diff --git a/packages/cli/src/auth.ts b/packages/cli/src/auth.ts
new file mode 100644
index 0000000..5279502
--- /dev/null
+++ b/packages/cli/src/auth.ts
@@ -0,0 +1,115 @@
+import { readFileSync, writeFileSync, mkdirSync, existsSync } from 'node:fs';
+import { resolve } from 'node:path';
+import { homedir } from 'node:os';
+
+const SESSION_DIR = resolve(homedir(), '.mosaic');
+const SESSION_FILE = resolve(SESSION_DIR, 'session.json');
+
+interface StoredSession {
+  gatewayUrl: string;
+  cookie: string;
+  userId: string;
+  email: string;
+  expiresAt: string;
+}
+
+export interface AuthResult {
+  cookie: string;
+  userId: string;
+  email: string;
+}
+
+/**
+ * Sign in to the gateway and return the session cookie.
+ */
+export async function signIn(
+  gatewayUrl: string,
+  email: string,
+  password: string,
+): Promise<AuthResult> {
+  const res = await fetch(`${gatewayUrl}/api/auth/sign-in/email`, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ email, password }),
+    redirect: 'manual',
+  });
+
+  if (!res.ok) {
+    const body = await res.text().catch(() => '');
+    throw new Error(`Sign-in failed (${res.status}): ${body}`);
+  }
+
+  // Extract set-cookie header
+  const setCookieHeader = res.headers.getSetCookie?.() ?? [];
+  const sessionCookie = setCookieHeader
+    .map((c) => c.split(';')[0]!)
+    .filter((c) => c.startsWith('better-auth.session_token='))
+    .join('; ');
+
+  if (!sessionCookie) {
+    throw new Error('No session cookie returned from sign-in');
+  }
+
+  // Parse the response body for user info
+  const data = (await res.json()) as { user?: { id: string; email: string } };
+  const userId = data.user?.id ?? 'unknown';
+  const userEmail = data.user?.email ?? email;
+
+  return { cookie: sessionCookie, userId, email: userEmail };
+}
+
+/**
+ * Save session to ~/.mosaic/session.json
+ */
+export function saveSession(gatewayUrl: string, auth: AuthResult): void {
+  if (!existsSync(SESSION_DIR)) {
+    mkdirSync(SESSION_DIR, { recursive: true });
+  }
+
+  const session: StoredSession = {
+    gatewayUrl,
+    cookie: auth.cookie,
+    userId: auth.userId,
+    email: auth.email,
+    expiresAt: new Date(Date.now() + 7 * 24 * 60 * 60 * 1000).toISOString(), // 7 days
+  };
+
+  writeFileSync(SESSION_FILE, JSON.stringify(session, null, 2), 'utf-8');
+}
+
+/**
+ * Load a saved session. Returns null if no session, expired, or wrong gateway.
+ */
+export function loadSession(gatewayUrl: string): AuthResult | null {
+  if (!existsSync(SESSION_FILE)) return null;
+
+  try {
+    const raw = readFileSync(SESSION_FILE, 'utf-8');
+    const session = JSON.parse(raw) as StoredSession;
+
+    if (session.gatewayUrl !== gatewayUrl) return null;
+    if (new Date(session.expiresAt) < new Date()) return null;
+
+    return {
+      cookie: session.cookie,
+      userId: session.userId,
+      email: session.email,
+    };
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * Validate that a stored session is still active by hitting get-session.
+ */
+export async function validateSession(gatewayUrl: string, cookie: string): Promise<boolean> {
+  try {
+    const res = await fetch(`${gatewayUrl}/api/auth/get-session`, {
+      headers: { Cookie: cookie },
+    });
+    return res.ok;
+  } catch {
+    return false;
+  }
+}
diff --git a/packages/cli/src/cli.ts b/packages/cli/src/cli.ts
index 2833cf8..4e5dea2 100644
--- a/packages/cli/src/cli.ts
+++ b/packages/cli/src/cli.ts
@@ -8,12 +8,83 @@ const program = new Command();
 
 program.name('mosaic').description('Mosaic Stack CLI').version('0.0.0');
 
