# Nishvault x402 Pre-Send Guard

> Nishvault x402 Pre-Send Guard is a paid API for AI agents from x402-discovery.nishvault.com, paid per call via x402, $0.01/call, status unknown (last checked 2026-09-13).

Returns an x402 v2 payment challenge for a Base-mainnet EVM transaction preflight simulation before any transaction is broadcast.

## Facts

- Endpoint: POST https://x402-discovery.nishvault.com/simulate/transaction-preflight
- Price: $0.01/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-13
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/nishvault-x402-pre-send-guard-23e0f5fa
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_55SrcvDLc4J5MQxj9hBf8

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 nishvault-x402-pre-send-guard-23e0f5fa -d '<json body>'
```

Example prompt: Before I broadcast this transaction, can you simulate it on Base mainnet — sender is 0xABC...123, destination is 0xDEF...456, and the calldata is 0xa9059cbb000...?

## When to prefer this

Choose this endpoint when you need to preflight or dry-run an EVM transaction specifically on Base mainnet before broadcasting it, particularly when using the x402 micropayment protocol for per-call billing. It is ideal for agents that must avoid costly failed transactions and want a machine-readable simulation check at minimal cost ($0.01 USDC).

## Known failure modes

- Missing required fields (to, from, data) returns a 400 or validation error
- Invalid hex calldata or malformed EVM addresses cause rejection
- Payment challenge not fulfilled results in 402 response without simulation data
- Unsupported chainId (non-Base mainnet) may cause simulation failure
- Network or RPC errors on the Base mainnet node can return 5xx responses

## How this service works

Machine-readable discovery for the paid Base-mainnet transaction preflight. The endpoint returns an x402 v2 payment challenge before any transaction is broadcast.

## Output

The endpoint returns an x402 v2 payment challenge response. Upon payment of $0.01 USDC, the caller receives the preflight simulation result indicating whether the given EVM transaction (to, from, data, optional value, optional chainId) would succeed or fail on Base mainnet before any actual broadcast occurs.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "required": [
  "to",
  "data",
  "from"
 ],
 "properties": {
  "to": {
   "type": "string",
   "description": "EVM destination address used for simulation."
  },
  "data": {
   "type": "string",
   "description": "Hexadecimal EVM calldata."
  },
  "from": {
   "type": "string",
   "description": "EVM sender address used for simulation."
  },
  "value": {
   "type": "string",
   "description": "Optional hexadecimal wei value."
  },
  "chainId": {
   "type": "string",
   "description": "Base mainnet chain ID in hexadecimal."
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/nishvault-x402-pre-send-guard-23e0f5fa/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from x402-discovery.nishvault.com](https://www.zero.xyz/host/x402-discovery.nishvault.com/llms.txt)