diff --git a/src/app/api/conversations/load/route.js b/src/app/api/conversations/load/route.js
index dca60ff..9f852dc 100644
--- a/src/app/api/conversations/load/route.js
+++ b/src/app/api/conversations/load/route.js
@@ -10,18 +10,29 @@ export const POST = withAuth(async ({ locals }) => {
 	try {
 		const { supabase, user } = locals;
 
+		const { data: userData, error: userError } = await supabase
+			.from('users')
+			.select('id')
+			.eq('auth_user_id', user.id)
+			.single();
+
+		if (userError || !userData) {
+			console.error('Failed to resolve internal user:', userError);
+			return NextResponse.json({ error: 'User not found' }, { status: 404 });
+		}
+
 		// Load user's conversations with participants and last message
 		const { data: participations, error: partError } = await supabase
 			.from('conversation_participants')
 			.select('conversation_id')
-			.eq('user_id', user.id);
+			.eq('user_id', userData.id);
 
 		if (partError) {
 			console.error('Failed to load participations:', partError);
 			return NextResponse.json({ error: 'Failed to load conversations' }, { status: 500 });
 		}
 
-		const conversationIds = participations.map(p => p.conversation_id);
+		const conversationIds = (participations || []).map(p => p.conversation_id);
 
 		if (conversationIds.length === 0) {
 			return NextResponse.json({ success: true, conversations: [] });
@@ -52,4 +63,4 @@ export const POST = withAuth(async ({ locals }) => {
 		console.error('Load conversations error:', error);
 		return NextResponse.json({ error: 'Internal server error' }, { status: 500 });
 	}
-});
\ No newline at end of file
+});
diff --git a/src/app/api/conversations/load/route.test.js b/src/app/api/conversations/load/route.test.js
new file mode 100644
index 0000000..704f8e3
--- /dev/null
+++ b/src/app/api/conversations/load/route.test.js
@@ -0,0 +1,116 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+
+const mocks = vi.hoisted(() => ({
+	from: vi.fn(),
+	userEq: vi.fn(),
+	participationEq: vi.fn(),
+	conversationsIn: vi.fn()
+}));
+
+vi.mock('@/lib/api/middleware/auth.js', () => ({
+	withAuth: (handler) => (request, context) =>
+		handler({
+			request,
+			locals: {
+				supabase: {
+					from: mocks.from
+				},
+				user: { id: 'auth-user-id' }
+			},
+			context
+		})
+}));
+
+function createUserQuery(result) {
+	const query = {
+		select: vi.fn(() => query),
+		eq: mocks.userEq,
+		single: vi.fn().mockResolvedValue(result)
+	};
+	mocks.userEq.mockReturnValue(query);
+	return query;
+}
+
+function createParticipationsQuery(result) {
+	const query = {
+		select: vi.fn(() => query),
+		eq: mocks.participationEq
+	};
+	mocks.participationEq.mockResolvedValue(result);
+	return query;
+}
+
+function createConversationsQuery(result) {
+	const query = {
+		select: vi.fn(() => query),
+		in: mocks.conversationsIn,
+		order: vi.fn().mockResolvedValue(result)
+	};
+	mocks.conversationsIn.mockReturnValue(query);
+	return query;
+}
+
+describe('POST /api/conversations/load', () => {
+	beforeEach(() => {
+		vi.resetModules();
+		vi.clearAllMocks();
+
+		mocks.from.mockImplementation((table) => {
+			if (table === 'users') {
+				return createUserQuery({
+					data: { id: 'internal-user-id' },
+					error: null
+				});
+			}
+
+			if (table === 'conversation_participants') {
+				return createParticipationsQuery({
+					data: [{ conversation_id: 'conversation-1' }],
+					error: null
+				});
+			}
+
+			if (table === 'conversations') {
+				return createConversationsQuery({
+					data: [{ id: 'conversation-1' }],
+					error: null
+				});
+			}
+
+			throw new Error(`Unexpected table: ${table}`);
+		});
+	});
+
+	it('loads participations with the internal user id resolved from auth_user_id', async () => {
+		const { POST } = await import('./route.js');
+		const response = await POST(new Request('https://qrypt.chat/api/conversations/load', { method: 'POST' }));
+		const body = await response.json();
+
+		expect(response.status).toBe(200);
+		expect(body.conversations).toEqual([{ id: 'conversation-1' }]);
+		expect(mocks.userEq).toHaveBeenCalledWith('auth_user_id', 'auth-user-id');
+		expect(mocks.participationEq).toHaveBeenCalledWith('user_id', 'internal-user-id');
+		expect(mocks.conversationsIn).toHaveBeenCalledWith('id', ['conversation-1']);
+	});
+
+	it('returns 404 when the authenticated user has no internal profile', async () => {
+		mocks.from.mockImplementation((table) => {
+			if (table === 'users') {
+				return createUserQuery({
+					data: null,
+					error: { message: 'not found' }
+				});
+			}
+
+			throw new Error(`Unexpected table: ${table}`);
+		});
+
+		const { POST } = await import('./route.js');
+		const response = await POST(new Request('https://qrypt.chat/api/conversations/load', { method: 'POST' }));
+		const body = await response.json();
+
+		expect(response.status).toBe(404);
+		expect(body).toEqual({ error: 'User not found' });
+		expect(mocks.from).not.toHaveBeenCalledWith('conversation_participants');
+	});
+});