# Building PayWatcher: x402-native USDC Payment Verification on Base

*A solo founder building in public from Austria*

By [PayWatcher](https://paragraph.com/@paywatcher) · 2026-04-13

base, x402, web3, payments, stabelcoins, building-in-public, usdc, ai-agents

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We've been quietly shipping for the past few months. Here's what PayWatcher is and why we built it on Base.

**The problem**

Every developer building with stablecoins runs into the same wall: you need to know when a USDC payment has arrived on-chain. Polling the blockchain yourself is brittle. Existing payment processors take a percentage cut or hold your funds in custody. Neither makes sense for stablecoin-native applications.

**What we built**

PayWatcher is a USDC payment verification API. Non-custodial, developer-first, $0.05 flat fee per verification.

You tell PayWatcher which payment you expect — the amount, the deposit address, the chain. It watches the blockchain and fires a signed webhook the moment the payment is confirmed. That's it.

No percentage. No custody. No lock-in. Just: did the payment arrive?

**What's live today**

*   Multi-chain EVM backend: Base, Arbitrum, Ethereum, Optimism, Polygon
    
*   POST /v1/payments/verify — x402 native, no API key required
    
*   GET /v1/verifications — for dashboards and agent workflows
    
*   Frontend: landing page, dashboard, interactive demo at paywatcher.dev/demo
    
*   CI/CD integration tests across 5 testnets
    

**Why x402**

PayWatcher is one of the few projects running x402 verification in production. AI agents can verify USDC payments on Base without an API key — just HTTP 402 + txHash. We built the verification layer that makes x402-based payment flows reliable.

**What's next**

A PayWatcher MCP Server at api.paywatcher.dev/mcp — agents call PayWatcher tools directly and pay per call via x402. No account, no onboarding. Connect, pay, verify.

ChainSights (our governance data API) will use PayWatcher internally as its x402 verification layer — dogfooding the stack end-to-end.

**The ask**

We're applying for a Base Builder Grant (1-3 ETH) to cover infrastructure costs — Vercel Pro, Alchemy RPC, tooling — and accelerate the MCP server launch.

If you're building with x402 or stablecoin payments on Base, we'd love to connect.

→ [paywatcher.dev](http://paywatcher.dev) → @paywatcher on Farcaster, @X\_Sem\_pre on X

_Mario Semper — masemIT e.U., Austria. Solo founder, building in public._

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*Originally published on [PayWatcher](https://paragraph.com/@paywatcher/building-paywatcher-x402-native-usdc-payment-verification-on-base)*