deepMerge mutates its arguments, corrupting request payloads across calls

[richard-willis-chevin]

deepMerge mutates both arguments instead of returning a new object.

Still present on main.

// deepMerge as generated in src/api/<client>/utils.ts
function deepMerge(target, source) {
  const returnType = target || {};
  for (const key in source) {
    if (source[key] instanceof Object)
      Object.assign(source[key], deepMerge(returnType[key], source[key]));
  }
  Object.assign(returnType || {}, source);
  return returnType;
}

const fetcherOptions = {};                        // shared per hook instance
const send = (variables) => deepMerge(fetcherOptions, variables);

send({ body: { title: 'Fleet', archivedAt: '2026-01-01' } });
send({ body: { title: 'Costs' } });
// sends { title: 'Costs', archivedAt: '2026-01-01' }  — stale field leaked

The bug can be replicated using the generated (request query hooks) code like so:

export function DeepMergeDemoScreen() {
  const { mutateAsync: createDashboard, isPending } = useCreateDashboard();

  const onRun = async () => {
    await Promise.all(
      ['Demo A', 'Demo B', 'Demo C'].map((title) =>
        createDashboard({
          body: { title },
        }),
      ),
    );
  };

  return (
    <button onClick={onRun}>
      {isPending ? 'Creating…' : 'Create 3 dashboards at once'}
    </button>
  );
}

In the example above the request body payload is incorrect for some of the requests.