toISOString();
   const sessionFile = path.join(SESSIONS_DIR, sidebarSession.id, 'session.json');
-  try { fs.writeFileSync(sessionFile, JSON.stringify(sidebarSession, null, 2)); } catch (err: any) {
+  try { fs.writeFileSync(sessionFile, JSON.stringify(sidebarSession, null, 2), { mode: 0o600 }); } catch (err: any) {
     console.error('[browse] Failed to save session:', err.message);
   }
 }
@@ -558,7 +558,7 @@ function spawnClaude(userMessage: string, extensionUrl?: string | null, forTabId
     tabId: agentTabId,
   });
   try {
-    fs.mkdirSync(gstackDir, { recursive: true });
+    fs.mkdirSync(gstackDir, { recursive: true, mode: 0o700 });
     fs.appendFileSync(agentQueue, entry + '\n');
   } catch (err: any) {
     addChatEntry({ ts: new Date().toISOString(), role: 'agent', type: 'agent_error', error: `Failed to queue: ${err.message}` });
@@ -614,7 +614,7 @@ function startAgentHealthCheck(): void {
 
 // Initialize session on startup
 function initSidebarSession(): void {
-  fs.mkdirSync(SESSIONS_DIR, { recursive: true });
+  fs.mkdirSync(SESSIONS_DIR, { recursive: true, mode: 0o700 });
   sidebarSession = loadSession();
   if (!sidebarSession) {
     sidebarSession = createSession();
@@ -1331,7 +1331,7 @@ async function start() {
         chatBuffer = [];
         chatNextId = 0;
         if (sidebarSession) {
-          try { fs.writeFileSync(path.join(SESSIONS_DIR, sidebarSession.id, 'chat.jsonl'), ''); } catch (err: any) {
+          try { fs.writeFileSync(path.join(SESSIONS_DIR, sidebarSession.id, 'chat.jsonl'), '', { mode: 0o600 }); } catch (err: any) {
             console.error('[browse] Failed to clear chat file:', err.message);
           }
         }
@@ -1693,8 +1693,8 @@ start().catch((err) => {
   // stderr because the server is launched with detached: true, stdio: 'ignore'.
   try {
     const errorLogPath = path.join(config.stateDir, 'browse-startup-error.log');
-    fs.mkdirSync(config.stateDir, { recursive: true });
-    fs.writeFileSync(errorLogPath, `${new Date().toISOString()} ${err.message}\n${err.stack || ''}\n`);
+    fs.mkdirSync(config.stateDir, { recursive: true, mode: 0o700 });
+    fs.writeFileSync(errorLogPath, `${new Date().toISOString()} ${err.message}\n${err.stack || ''}\n`, { mode: 0o600 });
   } catch {
     // stateDir may not exist — nothing more we can do
   }
diff --git a/browse/src/sidebar-agent.ts b/browse/src/sidebar-agent.ts
index 2f13b243..67fe2750 100644
--- a/browse/src/sidebar-agent.ts
+++ b/browse/src/sidebar-agent.ts
@@ -49,7 +49,7 @@ function writeToInbox(message: string, pageUrl?: string, sessionId?: string): vo
   }
 
   const inboxDir = path.join(gitRoot, '.context', 'sidebar-inbox');
-  fs.mkdirSync(inboxDir, { recursive: true });
+  fs.mkdirSync(inboxDir, { recursive: true, mode: 0o700 });
 
   const now = new Date();
   const timestamp = now.toISOString().replace(/:/g, '-');
@@ -65,7 +65,7 @@ function writeToInbox(message: string, pageUrl?: string, sessionId?: string): vo
     sidebarSessionId: sessionId || 'unknown',
   };
 
-  fs.writeFileSync(tmpFile, JSON.stringify(inboxMessage, null, 2));
+  fs.writeFileSync(tmpFile, JSON.stringify(inboxMessage, null, 2), { mode: 0o600 });
   fs.renameSync(tmpFile, finalFile);
   console.log(`[sidebar-agent] Wrote inbox message: ${filename}`);
 }
@@ -413,8 +413,8 @@ function pollKillFile(): void {
 
 async function main() {
   const dir = path.dirname(QUEUE);
-  fs.mkdirSync(dir, { recursive: true });
-  if (!fs.existsSync(QUEUE)) fs.writeFileSync(QUEUE, '');
+  fs.mkdirSync(dir, { recursive: true, mode: 0o700 });
+  if (!fs.existsSync(QUEUE)) fs.writeFileSync(QUEUE, '', { mode: 0o600 });
 
   lastLine = countLines();
   await refreshToken();
diff --git a/setup b/setup
index 7e74c64f..9136bef0 100755
--- a/setup
+++ b/setup
@@ -1,6 +1,7 @@
 #!/usr/bin/env bash
 # gstack setup — build browser binary + register skills with Claude Code / Codex
 set -e
+umask 077  # Restrict new files to owner-only (0o600 files, 0o700 dirs)
 
 if ! command -v bun >/dev/null 2>&1; then
   echo "Error: bun is required but not installed." >&2