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Inference and Arena draw on the same billing account’s credits. Topping up means getting credits into that account, which is what buying or depositing OpenTokens does.
Depositing USDC on its own does not create credits, and a token transfer hash is not a receipt. A purchase only counts once the collector burns TOKEN and the indexer settles the conversion — check the account’s conversions for finalized: true and status: "credited".

Discovering the account

With a management credential:
With an Arena key:
Both return the same billing account’s paid TOKEN identity, deposit address, balances and conversion history. An optional deploymentId selects an advertised chain; Base is the default. Conversion availability is separate from inference availability — ordinary signup, key management and inference all work while conversion is off, in which case bootstrap returns depositAddress, chainId and token as null.

Buying with USDC

Recover recent orders with GET /paid-opentoken/purchases, and read pending work with GET /paid-opentoken/operations. destination: "credits" mints to your account’s receiver. destination: "wallet" sends TOKEN to the paying address instead.
Purchase terms expire about 15 minutes after they are issued, so sign and authorize promptly rather than holding them. The signature is a 65-byte hex string — 0x followed by 130 hex characters.Reusing an idempotency key with a different deploymentId, payer, amount or destination is rejected. Replay the same terms, or use a new key.

Depositing TOKEN you already hold

The response carries an ID and an x402 paymentRequired. Sign the exact Permit2 or EIP-2612 payload with the wallet, submit it to POST /paid-opentoken/deposits/{id}/settle, then poll GET /paid-opentoken/deposits/{id}.

Recovering an uncertain payment

Persist the creation body, the idempotency key and the exact signature before you submit. After a timeout, replay those original terms and query the same ID. Never replace an uncertain payment with a fresh one.