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

> Integrate Base Builder Codes into an x402 app to get onchain attribution for
transactions on Base. Use when the user wants to set up builder code
attribution, track x402 payments, or get credited for driving or serving
x402 transactions on Base.


# Builder code.skill

# x402 Builder Codes Setup

**First, ask the user:** Are you a buyer/client (you make payment requests) or a seller (you serve paid endpoints)? Then follow the corresponding path below.

***

## Path A: Buyer / Client

Integrate a Base Builder Code into your x402 client so every payment you drive on Base is attributed to your app onchain.

### Steps

1. **Check for a Base Builder Code**

   Ask the user if they already have a Base Builder Code. If not, direct them to:

   > Go to [https://dashboard.base.org](https://dashboard.base.org) and log in. Register your app name, add and verify your domain, then go to **Settings → Builder Codes** to get your code.

   The code looks like `bc_b7k3p9da` and must match `^[a-z0-9_]{1,32}$` (lowercase letters, digits, underscores, 1–32 characters).

2. **Install dependencies**

   ```bash theme={null}
   npm install @x402/fetch @x402/extensions @x402/evm
   ```

3. **Register the BuilderCodeClientExtension**

   In the file where you create your x402 client, import and register `BuilderCodeClientExtension`. Replace `"your_builder_code"` with the user's actual Base Builder Code.

   ```typescript theme={null}
   import { x402Client, wrapFetchWithPayment } from "@x402/fetch";
   import { ExactEvmScheme } from "@x402/evm/exact/client";
   import { BuilderCodeClientExtension } from "@x402/extensions/builder-code";

   const client = new x402Client();
   client.register("eip155:*", new ExactEvmScheme(signer));
   client.registerExtension(new BuilderCodeClientExtension("your_builder_code"));

   const fetchWithPayment = wrapFetchWithPayment(fetch, client);

   // All x402 payments made through fetchWithPayment will carry the Builder Code
   const response = await fetchWithPayment("https://example.com/paid-endpoint");
   ```

4. **Verify attribution**

   After an x402 payment settles on Base, confirm your Builder Code was recorded. Get the settlement transaction hash and run:

   ```typescript theme={null}
   import { createPublicClient, http } from "viem";
   import { base } from "viem/chains";
   import { parseBuilderCodeSuffixFromCalldata } from "@x402/extensions/builder-code";

   const publicClient = createPublicClient({ chain: base, transport: http() });
   const tx = await publicClient.getTransaction({ hash: "0xYourTxHash" });

   const attribution = parseBuilderCodeSuffixFromCalldata(tx.input);
   console.log(attribution);
   // { a: "seller_app_code_if_integrated", w: "cdp_facil", s: "your_builder_code" }
   // `s` appears if you completed setup above; `w` appears if using CDP Facilitator;
   // `a` is only present if the seller also integrated Builder Codes.
   ```

   Assert: `attribution?.s === "your_builder_code"`.

   Alternatively, paste the transaction hash into [https://buildercode-checker.vercel.app/](https://buildercode-checker.vercel.app/) to inspect attribution without writing any code.

***

## Path B: Seller / Resource Server

Declare a Base Builder Code on your x402 resource server so every payment your endpoint receives on Base is attributed to your app onchain.

### Steps

1. **Check for a Base Builder Code**

   Ask the user if they already have a Base Builder Code. If not, direct them to:

   > Go to [https://dashboard.base.org](https://dashboard.base.org) and log in. Register your app name, add and verify your domain, then go to **Settings → Builder Codes** to get your code.

   The code looks like `bc_b7k3p9da` and must match `^[a-z0-9_]{1,32}$` (lowercase letters, digits, underscores, 1–32 characters).

2. **Install dependencies**

   ```bash theme={null}
   npm install @x402/extensions
   ```

3. **Declare the BuilderCodeExtension on your routes**

   Add `declareBuilderCodeExtension` to the `extensions` field of each route you want attributed. Replace `"your_builder_code"` with the user's actual Base Builder Code.

   ```typescript theme={null}
   import { paymentMiddleware, x402ResourceServer } from "@x402/express";
   import { ExactEvmScheme } from "@x402/evm/exact/server";
   import { BUILDER_CODE, declareBuilderCodeExtension } from "@x402/extensions/builder-code";

   app.use(
     paymentMiddleware(
       {
         "GET /weather": {
           accepts: [
             {
               scheme: "exact",
               price: "$0.001",
               network: "eip155:8453",
               payTo: "0xYourAddress",
             },
           ],
           description: "Weather data",
           mimeType: "application/json",
           extensions: {
             [BUILDER_CODE]: declareBuilderCodeExtension("your_builder_code"),
           },
         },
       },
       new x402ResourceServer(facilitatorClient).register("eip155:8453", new ExactEvmScheme()),
     ),
   );
   ```

   For Go, import `github.com/x402-foundation/x402/go/v2/extensions/buildercode` and call `buildercode.DeclareBuilderCodeExtension("your_builder_code")` in the route's `Extensions` map.

4. **Verify attribution**

   After a payment settles, paste the transaction hash into [https://buildercode-checker.vercel.app/](https://buildercode-checker.vercel.app/) or parse the calldata programmatically.

   Assert: the `a` field in the parsed attribution matches your declared builder code.

***

## Notes

* **Dashboard:** Attribution data and app metrics are visible at [https://dashboard.base.org](https://dashboard.base.org).
* **Docs:** Full details at [https://docs.cdp.coinbase.com/x402/core-concepts/builder-codes](https://docs.cdp.coinbase.com/x402/core-concepts/builder-codes).