diff --git a/packages/shared/src/auth/__tests__/mockClient.ts b/packages/shared/src/auth/__tests__/mockClient.ts
new file mode 100644
index 0000000..79e4631
--- /dev/null
+++ b/packages/shared/src/auth/__tests__/mockClient.ts
@@ -0,0 +1,30 @@
+import { vi } from 'vitest';
+import type { AppSupabaseClient } from '../../supabase/client';
+
+export interface MockedRpcCall { name: string; params: unknown }
+
+export interface MockClient {
+  client: AppSupabaseClient;
+  rpcCalls: MockedRpcCall[];
+  setRpcResponse: (name: string, response: { data?: unknown; error?: unknown }) => void;
+}
+
+export function makeMockClient(initialUserId = '11111111-1111-1111-1111-111111111111'): MockClient {
+  const rpcCalls: MockedRpcCall[] = [];
+  const rpcResponses = new Map<string, { data?: unknown; error?: unknown }>();
+  const client = {
+    auth: {
+      getUser: vi.fn().mockResolvedValue({ data: { user: { id: initialUserId } }, error: null }),
+    },
+    rpc: vi.fn().mockImplementation((name: string, params: unknown) => {
+      rpcCalls.push({ name, params });
+      const r = rpcResponses.get(name) ?? { data: null, error: null };
+      return Promise.resolve(r);
+    }),
+  } as unknown as AppSupabaseClient;
+  return {
+    client,
+    rpcCalls,
+    setRpcResponse: (name, response) => rpcResponses.set(name, response),
+  };
+}