> ## Documentation Index
> Fetch the complete documentation index at: https://docs.x402.org/llms.txt
> Use this file to discover all available pages before exploring further.

# 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/x402-foundation/x402/issues) and let us know, we'd be happy to help!

### Facilitators

#### Who runs facilitators today?

Multiple organizations operate production facilitators. The protocol is **permissionless**—anyone can run a facilitator. See the [x402 Ecosystem](https://www.x402.org/ecosystem?filter=facilitators) for available options, including:

* Community‑run facilitators for various networks and assets
* Private facilitators for enterprises that need custom KYT / KYC flows.

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

Every x402 `PaymentPayload` is **signed by the buyer** and settled **directly on‑chain**.\
A facilitator that tampers with the transaction would fail signature checks and would **not be able to** settle the transaction.

### 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** (`scheme: "upto"`): The client authorizes a maximum amount but is only charged for actual usage (tokens, compute time, bandwidth, etc.). Available on EVM networks in TypeScript, Go, and Python. See the [Seller Quickstart](/getting-started/quickstart-for-sellers#payment-schemes-exact-vs-upto) for setup.

#### 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 ERC-20 token                  | fee-free | **Mainnet** |
| Base Sepolia  | `eip155:84532`                            | Any ERC-20 token                  | fee-free | **Testnet** |
| Solana        | `solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp` | Any SPL token or Token-2022 token | fee-free | **Mainnet** |
| Solana Devnet | `solana:EtWTRABZaYq6iMfeYKouRu166VU2xqa1` | Any SPL token or Token-2022       | fee-free | **Testnet** |

\* Gas paid on chain; many facilitators offer **zero** facilitator fees (see [ecosystem](https://www.x402.org/ecosystem?filter=facilitators) for details).

*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. For Solana-based payments, agents can use **@solana/kit** to sign transactions (see the [Solana buyer quickstart](https://docs.x402.org/getting-started/quickstart-for-buyers#solana-svm) for an example).

### Governance & Roadmap

#### Is there a formal spec or whitepaper?

* **Spec:** [GitHub Specification](https://github.com/x402-foundation/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 (`stream`)
* 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 does not exactly match the required `amount` in the payment requirements (the exact scheme requires strict equality - no overpayment or underpayment).
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/x402-foundation/x402)