Decode EVM calldata, return data, revert data or event logs against an ABI

$0.005 USDC per call · Developer and security · POST /api/v1/calldata-decode · Id calldata-decode

Decodes hex calldata (selector plus arguments), function return data, revert data or an event log against a caller-supplied ABI using ethers, for agents inspecting a transaction before signing or after it fails. Handles tuples, nested and dynamic arrays, bytes and strings, and the built-in Error(string) and Panic(uint256) reverts; a strict re-encode check flags trailing bytes, dirty padding and non-canonical offsets. Does not look up unknown selectors, fetch ABIs, simulate the call or judge whether the transaction is safe; a selector missing from the ABI returns unknown.

What you get

A 'transfer' that is really an approval to an attacker is visible once the calldata is decoded.

Call it

Send the JSON body below to https://agents.writerslogic.com/api/v1/calldata-decode. An unpaid request answers 402 with the price and how to pay; a request that fails with a 5xx status is not charged.

curl -i -X POST https://agents.writerslogic.com/api/v1/calldata-decode \
  -H 'content-type: application/json' \
  -d '{"abi":["function transfer(address to, uint256 amount) returns (bool)"],"data":"0xa9059cbb000000000000000000000000000000000000000000000000000000000000dead0000000000000000000000000000000000000000000000000000000000000005"}'

Input

Field	Type	Required	Description
`abi`	array<any>; max 200 items	yes	Fragments of the contract ABI (JSON objects or human-readable strings).
`mode`	string; one of "call", "return", "error", "event"	no	Default call: data is selector plus arguments.
`data`	string; length 0..40002	yes	0x-prefixed hex, whole bytes; up to 20000 bytes.
`function`	string; length 0..200	no	mode return: function name or signature.
`topics`	array<string; length 0..66>; max 4 items	no	mode event: 0x topics, topic0 first.

Example request body

{
  "abi": [
    "function transfer(address to, uint256 amount) returns (bool)"
  ],
  "data": "0xa9059cbb000000000000000000000000000000000000000000000000000000000000dead0000000000000000000000000000000000000000000000000000000000000005"
}

Result

The response is { "data": ..., "_provenance": ... }. data carries these fields; _provenance is a signed W3C Verifiable Credential with a C2PA-style manifest, checked free at POST /api/v1/verify-provenance.

Field	Type	Required	Description
`status`	string; one of "pass", "fail", "unknown", "unsupported"	no	pass: every rule held. fail: a rule was violated (see witnesses). unknown: the input lacks the evidence to decide. unsupported: outside what this engine covers. Missing evidence is never a pass.
`mode`	string	no
`selector`	string	no
`signature`	string	no
`name`	string	no
`args`	array<object>	no
`witnesses`	array<object>	no
`versions`	object	no	Every semantic dependency of this answer (engine, tables and their as-of dates, tokenizer, calendar, rounding profile).
`limits`	array<string>	no
`deterministic`	boolean	no	Always true: pure code, no model.

Pay and connect

Available through: x402 (USDC on Base), MCP. MCP clients reach it through the code-preflight-kit package.

Machine-readable

This agent as JSON (schemas, example, OpenAPI operation)
Free signed sample response
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