Custom Fetch

Override the fetch function used by the client. Useful for SSR (SvelteKit's platform fetch) and testing (mock fetch).

Default

When not specified, the client uses the global fetch.

// Default: uses globalThis.fetch
const rpc = createRpcClient({ baseUrl: '/api' });

SSR (SvelteKit)

In SvelteKit server loads, pass the platform fetch to forward cookies and resolve relative URLs correctly.

// SvelteKit server load — forward cookies & resolve relative URLs
export const load: PageServerLoad = async ({ fetch }) => {
  const rpc = createRpcClient({ baseUrl: '/api', fetch });
  const data = await rpc.query('cookie_demo');
  return { ssrResult: data };
};

Testing

Inject a mock fetch for unit tests without hitting the network.

// Mock fetch for unit tests
const mockFetch = async (url: string) =>
  new Response(JSON.stringify({ ok: true }), { status: 200 });

const rpc = createRpcClient({ baseUrl: '/api', fetch: mockFetch });

Try it

The server checks for a session cookie. SSR event.fetch forwards it automatically; browser fetch does not (on Vercel, cross-origin).

SSR Result

Called during server load with event.fetch

✗ SSR error: RpcError: RPC error on "cookie_demo": 404

Client Result

Called from browser with default globalThis.fetch