diff --git a/server.js b/server.js
index bed1528..c9edb0e 100644
--- a/server.js
+++ b/server.js
@@ -21,15 +21,70 @@ const MODELS = {
     haiku: 'claude-haiku-4-5-20251001',
 };
 
-function getAccessToken() {
-    const configDir = process.env.CLAUDE_CONFIG_DIR || path.join(os.homedir(), '.claude');
-    const credsPath = path.join(configDir, '.credentials.json');
-    try {
-        const creds = JSON.parse(fs.readFileSync(credsPath, 'utf8'));
-        return creds?.claudeAiOauth?.accessToken || null;
-    } catch (e) {
-        return null;
+const CONFIG_DIR = process.env.CLAUDE_CONFIG_DIR || path.join(os.homedir(), '.claude');
+const CREDS_PATH = path.join(CONFIG_DIR, '.credentials.json');
+const OAUTH_CLIENT_ID = '9d1c250a-e61b-44d9-88ed-5944d1962f5e';
+const REFRESH_SKEW_MS = 5 * 60 * 1000;
+
+function readCreds() {
+    try { return JSON.parse(fs.readFileSync(CREDS_PATH, 'utf8')); } catch { return null; }
+}
+
+function writeCreds(creds) {
+    const tmp = CREDS_PATH + '.tmp';
+    fs.writeFileSync(tmp, JSON.stringify(creds, null, 2));
+    fs.renameSync(tmp, CREDS_PATH);
+}
+
+async function refreshTokens(refreshToken) {
+    const resp = await fetch('https://console.anthropic.com/v1/oauth/token', {
+        method: 'POST',
+        dispatcher: anthropicAgent,
+        headers: {
+            'Content-Type': 'application/json',
+            'User-Agent': 'claude-cli/2.1.114 (external, cli)',
+            'Accept': '*/*',
+            'anthropic-beta': 'oauth-2025-04-20',
+        },
+        body: JSON.stringify({
+            grant_type: 'refresh_token',
+            refresh_token: refreshToken,
+            client_id: OAUTH_CLIENT_ID,
+        }),
+    });
+    if (!resp.ok) throw new Error(`refresh ${resp.status}: ${(await resp.text()).slice(0, 200)}`);
+    return resp.json();
+}
+
+let refreshInFlight = null;
+async function ensureFreshToken() {
+    const creds = readCreds();
+    if (!creds?.claudeAiOauth) return null;
+    const t = creds.claudeAiOauth;
+    if (t.expiresAt && t.expiresAt > Date.now() + REFRESH_SKEW_MS) return t.accessToken;
+
+    // Serialize concurrent refreshes
+    if (!refreshInFlight) {
+        refreshInFlight = (async () => {
+            // Re-read: another fetch may have refreshed while we waited
+            const latest = readCreds();
+            if (latest.claudeAiOauth.expiresAt > Date.now() + REFRESH_SKEW_MS) return;
+            console.log('[oauth] refreshing token…');
+            const r = await refreshTokens(latest.claudeAiOauth.refreshToken);
+            latest.claudeAiOauth.accessToken = r.access_token;
+            latest.claudeAiOauth.refreshToken = r.refresh_token;
+            latest.claudeAiOauth.expiresAt = Date.now() + r.expires_in * 1000;
+            writeCreds(latest);
+            console.log(`[oauth] ✓ refreshed, expires ${new Date(latest.claudeAiOauth.expiresAt).toISOString()}`);
+        })().finally(() => { refreshInFlight = null; });
     }
+    try { await refreshInFlight; } catch (e) { console.error('[oauth] refresh failed:', e.message); }
+    return readCreds()?.claudeAiOauth?.accessToken || null;
+}
+
+// keep old sync name for compatibility
+function getAccessToken() {
+    return readCreds()?.claudeAiOauth?.accessToken || null;
 }
 
 const SYSTEM_PROMPT = [
@@ -90,9 +145,9 @@ app.post('/api/chat', express.json({ limit: '50mb' }), async (req, res) => {
         content = text;
     }
 
-    const accessToken = getAccessToken();
+    const accessToken = await ensureFreshToken();
     if (!accessToken) {
-        return res.status(500).json({ error: 'No credentials in ' + (process.env.CLAUDE_CONFIG_DIR || '~/.claude') });
+        return res.status(500).json({ error: 'No credentials in ' + CONFIG_DIR });
     }
 
     res.setHeader('Content-Type', 'text/event-stream');