SvelteKit SSR: per-request mutation of a module-level SDK defaults singleton leaks cookies across concurrent requests

A common pattern for generated API clients (oazapfts, openapi-typescript-fetch, etc.) in SvelteKit is a module-level defaults object, configured per-request in hooks.server.ts:

export const handleSDK: Handle = async ({ event, resolve }) => {
  sdk_defaults.headers.cookie = event.request.headers.get('cookie'); // RACE
  sdk_defaults.fetch = wrap(fetch, event.route, event.url);          // RACE
  return resolve(event);
};

This works in dev and low traffic, but the singleton is shared across all in-flight requests in the Node process. Loads await between the mutation and the SDK calls, so concurrent request B overwrites the cookie header while request A's loads are still running — request A's API calls carry request B's cookies. This is a cross-user data leak, not just a correctness bug, and it's silent.

Don't count on SvelteKit's own cookie forwarding to save you either: event.fetch only auto-forwards cookies cross-origin when the target hostname equals the app hostname or is a subdomain of it (runtime/server/fetch.js, the .${url.hostname}.endsWith(...) check). Sibling domains (api-dev.example.dev vs app-dev.example.dev) and docker-internal hostnames (http://api:8081) get nothing.

Correct setups, in order of preference:

1. Per-call opts + handleFetch. Every oazapfts function accepts trailing opts?: RequestOpts with fetch/headers/baseUrl overrides. Pass { fetch: event.fetch } from loads; add a handleFetch hook that injects the raw incoming Cookie header (and rewrites public→internal API URL) for API-bound requests only. Per-request state lives on the event, nothing shared.

2. Request-scoped default fetch. Set defaults.fetch ONCE at server startup to a wrapper that resolves the current event: getRequestEvent() from $app/server (kit ≥ 2.20), or below that a self-rolled AsyncLocalStorage<RequestEvent> whose .run(event, () => resolve(event)) wraps the first handle in the sequence. The wrapper delegates to event.fetch when in a request, plain fetch otherwise. This makes bare SDK calls in endpoints/hooks per-request-correct with zero callsite churn (ALS context survives awaits on Node ≥ 20 / adapter-node).

Either way, delete every per-request write to the singleton; baseUrl/credentials can be set once at module scope.