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General

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What is x402 in a single sentence?

x402 is an open-source protocol that turns the dormant HTTP 402 Payment Required status code into a fully-featured, on-chain payment layer for APIs, websites, and autonomous agents.

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Why not use traditional payment rails or API keys?

Traditional rails require credit-card networks, user accounts, and multi-step UI flows.
x402 removes those dependencies, enabling programmatic, HTTP-native payments (perfect for AI agents) while dropping fees to near-zero and settling in ~1 second.

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Is x402 only for crypto-native projects?

No. Any web API or content provider—crypto or web2—can integrate x402 if it wants a lower-cost, friction-free payment path for small or usage-based transactions.

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Language & Framework Support

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What languages and frameworks are supported?

TypeScript and Python (coming soon) are the reference implementations, but x402 is an open protocol.

Nothing prevents you from implementing the spec in Go, Rust, etc. If you’re interested in building support for your favorite language, please open an issue and let us know, we’d be happy to help!

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Facilitators

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Who runs facilitators today?

Coinbase Developer Platform operates the first production facilitator. The protocol, however, is permissionless—anyone can run a facilitator. Expect:

  • Community-run facilitators for other networks or assets.
  • Private facilitators for enterprises that need custom KYT / KYC flows.

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What stops a malicious facilitator from stealing funds or lying about settlement?

Every x402PaymentPayload is signed by the buyer and settles directly on-chain.
A facilitator that tampers with the transaction will fail signature checks.

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Pricing & Schemes

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How should I price my endpoint?

There is no single answer, but common patterns are:

  • Flat per-call (e.g., $0.001 per request)
  • Tiered (/basic vs /pro endpoints with different prices)
  • Up-to (work in progress): “pay-up-to” where the final cost equals usage (tokens, MB, etc.)

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Can I integrate x402 with a usage / plan manager like Metronome?

Yes. x402 handles the payment execution. You can still meter usage, aggregate calls, or issue prepaid credits in Metronome and only charge when limits are exceeded. Example glue code is coming soon.

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Assets, Networks & Fees

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Which assets and networks are supported today?

NetworkAssetFees*Status
BaseUSDCfee-freeMainnet
Base SepoliaUSDCfee-freeTestnet
  • Gas paid on chain; Coinbase’s x402 facilitator adds zero facilitator fee.
  • Support for additional chains and assets is on the roadmap and community-driven.

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Security

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Do I have to expose my private key to my backend?

No. The recommended pattern is:

  1. Buyers (clients/agents) sign locally in their runtime (browser, serverless, agent VM). You can use CDP Wallet API to create a programmatic wallet.
  2. Sellers never hold the buyer’s key; they only verify signatures.

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How do refunds work?

The current exact scheme is a push payment—irreversible once executed. Two options:

  1. Business-logic refunds: Seller sends a new USDC transfer back to the buyer.
  2. Escrow schemes: Future spec could add conditional transfers (e.g., HTLCs or hold invoices).

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Usage by AI Agents

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How does an agent know what to pay?

Agents follow the same flow as humans:

  1. Make a request.
  2. Parse the 402 JSON (accepts array).
  3. Choose a suitable requirement and sign a payload via the x402 client SDKs.
  4. Retry with X-PAYMENT.

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Do agents need wallets?

Yes. Programmatic wallets (e.g., CDP Wallet API, viem, ethers-v6 HD wallets) let agents sign EIP-712 payloads without exposing seed phrases.

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Governance & Roadmap

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Is there a formal spec or whitepaper?

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How will x402 evolve?

Tracked in public GitHub issues + community RFCs. Major themes:

  • Multi-asset support
  • Additional schemes (upto, stream, permit2)
  • Discovery layer for service search & reputation

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Why is x402 hosted in the Coinbase GitHub?

We acknowledge that the repo is primarily under Coinbase ownership today. This is primarily to leverage our best-in-house security and auditing team to ensure the spec is safe and nobody accidentally creates legally ambiguous payment flows. We intend to eventually transfer ownership of the repo to a steering group or open-source committee.

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Troubleshooting

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I keep getting 402 Payment Required, even after attaching X-PAYMENT. Why?

  1. Signature is invalid (wrong chain ID or payload fields).
  2. Payment amount < maxAmountRequired.
  3. Address has insufficient USDC or was flagged by KYT.
    Check the error field in the server’s JSON response for details.

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My test works on Base Sepolia but fails on Base mainnet—what changed?

  • Ensure you set network: "base" (not "base-sepolia").
  • Confirm your wallet has mainnet USDC.
  • Gas fees are higher on mainnet; fund the wallet with a small amount of ETH for gas.

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Still have questions?