Platform Balance
Fiat credit card top-up with an API-key-scoped balance. The lowest-friction way for an agent to pay for tool calls.
API Key
Overview
Platform Balance is Custena's native, no-crypto payment method. Buyers top up their account balance with a credit card (via Stripe) and expose an API key to their agent. Every call deducts from that balance at settlement time.
No wallet, no signing, no on-chain fees — just an HTTP header.
Request format
GET /tools/search?q=hello HTTP/1.1
Host: seller.example.com
Authorization: Bearer pk_live_...Custena's middleware resolves the key, checks the agent's remaining balance and spending limits, debits the cost of the call, and forwards the request upstream.
Response on insufficient balance
HTTP/1.1 402 Payment Required
Content-Type: application/json
{
"error": "insufficient_balance",
"required": { "currency": "USD", "amount": 0.01 },
"remaining": { "currency": "USD", "amount": 0.004 },
"topup_url": "https://dashboard.custena.com/billing"
}When to use it
- Enterprise buyers who want clean invoicing and expense reports
- Consumer agents where crypto UX is a non-starter
- Platforms paying on behalf of many end-users