diff --git a/src/app/api/typing/route.test.js b/src/app/api/typing/route.test.js
new file mode 100644
index 0000000..f090a1b
--- /dev/null
+++ b/src/app/api/typing/route.test.js
@@ -0,0 +1,148 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+
+const mocks = vi.hoisted(() => ({
+	mockSupabase: {
+		from: vi.fn()
+	},
+	mockBroadcastToRoom: vi.fn(),
+	participantResult: {
+		data: { id: 'participant-row' },
+		error: null
+	}
+}));
+
+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: {
+		broadcastToRoom: mocks.mockBroadcastToRoom
+	}
+}));
+
+function createQuery(result) {
+	const query = {
+		select: vi.fn(() => query),
+		eq: vi.fn(() => query),
+		is: vi.fn(() => query),
+		single: vi.fn().mockResolvedValue(result)
+	};
+
+	return query;
+}
+
+function setupSupabase() {
+	mocks.mockSupabase.from.mockImplementation((table) => {
+		if (table === 'users') {
+			return createQuery({
+				data: {
+					id: 'internal-user-id',
+					username: 'cream',
+					display_name: 'Cream'
+				},
+				error: null
+			});
+		}
+
+		if (table === 'conversation_participants') {
+			return createQuery(mocks.participantResult);
+		}
+
+		throw new Error(`Unexpected table: ${table}`);
+	});
+}
+
+describe('typing API conversation access', () => {
+	beforeEach(() => {
+		vi.resetModules();
+		vi.clearAllMocks();
+		mocks.participantResult = {
+			data: { id: 'participant-row' },
+			error: null
+		};
+		setupSupabase();
+	});
+
+	it('rejects typing start for conversations the user cannot access', async () => {
+		mocks.participantResult = {
+			data: null,
+			error: { message: 'not found' }
+		};
+
+		const { POST } = await import('./start/route.js');
+		const response = await POST({
+			json: vi.fn().mockResolvedValue({ conversationId: 'conversation-1' })
+		});
+		const body = await response.json();
+
+		expect(response.status).toBe(403);
+		expect(body.error).toBe('Access denied to conversation');
+		expect(mocks.mockBroadcastToRoom).not.toHaveBeenCalled();
+	});
+
+	it('rejects typing stop for conversations the user cannot access', async () => {
+		mocks.participantResult = {
+			data: null,
+			error: { message: 'not found' }
+		};
+
+		const { POST } = await import('./stop/route.js');
+		const response = await POST({
+			json: vi.fn().mockResolvedValue({ conversationId: 'conversation-1' })
+		});
+		const body = await response.json();
+
+		expect(response.status).toBe(403);
+		expect(body.error).toBe('Access denied to conversation');
+		expect(mocks.mockBroadcastToRoom).not.toHaveBeenCalled();
+	});
+
+	it('broadcasts typing start after membership is verified', async () => {
+		const { POST } = await import('./start/route.js');
+		const response = await POST({
+			json: vi.fn().mockResolvedValue({ conversationId: 'conversation-1' })
+		});
+
+		expect(response.status).toBe(200);
+		expect(mocks.mockSupabase.from).toHaveBeenCalledWith('conversation_participants');
+		expect(mocks.mockBroadcastToRoom).toHaveBeenCalledWith(
+			'conversation-1',
+			expect.any(String),
+			expect.objectContaining({
+				userId: 'internal-user-id',
+				conversationId: 'conversation-1',
+				isTyping: true
+			}),
+			'internal-user-id'
+		);
+	});
+
+	it('broadcasts typing stop after membership is verified', async () => {
+		const { POST } = await import('./stop/route.js');
+		const response = await POST({
+			json: vi.fn().mockResolvedValue({ conversationId: 'conversation-1' })
+		});
+
+		expect(response.status).toBe(200);
+		expect(mocks.mockSupabase.from).toHaveBeenCalledWith('conversation_participants');
+		expect(mocks.mockBroadcastToRoom).toHaveBeenCalledWith(
+			'conversation-1',
+			expect.any(String),
+			expect.objectContaining({
+				userId: 'internal-user-id',
+				conversationId: 'conversation-1',
+				isTyping: false
+			}),
+			'internal-user-id'
+		);
+	});
+});
diff --git a/src/app/api/typing/start/route.js b/src/app/api/typing/start/route.js
index bf43ba5..7487fe6 100644
--- a/src/app/api/typing/start/route.js
+++ b/src/app/api/typing/start/route.js
@@ -32,6 +32,18 @@ export const POST = withAuth(async ({ request, locals }) => {
 
 		const userId = userData.id;
 
+		const { data: participant, error: participantError } = await supabase
+			.from('conversation_participants')
+			.select('id')
+			.eq('conversation_id', conversationId)
+			.eq('user_id', userId)
+			.is('left_at', null)
+			.single();
+
+		if (participantError || !participant) {
+			return NextResponse.json({ error: 'Access denied to conversation' }, { status: 403 });
+		}
+
 		// Broadcast typing indicator to conversation
 		sseManager.broadcastToRoom(conversationId, MESSAGE_TYPES.USER_TYPING, {
 			userId,
@@ -46,4 +58,4 @@ export const POST = withAuth(async ({ request, locals }) => {
 		console.error('Start typing error:', error);
 		return NextResponse.json({ error: 'Internal server error' }, { status: 500 });
 	}
-});
\ No newline at end of file
+});
diff --git a/src/app/api/typing/stop/route.js b/src/app/api/typing/stop/route.js
index 1dd97b9..05ea45c 100644
--- a/src/app/api/typing/stop/route.js
+++ b/src/app/api/typing/stop/route.js
@@ -32,6 +32,18 @@ export const POST = withAuth(async ({ request, locals }) => {
 
 		const userId = userData.id;
 
+		const { data: participant, error: participantError } = await supabase
+			.from('conversation_participants')
+			.select('id')
+			.eq('conversation_id', conversationId)
+			.eq('user_id', userId)
+			.is('left_at', null)
+			.single();
+
+		if (participantError || !participant) {
+			return NextResponse.json({ error: 'Access denied to conversation' }, { status: 403 });
+		}
+
 		// Broadcast typing stopped to conversation
 		sseManager.broadcastToRoom(conversationId, MESSAGE_TYPES.USER_TYPING, {
 			userId,
@@ -44,4 +56,4 @@ export const POST = withAuth(async ({ request, locals }) => {
 		console.error('Stop typing error:', error);
 		return NextResponse.json({ error: 'Internal server error' }, { status: 500 });
 	}
-});
\ No newline at end of file
+});