Getting Started with Data Feeds (using Foundry)

Chainlink Data Feeds are the fastest way to connect your smart contracts to real-world data such as asset prices, reserve balances, and L2 sequencer health. Each feed is aggregated by many independent Chainlink node operators and published onchain through a decentralized oracle network, giving your contracts a reliable, manipulation-resistant source of data.

Each price feed has an onchain address and functions that enable contracts to read pricing data from that address, for example the ETH / USD feed.

This guide walks you through reading a Data Feed end-to-end using the Foundry CLI. You'll deploy a consumer contract on the Sepolia testnet with forge, read the BTC / USD price feed onchain, and read the same feed offchain with cast call.

What you'll do

• Deploy a Solidity consumer contract that reads the BTC / USD price feed on Sepolia using forge script.
• Retrieve the latest price onchain by calling the consumer contract with cast call.
• Read the same feed offchain directly from the feed proxy with cast call — no consumer contract required.
• Learn the key safety checks to apply before moving to production.

The code for reading Data Feeds on Ethereum and other EVM-compatible blockchains is the same for every chain and every feed type. You choose different feeds for different use cases, but the request and response format is always the same. The answer's decimal length and expected value range may differ depending on the feed.

Guide Versions

This guide is available in multiple versions. Choose the one that matches your needs.

Foundry

Low code (Remix) Foundry Hardhat

Before you begin

If you are new to smart contract development, complete the Deploy Your First Smart Contract quickstart first.

You will need:

• Foundry installed (forge, cast).
• A funded wallet on the Sepolia testnet. Get testnet ETH from a Sepolia faucet.
• A Sepolia RPC URL (e.g. from a node provider).
• A funded deployer private key for Sepolia.

Step 1: Examine the sample contract

The example contract below reads the latest answer from the BTC / USD feed on Sepolia. You can modify it to read any of the Types of Data Feeds.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.7;

import {AggregatorV3Interface} from "@chainlink/contracts/src/v0.8/shared/interfaces/AggregatorV3Interface.sol";

/**
 * THIS IS AN EXAMPLE CONTRACT THAT USES HARDCODED
 * VALUES FOR CLARITY.
 * THIS IS AN EXAMPLE CONTRACT THAT USES UN-AUDITED CODE.
 * DO NOT USE THIS CODE IN PRODUCTION.
 */

/**
 * If you are reading data feeds on L2 networks, you must
 * check the latest answer from the L2 Sequencer Uptime
 * Feed to ensure that the data is accurate in the event
 * of an L2 sequencer outage. See the
 * https://docs.chain.link/data-feeds/l2-sequencer-feeds
 * page for details.
 */
contract DataConsumerV3 {
  AggregatorV3Interface internal dataFeed;

  /**
   * Network: Sepolia
   * Aggregator: BTC/USD
   * Address: 0x1b44F3514812d835EB1BDB0acB33d3fA3351Ee43
   */
  constructor() {
    dataFeed = AggregatorV3Interface(0x1b44F3514812d835EB1BDB0acB33d3fA3351Ee43);
  }

  /**
   * Returns the latest answer.
   */
  function getChainlinkDataFeedLatestAnswer() public view returns (int256) {
    // prettier-ignore
    (
      /* uint80 roundId */
      ,
      int256 answer,
      /*uint256 startedAt*/
      ,
      /*uint256 updatedAt*/
      ,
      /*uint80 answeredInRound*/
    ) = dataFeed.latestRoundData();
    return answer;
  }
}

The contract has the following components:

• The import line brings in AggregatorV3Interface — the Solidity interface that every Chainlink v3 price feed proxy implements. It exposes latestRoundData(), getRoundData(), decimals(), and version(). The sample uses latestRoundData to fetch the current price.
• The constructor() initializes a dataFeed object that uses AggregatorV3Interface pointing at the proxy aggregator deployed at 0x1b44F3514812d835EB1BDB0acB33d3fA3351Ee43. This is the proxy address for the Sepolia BTC / USD feed. The proxy lets the aggregator be upgraded without breaking consumer contracts.
• The getChainlinkDataFeedLatestAnswer() function calls your dataFeed object and runs the latestRoundData() function and returns the answer variable. When you deploy the contract, it initializes the dataFeed object to point to the aggregator at 0x1b44F3514812d835EB1BDB0acB33d3fA3351Ee43, which is the proxy address for the Sepolia BTC / USD data feed. Your contract connects to that address and executes the function. The aggregator connects with several oracle nodes and aggregates the pricing data from those nodes. The response from the aggregator includes several variables, but getChainlinkDataFeedLatestAnswer() returns only the answer variable. The full response includes roundId, startedAt, updatedAt, and answeredInRound — see the API Reference for details.

