Read the credit headers: audit every charge from the response

curl -i -X POST https://api.qannasapi.com/v1/data/contacts/verify-email \
  -H "Authorization: Bearer $QANNAS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"query": "jane@acme.example"}'

# < HTTP/1.1 200 OK
# < X-Credits-Charged: 1      # what this call cost
# < X-Credits-Balance: 94     # what is left afterwards
# < X-Price-Book: public    # which rules were applied

export async function call(action: string, body: unknown) {
  const res = await fetch(`${API}/v1/data/${action}`, {
    method: "POST",
    headers: { Authorization: `Bearer ${key}`, "Content-Type": "application/json" },
    body: JSON.stringify(body),
  });

  const json = await res.json();
  // Your logs become the billing audit trail.
  logger.info("qannas.call", {
    action,
    request_id: json.request_id,
    charged: Number(res.headers.get("X-Credits-Charged")),
    balance: Number(res.headers.get("X-Credits-Balance")),
  });

  return json;
}

// The balance arrives on the call you already made —
// there is no quota endpoint to poll and no lag.
const balance = Number(res.headers.get("X-Credits-Balance"));
if (balance < LOW_WATER_MARK) {
  pauseNonUrgentJobs();
  alertOps(`credits low: ${balance}`);
}