JSON API Request

Fetches JSON data from any public API endpoint and extracts specific values using a selector path. This agent is the fundamental building block for creating on-chain oracles.

Methods

Function	Inputs	Output	Use Case
fetchString	url, selector	string	Text values, names
fetchUint	url, selector, decimals	uint256	Prices, positive numbers
fetchInt	url, selector, decimals	int256	Temperatures, signed numbers
fetchBool	url, selector	bool	Flags, status checks
fetchStringArray	url, selector	string[]	Lists of names/IDs
fetchUintArray	url, selector, decimals	uint256[]	Price lists, numeric arrays

Parameters

Input	Type	Description
url	string	The URL of the JSON API endpoint to fetch
selector	string	A dot-notation path to extract from the response (e.g., data.price or items[0].name)
decimals	uint8	Number of decimal places to scale the value by (multiplies by 10^decimals)

Decimal Scaling

The decimals parameter is used by fetchUint, fetchInt, and fetchUintArray to convert floating-point values from APIs into blockchain-compatible integers. The value is multiplied by 10^decimals.

For example, with decimals=8:

• 42000.50 becomes 4200050000000

• 0.00001234 becomes 1234

This follows the standard pattern for handling decimals in smart contracts (e.g., ERC-20 tokens typically use 18 decimals).

Method Signatures

function fetchString(string url, string selector) returns (string)
function fetchUint(string url, string selector, uint8 decimals) returns (uint256)
function fetchInt(string url, string selector, uint8 decimals) returns (int256)
function fetchBool(string url, string selector) returns (bool)
function fetchStringArray(string url, string selector) returns (string[])
function fetchUintArray(string url, string selector, uint8 decimals) returns (uint256[])

Example Use Cases

• Fetching cryptocurrency prices from CoinGecko or similar APIs

• Retrieving weather data for parametric insurance contracts

• Pulling sports scores for prediction markets

• Accessing any REST API that returns JSON

Usage Examples

Fetch a Price as uint256

import { encodeFunctionData } from 'viem';

const abi = [{
  type: 'function',
  name: 'fetchUint',
  inputs: [
    { name: 'url', type: 'string' },
    { name: 'selector', type: 'string' },
    { name: 'decimals', type: 'uint8' }
  ],
  outputs: [{ name: 'result', type: 'uint256' }]
}];

// Fetch Bitcoin price with 8 decimal places
const calldata = encodeFunctionData({
  abi,
  functionName: 'fetchUint',
  args: [
    'https://api.coingecko.com/api/v3/simple/price?ids=bitcoin&vs_currencies=usd',
    'bitcoin.usd',
    8
  ]
});

Fetch a String Value

const abi = [{
  type: 'function',
  name: 'fetchString',
  inputs: [
    { name: 'url', type: 'string' },
    { name: 'selector', type: 'string' }
  ],
  outputs: [{ name: 'result', type: 'string' }]
}];

// Fetch a token symbol
const calldata = encodeFunctionData({
  abi,
  functionName: 'fetchString',
  args: [
    'https://api.example.com/token/info',
    'data.symbol'
  ]
});

Fetch a Boolean Value

const abi = [{
  type: 'function',
  name: 'fetchBool',
  inputs: [
    { name: 'url', type: 'string' },
    { name: 'selector', type: 'string' }
  ],
  outputs: [{ name: 'result', type: 'bool' }]
}];

// Check if a market is open
const calldata = encodeFunctionData({
  abi,
  functionName: 'fetchBool',
  args: [
    'https://api.example.com/market/status',
    'isOpen'
  ]
});