x402 Bazaar — Pay-per-call API marketplace with Base Builder Codes

A live pay-per-call API marketplace on Base mainnet — 402.com.tr. 143 public services as of 2026-09-06 (live catalogue) — token safety, a 29-tool B20 protection suite incl. real-time seizure alerts and cover for all 13 of Coinbase's tokenized equities, wallet intelligence, OFAC screening, AI-written reports — sold to AI agents and humans over x402, settled in USDC via the Coinbase CDP facilitator, with onchain attribution via Builder Codes (ERC-8021 Schema 2). Listed in the CDP x402 discovery index; consumable via MCP (x402-bazaar-mcp), plain HTTP, or the Farcaster/Base App mini-app.

One Next.js app plays all three roles in the x402 flow:

Role	Builder Code	Where
Seller (resource server)	a (app)	src/app/api/x402/[service]/route.ts
Buyer (client)	s (service)	src/app/api/buy/route.ts
Facilitator (Coinbase CDP)	w = cdp_facil	settles + writes the calldata suffix

What it does

• Marketplace (/): every service is a real x402-protected, pay-per-call endpoint — pay from your own browser wallet (or the server buyer), a USDC micro-payment settles on Base, you get the data + the settlement tx. Safety responses include an auditable pre-spend receipt (GO/HOLD/STOP).
• B20 protection suite: 29 tools reading Base's native-token precompiles — freeze/seize risk, "when did it turn seizable", real-time PolicyUpdated alerts (CDP webhooks), launch radar.
• Tokenized equities: Coinbase's tokenized stocks are B20 Asset tokens, so the same reads cover them. b20_safety tells a real issuance apart from a token merely wearing an equity ticker, by checking who administers the transfer policy on chain — not by consulting a list of addresses. That recognises all 13 (AAPLc, AMZNc, COINc, CRCLc, GOOGLc, INTCc, METAc, MSFTc, MSTRc, NVDAc, SNDKc, SPCXc, TSLAc) with no per-token configuration, and a 14th the day it is issued. It also reports holder-eligibility gating and gated mint as the regulated shape they are, rather than scoring a compliant issuer as dangerous. What it does not claim is a corporate-action feed: across all thirteen the number of multiplier changes to date is zero, so there is nothing yet to serve — a watcher records the first one when it happens.
• Protect wallet (/app?mode=wallet): scan approvals, revoke the risky ones gas-free (sponsored via CDP Paymaster; one-signature Revoke All on smart wallets).
• Attribution dashboard (/dashboard): paste any Base settlement tx hash; we read its calldata and decode the a / w / s Builder Codes straight from chain (no DB, no trust).

How Builder Codes are wired

Seller declares the app code per route:

import { BUILDER_CODE, declareBuilderCodeExtension } from "@x402/extensions/builder-code";

extensions: { [BUILDER_CODE]: declareBuilderCodeExtension(appBuilderCode) }

Buyer attaches the client code (and auto-echoes the seller's a):

import { BuilderCodeClientExtension } from "@x402/extensions/builder-code";

client.registerExtension(new BuilderCodeClientExtension(clientBuilderCode));

Verification reads it back from the chain:

import { parseBuilderCodeSuffixFromCalldata } from "@x402/extensions/builder-code";

const attribution = parseBuilderCodeSuffixFromCalldata(tx.input); // { a, w, s }

Setup

1. Install

   npm install

2. Get a Builder Code at dashboard.base.org: register your app → verify your domain → Settings → Builder Codes.

3. Configure — copy .env.example to .env.local and fill in:

   • APP_BUILDER_CODE / CLIENT_BUILDER_CODE — your codes (^[a-z0-9_]{1,32}$)
   • PAY_TO_ADDRESS — wallet that receives USDC
   • CDP_API_KEY_ID / CDP_API_KEY_SECRET — from portal.cdp.coinbase.com (needed for mainnet settlement)
   • BUYER_PRIVATE_KEY — a wallet with USDC + a little ETH on Base, used by the server-side Pay & call button
   • NEXT_PUBLIC_BASE_APP_ID — your base:app_id (renders the Base App verification meta tag). Not the Builder Code — that's a separate short code under base.dev → Settings → Builder Code.
   • ENABLE_BUYER (true/false) and BUY_ACCESS_TOKEN — public-deploy safety (see below)

4. Run

   npm run dev

   Open http://localhost:3000. The status bar shows whether the seller and buyer are fully wired.

Mainnet = real money. Prices are small ($0.002–$0.75 USDC per call) but every Pay & call is a real onchain settlement. Keep the buyer wallet funded with only what you need.

Deploy to Vercel

The repo is Vercel-ready. Import sukrutkrdg/402 in Vercel, add the env vars above in Project → Settings → Environment Variables, and deploy.

Before exposing it publicly, read this:

• 🔑 The buyer wallet spends real USDC. /api/buy is the server-side spend endpoint. On a public URL, anyone could trigger payments. Protect it:
  • ENABLE_BUYER=false → fully view-only showcase (browse + on-chain dashboard, no spending), or
  • BUY_ACCESS_TOKEN=<secret> → callers must supply the token (UI field / x-buy-token header).
  • A per-IP rate limit (5/min) is always on as a backstop.
  • Keep only a small balance in the buyer wallet regardless.
• 💾 Recent settlements are not durable on serverless. The store keeps an in-memory cache + best-effort temp file; it won't persist across instances on Vercel. The dashboard's on-chain lookup is unaffected. For durable history, swap src/lib/store.ts for Vercel KV / Upstash (same function signatures).

Verifying attribution

• In-app: every purchase shows its tx + decoded a/w/s, with links to BaseScan and the Coinbase Builder Code checker.
• In the Base dashboard: x402 traffic with your a code is attributed to your project (note: early user counts may be overstated since they reflect facilitator relayer addresses).

Stack

Next.js 16 (App Router) · @x402/next · @x402/fetch · @x402/evm · @x402/extensions · @coinbase/x402 (CDP facilitator) · viem · Tailwind CSS.

Project layout

src/
  lib/
    config.ts        env + network constants
    services.ts      the marketplace catalog (+ handlers)
    x402-server.ts   seller: resource server + CDP facilitator
    x402-client.ts   buyer: signer + payment-enabled fetch
    store.ts         flat-file payment log
  app/
    api/x402/[service]/route.ts   seller endpoints (declares `a`)
    api/buy/route.ts              buyer (attaches `s`, settles)
    api/attribution/route.ts      decode `a/w/s` from calldata
    api/payments/route.ts         recent settlements
    api/status/route.ts           config status for the UI
    page.tsx                      marketplace
    dashboard/page.tsx            attribution dashboard