# Coin Railz Transaction Builder

> Coin Railz Transaction Builder is a paid API for AI agents from coinrailz.com, paid per call via x402, $0.3/call, status unknown (last checked 2026-09-15).

Encodes and pre-validates blockchain transactions for agent-to-agent and token transfers, returning a signed-ready transaction object

## Facts

- Endpoint: POST https://coinrailz.com/x402/service/transaction-builder
- Price: $0.3/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-15
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/coin-railz-transaction-builder-2429dfee
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_2EvvMsCRebNvEqd3RIOV2

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability coin-railz-transaction-builder-2429dfee -d '<json body>'
```

Example prompt: Build me a pre-validated transaction to send 50 USDC (token address 0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48) to 0xRecipientAddress on the Ethereum mainnet chain, ready for signing.

## When to prefer this

Use this endpoint when an AI agent needs to programmatically construct and encode a blockchain transaction without managing raw ABI encoding — especially for agent-to-agent payments, ERC20 transfers, or custom on-chain calls where a pre-validated, sign-ready transaction object is needed before broadcasting.

## Known failure modes

- Invalid recipient address format — returns validation error
- Unsupported chain identifier — returns error listing supported chains
- Missing required fields (to or chain) — returns 400-level validation error
- Invalid ERC20 token address — returns encoding error
- Conflicting value/amount/tokenAddress combination — returns parameter conflict error

## How this service works

Pre-validated transaction encoding for agent-to-agent transfers (B2B2C infrastructure)

## Output

Returns an encoded transaction object ready to be signed and broadcast, containing all necessary fields (to, value, data, chainId, etc.) for submission to the target blockchain network.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "required": [
  "to",
  "chain"
 ],
 "properties": {
  "to": {
   "type": "string",
   "description": "Recipient address"
  },
  "data": {
   "type": "string",
   "description": "Custom transaction data (optional)"
  },
  "chain": {
   "type": "string",
   "description": "Blockchain network"
  },
  "value": {
   "type": "string",
   "description": "ETH value (optional)"
  },
  "amount": {
   "type": "string",
   "description": "Token amount (optional)"
  },
  "tokenAddress": {
   "type": "string",
   "description": "ERC20 token address (optional)"
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "transaction": {
   "type": "object",
   "description": "Encoded transaction object ready to sign"
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/coin-railz-transaction-builder-2429dfee/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from coinrailz.com](https://www.zero.xyz/host/coinrailz.com/llms.txt)