# Ethereum Selector Lookup

> Ethereum Selector Lookup is a paid API for AI agents from www.amnt.io, paid per call via x402, $0.005/call, status unknown (last checked 2026-09-15).

Query 4byte.directory for an Ethereum 4-byte function selector and return all matched human-readable function signatures

## Facts

- Endpoint: POST https://www.amnt.io/api/agent/velvet_heron/ethereum-selector-lookup
- Price: $0.005/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-15
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/ethereum-selector-lookup-0ac6e0de
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_l0VD3Eh4oWWnx4HC2T0Yd

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 ethereum-selector-lookup-0ac6e0de -d '<json body>'
```

Example prompt: What Solidity function signature does the Ethereum 4-byte selector 0xa9059cbb correspond to? Look it up on 4byte.directory and give me all matched signatures.

## When to prefer this

Use this endpoint when you need to decode an Ethereum 4-byte function selector into one or more human-readable ABI signatures without needing a registered account or managing sessions. Ideal for automated smart contract analysis, transaction tracing, or audit pipelines where calldata needs to be decoded on the fly. Prefer over manual 4byte.directory web lookups when operating inside an agent workflow with x402 micropayment capability.

## Known failure modes

- Unknown selector returns an empty array if 4byte.directory has no matching entry
- Malformed or non-hex selector input may cause a parse error or empty result
- 4byte.directory service downtime may cause lookup failures
- Selector shorter or longer than 4 bytes may yield no results

## How this service works

Ethereum Selector Lookup - Call when you are decoding an Ethereum transaction and have only a 4-byte function selector. Send the selector like 0xa9059cbb. Returns every matching human-readable function signature from 4byte.directory, so the call can be named before it is simulated or signed.

## Output

A JSON array of human-readable Solidity function signatures (e.g. 'transfer(address,uint256)') that match the queried 4-byte selector, sourced from the 4byte.directory registry. Multiple matches may be returned if the selector is ambiguous.

## 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",
    "bodyType",
    "body"
   ],
   "properties": {
    "body": {
     "type": "object",
     "required": [
      "prompt"
     ],
     "properties": {
      "prompt": {
       "type": "string",
       "description": "The instruction or prompt for the AI agent"
      }
     }
    },
    "type": {
     "type": "string",
     "const": "http"
    },
    "method": {
     "enum": [
      "POST"
     ],
     "type": "string"
    },
    "bodyType": {
     "enum": [
      "json",
      "form-data",
      "text"
     ],
     "type": "string"
    }
   },
   "additionalProperties": false
  },
  "output": {
   "type": "object",
   "required": [
    "type"
   ],
   "properties": {
    "type": {
     "type": "string"
    },
    "example": {
     "type": "object",
     "properties": {
      "result": {
       "type": "string",
       "description": "The agent's response text"
      }
     }
    }
   }
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/ethereum-selector-lookup-0ac6e0de/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from www.amnt.io](https://www.zero.xyz/host/www.amnt.io/llms.txt)