+// ─── login ──────────────────────────────────────────────────────────────
+
+program
+  .command('login')
+  .description('Sign in to a Mosaic gateway')
+  .option('-g, --gateway <url>', 'Gateway URL', 'http://localhost:4000')
+  .option('-e, --email <email>', 'Email address')
+  .option('-p, --password <password>', 'Password')
+  .action(async (opts: { gateway: string; email?: string; password?: string }) => {
+    const { signIn, saveSession } = await import('./auth.js');
+
+    let email = opts.email;
+    let password = opts.password;
+
+    if (!email || !password) {
+      const readline = await import('node:readline');
+      const rl = readline.createInterface({ input: process.stdin, output: process.stdout });
+      const ask = (q: string): Promise<string> => new Promise((resolve) => rl.question(q, resolve));
+
+      if (!email) email = await ask('Email: ');
+      if (!password) password = await ask('Password: ');
+      rl.close();
+    }
+
+    try {
+      const auth = await signIn(opts.gateway, email, password);
+      saveSession(opts.gateway, auth);
+      console.log(`Signed in as ${auth.email} (${opts.gateway})`);
+    } catch (err) {
+      console.error(err instanceof Error ? err.message : String(err));
+      process.exit(1);
+    }
+  });
+
+// ─── tui ────────────────────────────────────────────────────────────────
+
 program
   .command('tui')
   .description('Launch interactive TUI connected to the gateway')
   .option('-g, --gateway <url>', 'Gateway URL', 'http://localhost:4000')
   .option('-c, --conversation <id>', 'Resume a conversation by ID')
   .action(async (opts: { gateway: string; conversation?: string }) => {
+    const { loadSession, validateSession, signIn, saveSession } = await import('./auth.js');
+
+    // Try loading saved session
+    let session = loadSession(opts.gateway);
+
+    if (session) {
+      const valid = await validateSession(opts.gateway, session.cookie);
+      if (!valid) {
+        console.log('Session expired. Please sign in again.');
+        session = null;
+      }
+    }
+
+    // No valid session — prompt for credentials
+    if (!session) {
+      const readline = await import('node:readline');
+      const rl = readline.createInterface({ input: process.stdin, output: process.stdout });
+      const ask = (q: string): Promise<string> => new Promise((resolve) => rl.question(q, resolve));
+
+      console.log(`Sign in to ${opts.gateway}`);
+      const email = await ask('Email: ');
+      const password = await ask('Password: ');
+      rl.close();
+
+      try {
+        const auth = await signIn(opts.gateway, email, password);
+        saveSession(opts.gateway, auth);
+        session = auth;
+        console.log(`Signed in as ${auth.email}\n`);
+      } catch (err) {
+        console.error(err instanceof Error ? err.message : String(err));
+        process.exit(1);
+      }
+    }
+
     // Dynamic import to avoid loading React/Ink for other commands
     const { render } = await import('ink');
     const React = await import('react');
@@ -23,28 +94,29 @@ program
       React.createElement(TuiApp, {
         gatewayUrl: opts.gateway,
         conversationId: opts.conversation,
+        sessionCookie: session.cookie,
       }),
     );
   });
 
-// prdy subcommand
-// buildPrdyCli() returns a wrapper Command; extract the 'prdy' subcommand from it.
-// Type cast is required because @mosaic/prdy uses commander@12 while @mosaic/cli uses commander@13.
+// ─── prdy ───────────────────────────────────────────────────────────────
+
 const prdyWrapper = buildPrdyCli();
 const prdyCmd = prdyWrapper.commands.find((c) => c.name() === 'prdy');
 if (prdyCmd !== undefined) {
   program.addCommand(prdyCmd as unknown as Command);
 }
 
-// quality-rails subcommand
-// createQualityRailsCli() returns a wrapper Command; extract the 'quality-rails' subcommand.
+// ─── quality-rails ──────────────────────────────────────────────────────
+
 const qrWrapper = createQualityRailsCli();
 const qrCmd = qrWrapper.commands.find((c) => c.name() === 'quality-rails');
 if (qrCmd !== undefined) {
   program.addCommand(qrCmd as unknown as Command);
 }
 
-// wizard subcommand — wraps @mosaic/mosaic installation wizard
+// ─── wizard ─────────────────────────────────────────────────────────────
+
 program
   .command('wizard')
   .description('Run the Mosaic installation wizard')
@@ -60,7 +132,6 @@ program
   .option('--pronouns <pronouns>', 'Your pronouns')
   .option('--timezone <tz>', 'Your timezone')
   .action(async (opts: Record<string, string | boolean | undefined>) => {
-    // Dynamic import to avoid loading wizard deps for other commands
     const {
       runWizard,
       ClackPrompter,
diff --git a/packages/cli/src/tui/app.tsx b/packages/cli/src/tui/app.tsx
index 831d4a8..3a6698f 100644
--- a/packages/cli/src/tui/app.tsx
+++ b/packages/cli/src/tui/app.tsx
@@ -12,9 +12,14 @@ interface Message {
 interface TuiAppProps {
   gatewayUrl: string;
   conversationId?: string;
+  sessionCookie?: string;
 }
 
-export function TuiApp({ gatewayUrl, conversationId: initialConversationId }: TuiAppProps) {
+export function TuiApp({
+  gatewayUrl,
+  conversationId: initialConversationId,
+  sessionCookie,
+}: TuiAppProps) {
   const { exit } = useApp();
   const [messages, setMessages] = useState<Message[]>([]);
   const [input, setInput] = useState('');
@@ -27,6 +32,7 @@ export function TuiApp({ gatewayUrl, conversationId: initialConversationId }: Tu
   useEffect(() => {
     const socket = io(`${gatewayUrl}/chat`, {
       transports: ['websocket'],
+      extraHeaders: sessionCookie ? { Cookie: sessionCookie } : undefined,
     });
 
     socketRef.current = socket;