x402 Inspector

Live: https://dansgit2b.github.io/x402-inspector/

This browser tool decodes and lints x402 payment headers: PAYMENT-REQUIRED (base64 or JSON 402 bodies), X-PAYMENT / PAYMENT-SIGNATURE payloads, and PAYMENT-RESPONSE settlement receipts. It's handy when you're building or debugging a pay-per-call API or an agent that pays for one.

• Decodes v1 and v2 payloads and shows USDC amounts in human units.
• Flags missing fields, non-CAIP-2 networks in v2, malformed payTo, non-integer amounts, missing EIP-712 extra, and expired EIP-3009 authorizations.
• Everything runs locally as a static page, with no backend and no tracking.

Try it: open the page and click Load sample, or paste the PAYMENT-REQUIRED header from any 402 response (curl -si https://your-api/endpoint | grep -i payment-required).

Pro pass (0.05 USDC)

Decoding and linting are free. A one-time 0.05 USDC pass unlocks batch mode and Markdown report export. You pay in USDC on Base or Arc straight to the Lumen Works wallet 0xee9790cB3960952F585379F831a89b4e00B4029A, and the page checks the transaction on-chain. Since the code is MIT you could remove the check yourself; the pass is a fair way to fund upkeep.

Run locally

Open docs/index.html. There's no build step.

More from Lumen Works

• x402 Inspector: decode and lint x402 payment headers (repo)
• EIP-712 Inspector: see what a typed-data signature authorises, with hashes and signer recovery (repo)
• Calldata Decoder: decode EVM calldata and multicalls without an ABI, with risk flags (repo)
• Lumen Tab: pay-as-you-go micropayment tabs for agents on Arc (repo)

MIT © Lumen Works. Not affiliated with Coinbase or the x402 Foundation.

Support this project

This tool is free and MIT-licensed, maintained by one developer. If it saved you time, a tip of any size keeps it maintained:

• USDC on Base or Arc: 0xee9790cB3960952F585379F831a89b4e00B4029A
• Drips: this repo has a FUNDING.json, so it can receive funds and dependency splits through Drips.

Thank you!