diff --git a/src/app/api/auth/verify-sms/route.js b/src/app/api/auth/verify-sms/route.js
index bd79e8e..3cb90c6 100644
--- a/src/app/api/auth/verify-sms/route.js
+++ b/src/app/api/auth/verify-sms/route.js
@@ -125,7 +125,25 @@ export async function POST(request, { params } = {}) {
 					userId: user.id,
 					userPhone: user.phone
 				});
-				
+
+				// Bind the requested phone number to the JWT-verified phone.
+				// The session branch skips OTP, so without this a caller holding
+				// ANY valid session (their own verified number, or an anonymous
+				// session) could complete signup claiming a phone they never
+				// verified — that value is persisted as `phone_number` below.
+				// Compare bare digits so a leading '+' difference doesn't matter.
+				const verifiedPhone = (user.phone || '').replace(/\D/g, '');
+				const requestedPhone = phoneNumber.replace(/\D/g, '');
+				if (!verifiedPhone || verifiedPhone !== requestedPhone) {
+					logger.error('Session phone does not match verified phone', {
+						hasVerifiedPhone: !!user.phone
+					});
+					return NextResponse.json(
+						{ error: 'Phone number does not match verified session', code: 'PHONE_SESSION_MISMATCH' },
+						{ status: 403 }
+					);
+				}
+
 				// Use the authenticated user data
 				verifyData = {
 					user,
diff --git a/src/app/api/auth/verify-sms/route.test.js b/src/app/api/auth/verify-sms/route.test.js
new file mode 100644
index 0000000..4e52fe2
--- /dev/null
+++ b/src/app/api/auth/verify-sms/route.test.js
@@ -0,0 +1,77 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+
+// Mock the server + service Supabase clients. The phone-binding guard runs
+// right after getUser(), before any DB work, so only getUser needs behaviour.
+const mocks = vi.hoisted(() => ({
+	getUser: vi.fn(),
+	serverClient: vi.fn(),
+	createClient: vi.fn(() => ({}))
+}));
+
+vi.mock('@/lib/supabase.js', () => ({
+	createSupabaseServerClient: mocks.serverClient
+}));
+
+vi.mock('@supabase/supabase-js', () => ({
+	createClient: mocks.createClient
+}));
+
+function sessionRequest(body) {
+	return new Request('https://example.com/api/auth/verify-sms', {
+		method: 'POST',
+		headers: {
+			authorization: 'Bearer test-session-token',
+			'content-type': 'application/json'
+		},
+		body: JSON.stringify(body)
+	});
+}
+
+describe('verify-sms session phone binding', () => {
+	beforeEach(() => {
+		vi.resetModules();
+		vi.clearAllMocks();
+		process.env.NEXT_PUBLIC_SUPABASE_URL = 'https://example.supabase.co';
+		process.env.NEXT_PUBLIC_SUPABASE_ANON_KEY = 'anon-key';
+		process.env.SUPABASE_SERVICE_ROLE_KEY = 'service-role-key';
+		mocks.serverClient.mockResolvedValue({
+			auth: { getUser: mocks.getUser }
+		});
+	});
+
+	it('rejects session signup when the requested phone != JWT-verified phone', async () => {
+		// The JWT belongs to +15550000000, but the client claims +15559999999.
+		mocks.getUser.mockResolvedValue({
+			data: { user: { id: 'user-1', phone: '+15550000000' } },
+			error: null
+		});
+
+		const { POST } = await import('./route.js');
+		const res = await POST(sessionRequest({
+			useSession: true,
+			phoneNumber: '+15559999999',
+			username: 'alice'
+		}));
+		const body = await res.json();
+
+		expect(res.status).toBe(403);
+		expect(body.code).toBe('PHONE_SESSION_MISMATCH');
+		expect(mocks.getUser).toHaveBeenCalledWith('test-session-token');
+	});
+
+	it('rejects anonymous / phoneless sessions', async () => {
+		mocks.getUser.mockResolvedValue({
+			data: { user: { id: 'anon', phone: null } },
+			error: null
+		});
+
+		const { POST } = await import('./route.js');
+		const res = await POST(sessionRequest({
+			useSession: true,
+			phoneNumber: '+15559999999',
+			username: 'bob'
+		}));
+
+		expect(res.status).toBe(403);
+	});
+});