Skill: Cheapest rail right now

Where should an agent transact this minute? Send POST /api/skill/cheapest-rail with no required fields (1 optional) and pay $0.009 per call over x402 or MPP (there is no free tier). It returns a JSON object with pack, args, steps and summary.

Live gas on Ethereum + every major L2 side by side, Base gas tiers, a fee estimate for your transaction type, and ETH spot to price it all in dollars. Cheapest rail right now skill pack: one x402 payment runs 4 underlying tools (l2-gas-comparison, gas-snapshot, gas-estimate, crypto-price); partial-success per step.

Category: Skill packs · Tags: skill-pack workflow cheapest-rail · Pack overview

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Parameters

Example request

curl -i -X POST https://agent402.tools/api/skill/cheapest-rail \
  -H "Content-Type: application/json" \
  -d '{"networks":"ethereum,base,arbitrum,optimism,polygon"}'

Without payment this returns HTTP 402 Payment Required with the exact price for skill-cheapest-rail; any x402 v2 or MPP client pays it and retries.

Example response

{
  "pack": "cheapest-rail",
  "args": {
    "networks": "ethereum,base,arbitrum,optimism,polygon"
  },
  "steps": [
    {
      "slug": "l2-gas-comparison",
      "ok": true,
      "result": {}
    },
    {
      "slug": "gas-snapshot",
      "ok": true,
      "result": {}
    },
    {
      "slug": "gas-estimate",
      "ok": true,
      "result": {}
    },
    {
      "slug": "crypto-price",
      "ok": true,
      "result": {}
    }
  ],
  "summary": "4/4 steps succeeded"
}

From an MCP client

catalog.call {
  "slug": "skill-cheapest-rail",
  "params": {
    "networks": "ethereum,base,arbitrum,optimism,polygon"
  }
}

The hosted connector at https://agent402.tools/mcp needs a payment for skill-cheapest-rail; the stdio package pays it from a wallet or from AGENT402_CREDITS_KEY. Local install: npx -y agent402-mcp.

Errors and behavior

• Arguments left out fall back to the pack's own defaults. Each step reports on its own; the call succeeds when at least one step does, and a run where every step fails is refused (400 when the input caused it, 502 otherwise).
• A paid call that ends in any status of 400 or above is not charged: settlement is cancelled when the tool fails.
• Wallet-only: this tool reaches the network or stored state, so it has no proof-of-work tier. A prepaid card-credits key (Authorization: Bearer a402_...) also pays it.
• A GET or HEAD to /api/skill/cheapest-rail returns the same 402 quote, so the price can be read without a body.
• An Idempotency-Key header makes a retried paid call replay the first 200 instead of charging again.

Paid call (JavaScript agent)

import { wrapFetchWithPayment } from "@x402/fetch";
import { x402Client } from "@x402/core/client";
import { registerExactEvmScheme } from "@x402/evm/exact/client";
import { privateKeyToAccount } from "viem/accounts";

const client = new x402Client();
client.setSpendControls?.(false); // keep your own spending ceiling in code
registerExactEvmScheme(client, { signer: privateKeyToAccount(KEY) });
const payFetch = wrapFetchWithPayment(fetch, client);

const res = await payFetch("https://agent402.tools/api/skill/cheapest-rail", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({
    "networks": "ethereum,base,arbitrum,optimism,polygon"
  }),
});

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