# Xerxes ERC20 Allowance Lookup

> Xerxes ERC20 Allowance Lookup is a paid API for AI agents from ari001.tailb4ac3a.ts.net, paid per call via x402, $0.003/call, status unknown (last checked 2026-10-02).

Returns the uint256 allowance amount that a spender is approved to withdraw from an ERC20 token owner's account on Ethereum mainnet.

## Facts

- Endpoint: GET https://ari001.tailb4ac3a.ts.net/v1/ventures/ethereum_contract_abi_service/erc20_allowance?utm_source=zero.xyz
- Price: $0.003/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-02
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/xerxes-erc20-allowance-lookup-804ad897
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_HbP_aeDruaH7NWpdxGkIL

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 xerxes-erc20-allowance-lookup-804ad897
```

Example prompt: What's the current allowance that Uniswap's router (0x7a250d5630B4cF539739dF2C5dAcb4c659F2488D) is approved to spend from 0xd8dA6BF26964aF9D7eEd9e03E53415D37aA96045's USDC (0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48) holdings on Ethereum?

## When to prefer this

Use this endpoint when you need to read the current ERC20 allowance between a specific owner and spender on Ethereum mainnet without requiring a full node setup. Prefer this over general blockchain RPC calls when you want a clean, typed REST interface that returns the uint256 allowance as a human-readable decimal string with minimal setup. Ideal for pre-transaction validation, approval auditing, and DeFi automation workflows.

## Known failure modes

- Invalid address format (not 0x + 40 hex) returns a validation error
- Contract address is not a valid ERC20 contract — call may revert or return unexpected data
- Network/RPC connectivity issues causing timeout or error response
- ok: false with error field when the on-chain call reverts
- Missing required query parameters (token, owner, or spender) results in 400-level error

## How this service works

Returns the amount of ERC-20 tokens that a spender is allowed to withdraw from an owner's account. Provide token (ERC-20 contract address), owner (owner address), and spender (spender address). Query: token (ERC-20 contract, 0x + 40 hex), owner (0x + 40 hex), spender (0x + 40 hex).

## Output

Returns a JSON object with an 'ok' boolean indicating success, a 'result' field containing the allowance as a uint256 decimal string, and metadata fields including 'chain', 'method', and 'service'. On failure, 'ok' is false and an 'error' string is present describing the issue.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "$schema": "https://json-schema.org/draft/2020-12/schema",
 "required": [
  "input"
 ],
 "properties": {
  "input": {
   "type": "object",
   "required": [
    "type",
    "method"
   ],
   "properties": {
    "type": {
     "type": "string",
     "const": "http"
    },
    "method": {
     "enum": [
      "GET",
      "HEAD",
      "DELETE"
     ],
     "type": "string"
    },
    "queryParams": {
     "type": "object",
     "required": [
      "token",
      "owner",
      "spender"
     ],
     "properties": {
      "owner": {
       "type": "string",
       "pattern": "^0x[a-fA-F0-9]{40}$",
       "description": "0x + 40 hex"
      },
      "token": {
       "type": "string",
       "pattern": "^0x[a-fA-F0-9]{40}$",
       "description": "ERC-20 contract, 0x + 40 hex"
      },
      "spender": {
       "type": "string",
       "pattern": "^0x[a-fA-F0-9]{40}$",
       "description": "0x + 40 hex"
      }
     }
    }
   },
   "additionalProperties": false
  },
  "output": {
   "type": "object",
   "required": [
    "type"
   ],
   "properties": {
    "type": {
     "type": "string"
    },
    "example": {
     "type": "object",
     "required": [
      "ok",
      "service"
     ],
     "properties": {
      "ok": {
       "type": "boolean"
      },
      "chain": {
       "type": "string"
      },
      "error": {
       "type": "string",
       "description": "present when ok is false"
      },
      "method": {
       "type": "string"
      },
      "result": {
       "type": "string",
       "description": "the uint256 as a decimal string"
      },
      "service": {
       "type": "string"
      }
     }
    }
   }
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/xerxes-erc20-allowance-lookup-804ad897/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from ari001.tailb4ac3a.ts.net](https://www.zero.xyz/host/ari001.tailb4ac3a.ts.net/llms.txt)