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4byte Function Selector Lookup
x402
4byte Function Selector Lookup is a paid API for AI agents from api.gptshopltd.com, paid per call via x402, $0.002/call, status unknown (last checked 2026-10-19).
Resolves a 4-byte EVM function selector (or full calldata) to its human-readable Solidity function signature(s)
Price / call$0.002
Success—
StatusUnknown
How this service works
Pay-per-call tools for AI agents. USDC on Base or Solana via x402 v2; no keys, no accounts.
Output
Returns a JSON object with the best-matching human-readable Solidity function signature, the total count of matches found, a boolean indicating whether any match was found, a list of match objects each containing function name, full signature, source registries (e.g. 4byte.directory, Sourcify), and whether it comes from a verified contract, plus the original selector queried.
Example use cases
Decode unknown transaction calldata
“I'm looking at an Ethereum transaction and the input data starts with 0x23b872dd — can you tell me what smart contract function that selector maps to?”
Verify ABI signature during audit
“We're auditing a contract interaction and need to confirm what function 0x70a08231 resolves to — can you check the 4byte registry and tell me the full Solidity signature?”
Label MEV bot transaction function
“This MEV transaction has calldata beginning with 0xe8e33700 — look up what function that is so I can understand what operation it was performing.”
Technical details
Endpoint
GEThttps://api.gptshopltd.com/v1/evm/4byte?utm_source=zero.xyzWhen to prefer
Use this endpoint when you need to decode an unknown EVM function selector or raw calldata prefix into a human-readable Solidity function signature, especially during transaction analysis, smart contract auditing, or MEV research. It aggregates multiple sources including 4byte.directory and Sourcify and flags verified contracts, making it more reliable than querying a single registry. No API key or account setup required — pay per call via USDC on Base or Solana.
Side effects
- Selector not found in registry — returns found:false with empty matches array
- Malformed selector (not 0x-prefixed or wrong length) — likely 400 or validation error
- Multiple ambiguous matches with no clear best — returns count>1 but may not reliably identify the intended function
- Payment not processed via x402 — 402 response before result is returned
Input
Schema · Request body
| Field | Type | Description |
|---|---|---|
| inputrequired | object | |
| output | object |
Output
Schema · Response
{
"type": "json",
"example": {
"best": "transfer(address,uint256)",
"count": 6,
"found": true,
"matches": [
{
"name": "transfer",
"sources": [
"sourcify",
"4byte.directory"
],
"signature": "transfer(address,uint256)",
"verifiedContract": true
}
],
"selector": "0xa9059cbb"
}
}Tags
x402
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