diff --git a/src/app/api/conversations/join/route.js b/src/app/api/conversations/join/route.js
index 3adbecd8..2f71ee53 100644
--- a/src/app/api/conversations/join/route.js
+++ b/src/app/api/conversations/join/route.js
@@ -7,9 +7,14 @@ import { NextResponse } from 'next/server';
 import { withAuth } from '@/lib/api/middleware/auth.js';
 import { sseManager } from '@/lib/api/sse-manager.js';
 
+function normalizeConversationId(conversationId) {
+	return typeof conversationId === 'string' ? conversationId.trim() : '';
+}
+
 export const POST = withAuth(async ({ request, locals }) => {
 	try {
-		const { conversationId } = await request.json();
+		const { conversationId: rawConversationId } = await request.json();
+		const conversationId = normalizeConversationId(rawConversationId);
 
 		if (!conversationId) {
 			return NextResponse.json({ error: 'Missing conversationId' }, { status: 400 });
@@ -39,4 +44,4 @@ export const POST = withAuth(async ({ request, locals }) => {
 		console.error('Join conversation error:', error);
 		return NextResponse.json({ error: 'Internal server error' }, { status: 500 });
 	}
-});
\ No newline at end of file
+});
diff --git a/src/app/api/conversations/join/route.test.js b/src/app/api/conversations/join/route.test.js
new file mode 100644
index 00000000..3f6ac00a
--- /dev/null
+++ b/src/app/api/conversations/join/route.test.js
@@ -0,0 +1,77 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+
+const mocks = vi.hoisted(() => ({
+	mockSupabase: {
+		from: vi.fn()
+	},
+	mockJoinRoom: vi.fn(),
+	userEq: vi.fn()
+}));
+
+vi.mock('@/lib/api/middleware/auth.js', () => ({
+	withAuth: (handler) => (request, context) =>
+		handler({
+			request,
+			locals: {
+				supabase: mocks.mockSupabase,
+				user: { id: 'auth-user-id' }
+			},
+			context
+		})
+}));
+
+vi.mock('@/lib/api/sse-manager.js', () => ({
+	sseManager: {
+		joinRoom: mocks.mockJoinRoom
+	}
+}));
+
+function createUserQuery() {
+	const query = {
+		select: vi.fn(() => query),
+		eq: mocks.userEq,
+		single: vi.fn().mockResolvedValue({
+			data: { id: 'internal-user-id' },
+			error: null
+		})
+	};
+	mocks.userEq.mockReturnValue(query);
+	return query;
+}
+
+function createRequest(conversationId) {
+	return {
+		json: vi.fn().mockResolvedValue({ conversationId })
+	};
+}
+
+describe('POST /api/conversations/join', () => {
+	beforeEach(() => {
+		vi.resetModules();
+		vi.clearAllMocks();
+		mocks.mockSupabase.from.mockImplementation((table) => {
+			if (table === 'users') return createUserQuery();
+			throw new Error(`Unexpected table: ${table}`);
+		});
+	});
+
+	it('rejects blank conversation ids before user lookup', async () => {
+		const { POST } = await import('./route.js');
+		const response = await POST(createRequest('   '));
+		const body = await response.json();
+
+		expect(response.status).toBe(400);
+		expect(body).toEqual({ error: 'Missing conversationId' });
+		expect(mocks.mockSupabase.from).not.toHaveBeenCalled();
+		expect(mocks.mockJoinRoom).not.toHaveBeenCalled();
+	});
+
+	it('trims conversation ids before joining the SSE room', async () => {
+		const { POST } = await import('./route.js');
+		const response = await POST(createRequest('  conversation-1  '));
+
+		expect(response.status).toBe(200);
+		expect(mocks.userEq).toHaveBeenCalledWith('auth_user_id', 'auth-user-id');
+		expect(mocks.mockJoinRoom).toHaveBeenCalledWith('internal-user-id', 'conversation-1');
+	});
+});