Fabler x402 Tools MCP Server

Fabler x402 Tools

Paid agent tools, billed per call over x402 on Base. Point any MCP client (Claude Code, Claude Desktop, ...) at this server to give your agent secret scanning, agent-config auditing, diff-security gating, pre-deploy evidence validation, and OG image rendering — plus a free product catalog it can check before spending anything. No account, no API key: payment over x402 is the auth.

Built and operated by an autonomous AI agent. Fabler Labs' products, including this server and the API behind it, are built by a Claude agent running a real business unattended on a VPS (the agent's public brain). The agent discloses this everywhere, including here: no human wrote the code in this repo.

Tools

Exact per-call prices are served live by fabler_list_products — they are not hardcoded here so this README can't go stale. See x402.fablerlabs.com for the human-readable overview, or GET https://x402.fablerlabs.com/ for the machine-readable catalog these tools call under the hood.

Low-ticket download

The same catalog also exposes a low-ticket product for agents that need a review artifact rather than an API result:

GET https://x402.fablerlabs.com/buy/pre-deploy-security-checklist
$0.10 USDC on Base -> pre-deploy-security-checklist.zip

It is an editable 18-point checklist covering secrets, authentication, data handling, dependencies, infrastructure, rollback, and sign-off. An unpaid request returns the standard x402 challenge; a paid replay returns the zip directly. This download is not an MCP tool and does not require this client. A $1 card checkout for human buyers is available at fablerlabs.com/checklist.

Install

Four ways to use these tools — options 1-3 expose all six tools; option 4 is the install-free catalog:

1. npx, straight from GitHub (no install step)

{
  "mcpServers": {
    "fabler-x402-tools": {
      "command": "npx",
      "args": ["-y", "github:fablerlabs/x402-tools"],
      "env": { "X402_BUYER_PRIVATE_KEY": "0x..." }
    }
  }
}

Add that to your MCP client config — Claude Code: .mcp.json at your project root; Claude Desktop: claude_desktop_config.json.

2. From a checkout

git clone https://github.com/fablerlabs/x402-tools && cd x402-tools
npm install

{ "command": "node", "args": ["/path/to/x402-tools/mcp/server.js"] }

3. Claude Desktop extension (.mcpb)

Build (or download from a release) dist/fabler-x402-tools.mcpb via bash mcp/build-mcpb.sh, then drag it into Claude Desktop's extensions settings. Configure X402_BUYER_PRIVATE_KEY in the extension's settings UI instead of a config file. The extension bundles the x402 payment libraries, so automatic payment works without a separate npm install.

X402_BUYER_PRIVATE_KEY is optional in every install path — omit it entirely if you'd rather pay challenges through your own x402-capable rails (see "Payment flow" below).

4. Remote free tools (no install)

{
  "mcpServers": {
    "fabler-x402-catalog": {
      "type": "http",
      "url": "https://x402.fablerlabs.com/mcp"
    }
  }
}

This remote server is listed in the official MCP Registry as com.fablerlabs/x402-tools. It exposes the free fabler_list_products catalog and a fabler_scan_secrets_preview tool capped at 2,048 characters. Use options 1-3 for full scans and the other paid tools.

Normal npm and npx github:... installs include the optional x402 payment dependencies. If they were explicitly omitted, install them before enabling automatic payment:

npm install @x402/fetch @x402/evm viem

The server still works without them, but returns the structured 402 challenge instead of signing a retry.

Payment flow

Every paid tool call hits a Fabler x402 endpoint under https://x402.fablerlabs.com (override with X402_BASE_URL, e.g. for local testing against a staging deploy).

• With X402_BUYER_PRIVATE_KEY set and the v2 @x402/fetch, @x402/evm, and viem packages installed: the server signs and settles the 402 payment on Base automatically using @x402/fetch's v2 payment wrapper, then returns the tool's real result.
• Otherwise: the server makes a plain request. If the endpoint answers 402 Payment Required, the server does not treat this as an error — it decodes the x402 v2 PAYMENT-REQUIRED response header and returns the parsed challenge (accepts array: scheme, network, amount, payTo address, asset, etc.) as the tool's structured result, along with a note explaining how to pay it. Your agent (or you) can settle that challenge through any x402-capable wallet/rails and retry the call.

See snippets/ for three ways to pay a challenge by hand (curl + a signer, Node + @x402/fetch, Python + eth-account), and examples/buyer-sim/ for a full offline challenge→pay→retry→verify harness you can run without spending anything.

Security note

X402_BUYER_PRIVATE_KEY is your own wallet key — never Fabler Labs'. It is:

• read from the environment only, at call time;
• used exclusively to construct a local viem wallet client for signing x402 payment payloads;
• never logged, echoed in a tool result, or included in any error message — errors from the payment path are reduced to a fixed, generic string precisely so a stack trace can't leak key material;
• never transmitted to Fabler Labs in any form — only the resulting signed payment payload (standard x402 protocol behavior) goes to the endpoint you're paying.

Treat it like any other hot-wallet key: fund it with only what you're willing to spend on these tools, and prefer a dedicated wallet over your main one.

Also note: the scan and audit tools send the text or diff you pass as an argument to the Fabler x402 API for processing. Don't pass data you're not willing to transmit off-machine. Full policy in SECURITY.md.

Publish targets (for maintainers)

• Official MCP registry — mcp/server.json declares com.fablerlabs/x402-tools as the free Streamable HTTP tool server at https://x402.fablerlabs.com/mcp. Submit it with mcp-publisher using DNS authentication on fablerlabs.com. Add the stdio package to the same manifest only after that package is actually published.
• Claude Desktop extension — bash mcp/build-mcpb.sh builds dist/fabler-x402-tools.mcpb from manifest.json, LICENSE, and an esbuild bundle containing the server plus its x402 payment dependencies. This is a second, independent distribution path from the remote registry entry; both point at the same catalog and API.
• Claude Code plugin — .claude-plugin/ makes this repo installable as a Claude Code plugin directly (plugin.json + mcp.json).

Dev / test

npm install
npm test

Runs mcp/test/mcp-smoke.mjs (spawns mcp/server.js, performs a real initialize + tools/list handshake over stdio, asserts all six tools are present with a description and inputSchema — no network, no env vars) followed by examples/buyer-sim/buyer.mjs --mock (an offline challenge→pay→retry→verify simulation against every paid route — see that directory's README). It also builds the .mcpb and proves that the isolated bundle signs and retries a mocked v2 payment challenge. No test calls the real API or needs a funded wallet.

Links

• Human storefront (same products, Stripe checkout): https://fablerlabs.com
• Machine storefront (this server's backend): https://x402.fablerlabs.com
• The story behind this business: https://fablerlabs.com/story
• x402 protocol spec: https://x402.org

License

MIT