The auto-recharge mechanic behind your first pay-per-use bill

Five settings to change on day one

1. Set a spending limit in the Developer Console. Before you deploy anything. This is the only hard control you have, and the $400 cap is not it, because $400 a cycle is probably not your intended budget.
2. Count your link posts. Any post containing a URL bills $0.20 against $0.015 for a plain post. For most migrated accounts this one number explains the entire bill. Ten links a day is $60 a month on its own.
3. Replace polling with events where you can. Reads bill per post returned, not per request, so a search call returning 100 posts costs $0.50. Where an Activity API webhook event exists for what you are polling for, use it. post.delete and dm.sent are free.
4. Never retry a permanent error. A 403 will still be a 403 on the fiftieth attempt, and every attempt may bill. This is the single most common way a migrated account reaches the cap.
5. Alert on your own burn rate. The recharge email arrives after the charge. If you want a control rather than a notification, track spend in your own monitoring and page yourself on the slope, not the balance.

// The retry policy that matters most after migration.
// A permanent failure retried in a loop is how a $400 cycle happens.
const PERMANENT = new Set([400, 401, 403, 404, 422]);

async function postToX(body: unknown, attempt = 0): Promise<Response> {
  const res = await fetch('https://api.x.com/2/tweets', {
    method: 'POST',
    headers: {
      Authorization: `Bearer ${process.env.X_USER_ACCESS_TOKEN}`,
      'Content-Type': 'application/json',
    },
    body: JSON.stringify(body),
  });

  if (res.ok) return res;
  if (PERMANENT.has(res.status)) return res;   // never retry, it will never succeed
  if (attempt >= 2) return res;                // bounded, not infinite

  await new Promise((r) => setTimeout(r, 2 ** attempt * 1000));
  return postToX(body, attempt + 1);
}