Agent A piece of AI software that acts autonomously for a user or another system. Within Opus Protocol, the agent is always the buyer: it holds the wallet and pays for services.

Allowance A capped, on-chain spending grant, opened by an agent wallet (the granter) and spendable by a service (the grantee). Where a subscription bills on a schedule, an allowance has none: the grantee draws from it whenever usage happens, up to the cap. See Allowances.

Billing cycle How often a subscription is charged. The Plan’s interval field sets it to monthly, weekly, daily, or per-request.

Collection A provider taking payment from a subscriber’s wallet at the close of a billing cycle. The right to do so comes from the Subscription Authority; the actual transfer is carried out by the Opus Protocol Subscriptions contract.

Facilitator The off-chain service that checks payment headers and submits the resulting transactions to Robinhood Chain. In the beta it is a single centralized service run by Opus Protocol. After the beta it becomes a permissionless network made up of staked operator nodes. See Facilitator Network.

Grantee Whoever an Allowance permits to spend the granter’s funds: a service, or the wallet it controls.

Granter The wallet on the agent side that opens an Allowance and decides its cap.

MPP (Machine Payments Protocol) An open standard, on the IETF track, for payments between machines over HTTP. Stripe and Tempo published it in March 2026. Agents that speak MPP are intended to work with Opus Protocol as-is, with no changes. See MPP Compatibility.

Payment gate Middleware, exposed as opusprotocol.paymentGate(), that catches requests to a service API that carry no credentials and replies with 402 Payment Required along with the terms for paying.

Payment proof A signed object the Facilitator issues once a payment has settled on-chain. The client sends it back in the X-PAYMENT header of the retried request; the service checks it and then serves the resource.

Payment session A token with a short lifetime that authorizes many requests in quick succession against a balance committed up front, with settlement happening in periodic batches. Built for agents that make requests at high frequency. Arrives with general availability. See Payment Sessions.

Plan A record of pricing that a provider publishes on-chain to the Opus Protocol Plan Registry. It sets the price, the billing interval, the token, and an optional trial period for a service, and cannot be changed after creation. See Plans.

Plan Registry The catalog on Robinhood Chain mainnet in which providers’ published Plans are stored. Any Robinhood Chain client can read it.

Provider A service or API that takes payment from agents via Opus Protocol. Providers publish Plans, put the payment gate in front of their API, and collect from their subscribers.

Opus Protocol SDK (@opusprotocol/sdk) The TypeScript library that wraps all protocol and on-chain interactions. It exposes OpusProtocolAgent for agents that pay and OpusProtocolProvider for services that collect.

Subscription An on-chain record that ties an agent wallet to a Plan and delegates billing authority to the provider. Because of that delegation, recurring charges go through automatically; the agent does not sign again each cycle. See Subscriptions.

Subscription Authority What the agent signs at subscription time: a delegation that lets the provider collect up to the authorized amount once per billing cycle, and the Opus Protocol Subscriptions contract enforces that limit on-chain.

Token Any fungible ERC-20 token on Robinhood Chain. USDG is the canonical one. All Opus Protocol payments are made in such a token.

Trial period A free period at the start of a subscription, optional and defined on the Plan. Nothing is collected while the trial runs; the first billing cycle starts when the trial ends.

USDG Global Dollar, a regulated US dollar stablecoin from Paxos (issued by Paxos Digital Singapore under the supervision of the Monetary Authority of Singapore). It is native to Robinhood Chain and is the primary payment token for Opus Protocol during the beta. On Robinhood Chain mainnet its address is 0x5fc5360D0400a0Fd4f2af552ADD042D716F1d168, and it uses 6 decimal places.

Webhook An HTTP POST that Opus Protocol sends to an endpoint you configure each time something happens in the payment lifecycle: a subscription renews, a payment fails, an allowance runs out, and similar events. See Webhooks.

x402 A payment protocol built into HTTP. It defines how a server tells a client that it must pay, and how the client then presents proof that it has done so. Coinbase created it in May 2025; today the x402 Foundation, part of the Linux Foundation, governs it. Opus Protocol is a first-class implementation of x402 on Robinhood Chain. See x402 Compatibility.