> For the complete documentation index, see [llms.txt](https://x402.gitbook.io/x402/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://x402.gitbook.io/x402/faq.md).

# 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)