# FAQ #### General **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. **Is x402 a CDP Product?** *No.* While Coinbase Developer Platform provides tooling and are the creators of the standard, it is an open protocol (Apache-2.0 license) and you don't need any Coinbase products to use it. We look forward to further clarifying this distinction and making x402 a credibly neutral payment standard. **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. **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. #### Language & Framework Support **What languages and frameworks are supported?** Typescript, Python, and Go are reference implementations, but x402 is an **open protocol**. Nothing prevents you from implementing the spec in Rust, Java, or other languages. If you're interested in building support for your favorite language, please [open an issue](https://github.com/coinbase/x402/issues) and let us know, we'd be happy to help! #### Facilitators **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. **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. #### Pricing & Schemes **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.) **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. #### Assets, Networks & Fees **Which assets and networks are supported today?** | Network | CAIP-2 ID | Asset | Fees\* | Status | | ------------- | ----------------------------------------- | ----------------------------------- | -------- | ----------- | | Base | `eip155:8453` | Any EIP-3009 token | fee-free | **Mainnet** | | Base Sepolia | `eip155:84532` | Any EIP-3009 token | fee-free | **Testnet** | | Solana | `solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp` | Any SPL token; Token-2022 (v2 only) | fee-free | **Mainnet** | | Solana Devnet | `solana:EtWTRABZaYq6iMfeYKouRu166VU2xqa1` | Any SPL token; Token-2022 (v2 only) | fee-free | **Testnet** | \* 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.* **Does x402 support fiat off‑ramps or credit‑card deposits?** Not natively. However, facilitators or third‑party gateways can wrap x402 flows with on‑ and off‑ramps. #### Security **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. **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). #### Usage by AI Agents **How does an agent know what to pay?** Agents follow the same flow as humans: 1. Make a request. 2. Parse the `PAYMENT-REQUIRED` header. 3. Choose a suitable requirement and sign a payload via the x402 client SDKs. 4. Retry with the `PAYMENT-SIGNATURE` header. **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. #### Governance & Roadmap **Is there a formal spec or whitepaper?** * **Spec:** [GitHub Specification](https://github.com/coinbase/x402/tree/main/specs) * [**Whitepaper**](https://www.x402.org/x402-whitepaper.pdf) **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 **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. #### Troubleshooting **I keep getting `402 Payment Required`, even after attaching `PAYMENT-SIGNATURE`. Why?** 1. Signature is invalid (wrong chain ID or payload fields). 2. Payment amount is less than the required `amount` in the payment requirements. 3. Address has insufficient USDC or was flagged by KYT.\ Check the `error` field in the server's JSON response for details. **My test works on Base Sepolia but fails on Base mainnet—what changed?** * Ensure you set `network: "eip155:8453"` (Base mainnet) instead of `"eip155:84532"` (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. #### Still have questions? • Reach out in the [Discord channel](https://discord.gg/invite/cdp)\ • Open a GitHub Discussion or Issue in the [x402 repo](https://github.com/coinbase/x402) --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. 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