--- id: agent-that-can-buy-things type: stack title: Agent that can buy things summary: The pieces for an agent that pays for APIs per call with x402 stablecoins and buys from merchants with controlled virtual cards, with spend limits, proof of user intent and a full audit trail. description: "Two payment paths cover most of what an agent needs to buy. Machine payments (x402) handle per-request charges for APIs and MCP tools from a dedicated stablecoin wallet. Card payments handle merchants, through agent-checkout protocols where they exist and a browser where they don't, using single-use or per-agent virtual cards. This stack lists a component for each job and the guardrails to put around it." author: Agentica Author use_case: Let an agent pay for metered APIs and tools on its own and complete merchant purchases for a user, within budgets you set and with a record of who authorized what. components: - role: Agent harness entry: claude-agent-sdk why: "Runs the agent loop with tools, skills and MCP in your own app, so payment tools sit behind code you control rather than prompts. Any harness with tool calling works." - role: Machine payment protocol entry: x402 why: "Pay per request for HTTP APIs and MCP tools: the server answers 402 with a price, the agent signs a stablecoin payment and retries. SDKs default to USD stablecoins and a $1 cap per payment." - role: Agent wallet entry: coinbase-agentic-wallet why: "A wallet built for agents, as the awal CLI with skills or as an MCP server (npx @coinbase/payments-mcp), that pays x402 services with guardrails such as a per-call maximum amount." - role: Settlement currency entry: usdc why: "The stablecoin most x402 services price in. Fund the agent's wallet with only what it may spend, and test on Base Sepolia with faucet USDC first." - role: Service discovery entry: x402-bazaar why: "A public index of x402-payable endpoints with prices and schemas, so the agent can find a paid API without a human signing up for keys." - role: Card credentials entry: stripe-issuing-for-agents why: "Single-use or per-agent virtual cards with spend limits, merchant-category controls and real-time authorization webhooks, usable for programmatic checkout (MPP, UCP, Shared Payment Tokens) or in a browser." - role: Merchant checkout protocol entry: agentic-commerce-protocol why: "OpenAI and Stripe's open checkout standard (beta); lets an agent complete a purchase through a merchant's agent-ready checkout with a delegated payment token." - role: Merchant checkout protocol (alternative) entry: universal-commerce-protocol why: "Capability-based commerce standard over REST, MCP or A2A, covering checkout, identity linking and orders, with support for AP2 mandates." - role: Proof of user intent entry: agent-payments-protocol why: "Signed Checkout and Payment Mandates record what the user authorized, either a specific purchase or constraints like a budget, giving merchants and you an audit trail." - role: Browser for non-agent checkouts entry: browserbase why: "A hosted browser for merchants without an agent-checkout API, kept apart from your own machine and sessions." - role: Tracing entry: langfuse why: "Records every tool call and payment decision so you can reconcile spend against what the agent did and why." tags: [payments, agent-commerce, x402, stablecoins, cards, guardrails] entries: [claude-agent-sdk, x402, coinbase-agentic-wallet, usdc, x402-bazaar, cdp-x402, stripe-issuing-for-agents, agentic-commerce-protocol, universal-commerce-protocol, agent-payments-protocol, stripe-agentic-commerce, machine-payments-protocol, browserbase, langfuse] sources: - title: x402 Protocol Specification v2 url: https://github.com/x402-foundation/x402/blob/main/specs/x402-specification-v2.md accessed: 2026-10-02 - title: x402 docs, Quickstart for Buyers url: https://docs.x402.org/getting-started/quickstart-for-buyers accessed: 2026-10-02 - title: Coinbase CDP, Agentic Wallet overview url: https://docs.cdp.coinbase.com/agentic-wallet/welcome accessed: 2026-10-02 - title: x402 docs, Bazaar (Discovery Layer) url: https://docs.x402.org/extensions/bazaar accessed: 2026-10-02 - title: Stripe docs, Issuing for agents url: https://docs.stripe.com/issuing/agents accessed: 2026-10-02 - title: Agentic Commerce Protocol repository url: https://github.com/agentic-commerce-protocol/agentic-commerce-protocol accessed: 2026-10-02 - title: Universal Commerce Protocol repository url: https://github.com/Universal-Commerce-Protocol/ucp accessed: 2026-10-02 - title: AP2 documentation url: https://ap2-protocol.org/ accessed: 2026-10-02 related: [pay-for-an-api-with-x402, pay-with-x402, agent-payment-rails, run-untrusted-code-in-a-sandbox] last_verified: 2026-10-02 published: 2026-10-02 --- ## Two ways an agent pays **Per call, in stablecoins.** For APIs and MCP tools that charge per request, the agent holds a small [USDC](https://indexagentica.com/entries/usdc/) balance in an [agent wallet](https://indexagentica.com/entries/coinbase-agentic-wallet/) and pays with [x402](https://indexagentica.com/entries/x402/): request, get 402 with a price, sign, retry. There are no accounts or API keys to create, and each payment is capped. The [x402 guide](https://indexagentica.com/guides/pay-for-an-api-with-x402/) walks through it, and the [pay-with-x402 skill](https://indexagentica.com/skills/pay-with-x402/) teaches a coding agent to do it safely. **At a merchant, by card.** For physical goods, subscriptions and anything sold through a normal checkout, the agent uses a card you control: a single-use or per-agent virtual card from [Stripe Issuing for agents](https://indexagentica.com/entries/stripe-issuing-for-agents/). Where the merchant supports an agent-checkout protocol ([ACP](https://indexagentica.com/entries/agentic-commerce-protocol/) or [UCP](https://indexagentica.com/entries/universal-commerce-protocol/)), the card travels as a delegated token and the agent never sees the number. Where it doesn't, the agent fills in the checkout in a hosted browser ([Browserbase](https://indexagentica.com/entries/browserbase/)). The [agent payment protocols comparison](https://indexagentica.com/compare/agent-payment-rails/) explains how these protocols relate, including [MPP](https://indexagentica.com/entries/machine-payments-protocol/), which can serve both stablecoin and card payments on one endpoint. ## Guardrails to set before the first purchase - **Separate money.** Give the agent its own wallet and its own cards. Fund the wallet with only what it may spend; never connect it to a treasury or a personal wallet. - **Caps at every layer.** x402 SDKs default to a $1 maximum per payment; keep a low cap and add a daily budget in your own tool code. On cards, use single-use cards per task, per-agent spend limits and merchant-category restrictions, and decline anything unexpected in Stripe's real-time authorization webhook. - **Human approval above a threshold.** Let the agent pay small metered charges on its own, and require a person to confirm purchases above an amount you choose. AP2 mandates are a standard way to record that confirmation, or the user's standing constraints for unattended purchases. - **Treat inputs as hostile.** Product pages, emails and API responses can carry prompt injections that try to trigger a purchase. Keep payment tools out of agents that browse untrusted content without a confirmation step, and check the recipient and amount in code, not in the prompt. - **Test first.** Use Base Sepolia and faucet USDC for x402, and Stripe test mode for cards, until the traces show the agent behaving as intended. - **Reconcile.** Trace every payment tool call with [Langfuse](https://indexagentica.com/entries/langfuse/) and match it against wallet transactions and card authorizations.