Step 2: Set up a Foundry project

1. Create a new Foundry project and add the Chainlink contracts dependency:

   forge init data-feeds-quickstart
   cd data-feeds-quickstart
   forge install smartcontractkit/chainlink-evm@contracts-v1.5.0
   

2. Add this remapping to remappings.txt so Solidity can resolve @chainlink/contracts:

   @chainlink/contracts/=lib/chainlink-evm/contracts/
   

   You can write it with a single command:

   echo '@chainlink/contracts/=lib/chainlink-evm/contracts/' > remappings.txt
   

3. Create the directories for the sample consumer contract and deploy script:

   mkdir -p src/DataFeeds script/DataFeeds
   

4. Create the following files and paste in the code from the rendered code blocks on this page:

   • Create src/DataFeeds/DataConsumerV3.sol and paste in the contract from Step 1: Examine the sample contract.
   • Create script/DataFeeds/DeployAndReadDataConsumerV3.s.sol and paste in the script from Step 3: Deploy the contract with a Forge script.

   Make sure the filenames match exactly — Foundry looks for the contract in src/ and the script in script/.

5. Verify the project compiles:

   forge build
   

   You should see Compiler run successful. If you see an import error, double-check the remappings.txt line from step 2.

Step 3: Deploy the contract with a Forge script

The deploy script (script/DataFeeds/DeployAndReadDataConsumerV3.s.sol) deploys DataConsumerV3 and immediately reads the latest price through it. For reference, here is the script:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.7;

/* solhint-disable no-console */

import {DataConsumerV3} from "../../src/DataFeeds/DataConsumerV3.sol";
import {AggregatorV3Interface} from "@chainlink/contracts/src/v0.8/shared/interfaces/AggregatorV3Interface.sol";
import {Script} from "forge-std/Script.sol";
import {console2} from "forge-std/console2.sol";

/**
 * THIS IS EXAMPLE CODE THAT USES HARDCODED VALUES FOR CLARITY.
 * THIS IS EXAMPLE CODE THAT USES UN-AUDITED CODE.
 * DO NOT USE THIS CODE IN PRODUCTION.
 *
 * Deploy DataConsumerV3 and read the latest BTC/USD price on Sepolia.
 *
 * Usage:
 *   forge script script/DataFeeds/DeployAndReadDataConsumerV3.s.sol \
 *     --rpc-url $SEPOLIA_RPC_URL \
 *     --broadcast \
 *     --private-key $PRIVATE_KEY
 */
contract DeployAndReadDataConsumerV3 is Script {
  // Sepolia BTC / USD price feed proxy address
  address public constant SEPOLIA_BTC_USD = 0x1b44F3514812d835EB1BDB0acB33d3fA3351Ee43;

  function run() public {
    vm.startBroadcast();

    // 1. Deploy the consumer contract
    DataConsumerV3 consumer = new DataConsumerV3();
    console2.log("DataConsumerV3 deployed at:", address(consumer));

    vm.stopBroadcast();

    // 2. Read the latest price through the consumer
    int256 answer = consumer.getChainlinkDataFeedLatestAnswer();
    console2.log("Latest answer (raw):", uint256(answer));

    // 3. Read decimals directly from the feed to scale the answer
    uint8 decimals = AggregatorV3Interface(SEPOLIA_BTC_USD).decimals();
    console2.log("Decimals:", decimals);
    console2.log("Latest price (scaled): %s", _scale(answer, decimals));
  }

  function _scale(
    int256 answer,
    uint8 decimals
  ) internal pure returns (string memory) {
    // Convert the integer answer to a human-readable price string.
    // For 8 decimals, 3030914000000 -> "30309.14000000"
    uint256 magnitude = uint256(answer);
    uint256 base = 10 ** decimals;
    uint256 whole = magnitude / base;
    uint256 fraction = magnitude % base;
    return string.concat(vm.toString(whole), ".", vm.toString(fraction));
  }
}

Run it against Sepolia. Replace $SEPOLIA_RPC_URL and $PRIVATE_KEY with your own values:

forge script script/DataFeeds/DeployAndReadDataConsumerV3.s.sol \
  --rpc-url $SEPOLIA_RPC_URL \
  --broadcast \
  --private-key $PRIVATE_KEY

The script logs the deployed contract address, the raw integer answer, the feed's decimals, and the human-readable price. Save the deployed contract address for the next step.

