Mint a key per customer.
In one API call.

// On every new customer signup in your app:
const r = await fetch("https://api.aigateway.sh/v1/sub-accounts", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${AIGATEWAY_KEY}`,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    name: `customer_${customerId}`,
    spend_cap_cents: 5000,         // $50/mo hard cap
    rate_limit_rpm: 300,
    allowed_models: ["openai/gpt-5.4-mini",
                     "anthropic/claude-haiku-4.5"],
    default_tag: `tenant:${customerId}`,
  }),
});
const { key, id } = await r.json();
// → key: "sk-aig-..."   id: "sa_9f3k..."
// Save id; hand key to customer (or proxy through your backend).

# Your customer uses it like any other OpenAI key:
from openai import OpenAI

client = OpenAI(
    base_url="https://api.aigateway.sh/v1",
    api_key="sk-aig-...",  # the sub-account key
)

r = client.chat.completions.create(
    model="openai/gpt-5.4-mini",
    messages=[{"role": "user",
               "content": "Hi"}],
)

# Spend lands on YOUR AIgateway bill, tagged tenant:abc
# Hits cap at $50 → returns 402 payment_required
# Webhook sub_account.budget.exhausted fires to your backend