Circle, the issuer of the USDC stablecoin, has introduced a new discovery layer within its Agent Stack that allows AI agents to find and pay for digital services automatically. The protocol enables agents to discover consumable services using USDC and x402, building on a marketplace that already lists more than 900 endpoints for agentic services.
The Discovery API gives builders programmatic access to Circle's Agent Marketplace catalogue through a single public endpoint, without requiring an API key, authentication, or a Circle account. An agent can query the catalogue for services based on criteria such as category, blockchain, and pricing structure, then call the relevant endpoint directly via HTTP, without prior registration.
When a service is called, the server responds with an HTTP 402 Payment Required status, a code historically reserved but unused, containing a machine-readable payload that specifies the price, accepted tokens, and payment destination. The agent's wallet then cryptographically signs a payment authorisation and retries the request. A facilitator verifies the payload and settles the USDC transaction on-chain, after which the service returns the requested output. Circle has positioned the mechanism as a way to let agents complete tasks such as filling out forms, bypassing CAPTCHA challenges, or signing up for recurring subscriptions without manual intervention.
Application for individual users
According to Circle, individuals could deploy a single AI assistant with a funded wallet to manage multiple software subscriptions on a pay-per-request basis. Potential use cases include on-demand computing, data gathering, pay-per-inference access to specialised AI models listed in the marketplace, and the consolidation of recurring payments into smaller, incremental transactions.
Circle has stated that users retain ownership and control of funds throughout. The service operates through Circle's Developer-Controlled Wallets, secured using multi-party computation technology that fragments cryptographic keys so agents cannot access the master seed phrase. Agents instead act as authorised signers under transaction rules defined by the user, including per-transaction caps, rolling budgets, and whitelists enforced at the smart contract or wallet policy layer.
Addressing transaction risk
Because blockchain transactions are irreversible, Circle has built risk controls into the system rather than relying on transaction reversal. An isolated funding model prevents agents from being linked to primary bank accounts, limiting potential losses to the capital held in a given agent wallet. The system also includes circuit breakers that detect an unusually high frequency of 402 payment requests within a short period; when triggered, the wallet software revokes the agent's signing permissions and notifies the user.
The launch adds to a broader industry effort to define payment standards for machine-to-machine commerce, an area where stablecoins such as USDC are increasingly positioned as a settlement layer for automated, low-value transactions between software agents.