Step 4: Read the latest price onchain

Use cast call to read the latest answer through your deployed consumer. Replace $CONSUMER_ADDRESS with the address logged by the deploy script:

cast call $CONSUMER_ADDRESS "getChainlinkDataFeedLatestAnswer()(int256)" --rpc-url $SEPOLIA_RPC_URL

Read the feed's decimals directly to scale the answer yourself:

cast call 0x1b44F3514812d835EB1BDB0acB33d3fA3351Ee43 "decimals()(uint8)" --rpc-url $SEPOLIA_RPC_URL

The BTC / USD feed returns 8 for decimals. To convert the raw integer answer to a human-readable price, divide by 10 ** decimals. For example, an answer of 3030914000000 with 8 decimals is 30309.14 USD.

Step 5: Read the same feed offchain (no consumer contract required)

You can also read Data Feeds directly from the feed proxy without deploying a consumer contract. This is useful for backends, bots, dashboards, and pre-trade checks.

Read the latest answer with a single cast call against the feed proxy at 0x1b44F3514812d835EB1BDB0acB33d3fA3351Ee43:

cast call 0x1b44F3514812d835EB1BDB0acB33d3fA3351Ee43 "latestRoundData()(uint80,int256,uint256,uint256,uint80)" --rpc-url $SEPOLIA_RPC_URL

Read the decimals so you can scale the answer:

cast call 0x1b44F3514812d835EB1BDB0acB33d3fA3351Ee43 "decimals()(uint8)" --rpc-url $SEPOLIA_RPC_URL

The BTC / USD feed returns 8 for decimals. Divide the raw integer answer by 10 ** decimals to get the human-readable price. For example, an answer of 3030914000000 with 8 decimals is 30309.14 USD.

Before you go to production

The example intentionally omits the safety checks a production integration needs. Before shipping, review the following points and the Developer Responsibilities page.

Check updatedAt and staleness

latestRoundData() returns updatedAt (the timestamp of the latest round) and answeredInRound (the round in which the answer was finalized). Always verify the feed is fresh:

(uint80 roundId, int256 answer, , uint256 updatedAt, uint80 answeredInRound) = dataFeed.latestRoundData();

require(answeredInRound >= roundId, "Stale price");
require(block.timestamp - updatedAt < TIMEOUT, "Stale price");

Choose a TIMEOUT that matches your application's risk tolerance — shorter for trading, longer for less time-sensitive use cases.

Use the right feed for your asset

Not all feeds are equal. Low-liquidity assets are more exposed to market manipulation. Review Selecting Quality Data Feeds and the Data Feed Categories before choosing a feed.

Handle L2 sequencer risk

On L2s, a sequencer outage can cause stale or incorrect prices. Always pair L2 price feeds with a check on the L2 Sequencer Uptime Feed. The DataConsumerWithSequencerCheck sample shows the pattern. Try it out in Remix below:

Audit your integration

The sample code is unaudited and hardcodes values for clarity. Before production, complete your own audit, review your dependencies, and apply the risk-mitigation practices described in Developer Responsibilities.

FAQ

Do I need a consumer contract to read a Data Feed?

Only if another smart contract needs the price onchain. A consumer contract exists to wrap a feed read in your own contract's logic so that your other contracts can use the price onchain — for collateral checks, settlements, circuit breakers, and so on. The read is atomic with your onchain action and verifiable on the blockchain.

If you only need the price in an offchain system (a backend, bot, dashboard, or pre-trade check), you do not need a consumer contract. The feed proxy is a public contract and latestRoundData() is a view function — anyone can call it directly from a script using cast or ethers.js. See Step 5: Read the same feed offchain on this page, or the Hardhat guide.