# 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 crypto transfer transactions for agent-to-agent (B2B2C) payment flows before submission to the blockchain

## Facts

- Endpoint: POST https://coinrailz.com/x402/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-9537f688
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_FQELs56902Daq6QDXBaNU

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-9537f688 -d '<json body>'
```

Example prompt: Build me a pre-validated transaction payload to transfer 50 USDC from my agent wallet 0xABC...123 to recipient 0xDEF...456 on Base mainnet so I can sign and broadcast it.

## When to prefer this

Choose this endpoint when your agent needs to programmatically construct and pre-validate a blockchain transaction before signing — especially for autonomous agent-to-agent (B2B2C) payment pipelines where pre-validation reduces broadcast failures. Prefer this over raw RPC calls when you want a structured, validated transaction object returned as JSON without handling raw ABI encoding yourself.

## Known failure modes

- Invalid sender or recipient address format returns validation error
- Unsupported chain ID results in error response
- Missing required fields (amount, token, addresses) causes 400-level error
- Insufficient USDC balance to pay the $0.30 per-call fee blocks the request
- Malformed token contract address triggers encoding failure

## How this service works

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

## Output

Returns a JSON object containing the encoded transaction payload ready for signing, a success flag, the service name ('transaction-builder'), and a timestamp. The agent receives a structured transaction object it can directly sign and submit to the target blockchain network.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "query": {
   "type": "string",
   "description": "Query parameter for the service"
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "type": "application/json",
 "format": "json",
 "example": {
  "result": "Service executed successfully",
  "service": "transaction-builder",
  "success": true,
  "timestamp": "2026-02-07T01:42:05.861Z"
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/coin-railz-transaction-builder-9537f688/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)