diff --git a/src/app/api/auth/debug-sms-config/route.js b/src/app/api/auth/debug-sms-config/route.js
index 967fd43b..b9f8bba7 100644
--- a/src/app/api/auth/debug-sms-config/route.js
+++ b/src/app/api/auth/debug-sms-config/route.js
@@ -81,13 +81,15 @@ export async function POST(request, { params } = {}) {
 	try {
 		const { testPhoneNumber } = await request.json();
 		
-		if (!testPhoneNumber) {
+		if (typeof testPhoneNumber !== 'string' || !testPhoneNumber.trim()) {
 			return NextResponse.json(
-				{ error: 'testPhoneNumber is required' },
+				{ error: 'testPhoneNumber must be a non-empty string' },
 				{ status: 400 }
 			);
 		}
 
+		const normalizedPhoneNumber = testPhoneNumber.trim();
+
 		// First run the diagnostic
 		const config = {
 			PROJECT_REF: process.env.PROJECT_REF,
@@ -118,7 +120,7 @@ export async function POST(request, { params } = {}) {
 		console.log(`From number: ${config.TWILIO_PHONE_NUMBER || 'Message Service: ' + config.TWILIO_MESSAGE_SERVICE_SID?.substring(0, 8) + '***'}`);
 		
 		const { error: smsError } = await supabase.auth.signInWithOtp({
-			phone: testPhoneNumber,
+			phone: normalizedPhoneNumber,
 			options: {
 				channel: 'sms',
 				shouldCreateUser: true
@@ -137,7 +139,7 @@ export async function POST(request, { params } = {}) {
 					code: smsError.code
 				},
 				diagnostic,
-				testPhoneNumber: testPhoneNumber.substring(0, 3) + '***' + testPhoneNumber.substring(testPhoneNumber.length - 2),
+				testPhoneNumber: normalizedPhoneNumber.substring(0, 3) + '***' + normalizedPhoneNumber.substring(normalizedPhoneNumber.length - 2),
 				timestamp: new Date().toISOString()
 			}, { status: 400 });
 		}
@@ -148,7 +150,7 @@ export async function POST(request, { params } = {}) {
 			success: true,
 			message: 'SMS test successful',
 			diagnostic,
-			testPhoneNumber: testPhoneNumber.substring(0, 3) + '***' + testPhoneNumber.substring(testPhoneNumber.length - 2),
+			testPhoneNumber: normalizedPhoneNumber.substring(0, 3) + '***' + normalizedPhoneNumber.substring(normalizedPhoneNumber.length - 2),
 			timestamp: new Date().toISOString()
 		});
 
@@ -165,4 +167,4 @@ export async function POST(request, { params } = {}) {
 			{ status: 500 }
 		);
 	}
-}
\ No newline at end of file
+}
diff --git a/src/app/api/auth/debug-sms-config/route.test.js b/src/app/api/auth/debug-sms-config/route.test.js
new file mode 100644
index 00000000..d94037ef
--- /dev/null
+++ b/src/app/api/auth/debug-sms-config/route.test.js
@@ -0,0 +1,56 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+
+const mocks = vi.hoisted(() => ({
+	diagnoseSMSConfig: vi.fn(),
+	generateDiagnosticReport: vi.fn(),
+	createSupabaseServerClient: vi.fn()
+}));
+
+vi.mock('@/lib/utils/sms-config-diagnostic.js', () => ({
+	diagnoseSMSConfig: mocks.diagnoseSMSConfig,
+	generateDiagnosticReport: mocks.generateDiagnosticReport
+}));
+
+vi.mock('@/lib/supabase.js', () => ({
+	createSupabaseServerClient: mocks.createSupabaseServerClient
+}));
+
+function postRequest(body) {
+	return new Request('https://example.com/api/auth/debug-sms-config', {
+		method: 'POST',
+		headers: { 'content-type': 'application/json' },
+		body: JSON.stringify(body)
+	});
+}
+
+describe('POST /api/auth/debug-sms-config', () => {
+	beforeEach(() => {
+		vi.resetModules();
+		vi.clearAllMocks();
+		process.env.NODE_ENV = 'development';
+		mocks.diagnoseSMSConfig.mockReturnValue({ isValid: true });
+		mocks.generateDiagnosticReport.mockReturnValue('ok');
+	});
+
+	it('rejects non-string phone values before diagnostics or Supabase work', async () => {
+		const { POST } = await import('./route.js');
+		const response = await POST(postRequest({ testPhoneNumber: 15551234567 }));
+		const body = await response.json();
+
+		expect(response.status).toBe(400);
+		expect(body.error).toBe('testPhoneNumber must be a non-empty string');
+		expect(mocks.diagnoseSMSConfig).not.toHaveBeenCalled();
+		expect(mocks.createSupabaseServerClient).not.toHaveBeenCalled();
+	});
+
+	it('rejects whitespace-only phone values before diagnostics or Supabase work', async () => {
+		const { POST } = await import('./route.js');
+		const response = await POST(postRequest({ testPhoneNumber: '   ' }));
+		const body = await response.json();
+
+		expect(response.status).toBe(400);
+		expect(body.error).toBe('testPhoneNumber must be a non-empty string');
+		expect(mocks.diagnoseSMSConfig).not.toHaveBeenCalled();
+		expect(mocks.createSupabaseServerClient).not.toHaveBeenCalled();
+	});
+});