Token Extensions: Mint Close Authority

With the original SPL token program, there was no option to close Mint Accounts owned by the Token Program and reclaim the SOL allocated to these accounts.

The MintCloseAuthority extension introduces a solution to this limitation by allowing a designated Close Authority to close a Mint Account if the supply of the mint is 0. This feature provides a mechanism to recover SOL allocated to Mint Accounts that are no longer in use.

In this guide, we'll walk through an example of using Solana Playground. Here is the final script.

Getting Started

Start by opening this Solana Playground link with the following starter code.

// Client
console.log("My address:", pg.wallet.publicKey.toString());
const balance = await pg.connection.getBalance(pg.wallet.publicKey);
console.log(`My balance: ${balance / web3.LAMPORTS_PER_SOL} SOL`);

If it is your first time using Solana Playground, you'll first need to create a Playground Wallet and fund the wallet with devnet SOL.

Info

If you do not have a Playground wallet, you may see a type error within the editor on all declarations of pg.wallet.publicKey. This type error will clear after you create a Playground wallet.

To get devnet SOL, run the solana airdrop command in the Playground's terminal, or visit this devnet faucet.

solana airdrop 5

Once you've created and funded the Playground wallet, click the "Run" button to run the starter code.

Add Dependencies

Let's start by setting up our script. We'll be using the @solana/web3.js and @solana/spl-token libraries.

Replace the starter code with the following:

import {
  Connection,
  Keypair,
  SystemProgram,
  Transaction,
  clusterApiUrl,
  sendAndConfirmTransaction,
} from "@solana/web3.js";
import {
  ExtensionType,
  TOKEN_2022_PROGRAM_ID,
  closeAccount,
  createInitializeMintCloseAuthorityInstruction,
  createInitializeMintInstruction,
  getMintLen,
} from "@solana/spl-token";
 
// Playground wallet
const payer = pg.wallet.keypair;
 
// Connection to devnet cluster
const connection = new Connection(clusterApiUrl("devnet"), "confirmed");
 
// Transaction signature returned from sent transaction
let transactionSignature: string;

Mint Setup

First, let's define the properties of the Mint Account we'll be creating in the following step.

// Generate new keypair for Mint Account
const mintKeypair = Keypair.generate();
// Address for Mint Account
const mint = mintKeypair.publicKey;
// Decimals for Mint Account
const decimals = 2;
// Authority that can mint new tokens
const mintAuthority = pg.wallet.publicKey;
// Authority that can close the Mint Account
const closeAuthority = pg.wallet.publicKey;

Next, let's determine the size of the new Mint Account and calculate the minimum lamports needed for rent exemption.

// Size of Mint Account with extension
const mintLen = getMintLen([ExtensionType.MintCloseAuthority]);
// Minimum lamports required for Mint Account
const lamports = await connection.getMinimumBalanceForRentExemption(mintLen);

With Token Extensions, the size of the Mint Account will vary based on the extensions enabled.

Build Instructions

Next, let's build the set of instructions to:

Create a new account
Initialize the MintCloseAuthority extension
Initialize the remaining Mint Account data

First, build the instruction to invoke the System Program to create an account and assign ownership to the Token Extensions Program.

// Instruction to invoke System Program to create new account
const createAccountInstruction = SystemProgram.createAccount({
  fromPubkey: payer.publicKey, // Account that will transfer lamports to created account
  newAccountPubkey: mint, // Address of the account to create
  space: mintLen, // Amount of bytes to allocate to the created account
  lamports, // Amount of lamports transferred to created account
  programId: TOKEN_2022_PROGRAM_ID, // Program assigned as owner of created account
});

Next, build the instruction to initialize the MintCloseAuthority extension for the Mint Account.

// Instruction to initialize the MintCloseAuthority Extension
const initializeMintCloseAuthorityInstruction =
  createInitializeMintCloseAuthorityInstruction(
    mint, // Mint Account address
    closeAuthority, // Designated Close Authority
    TOKEN_2022_PROGRAM_ID, // Token Extension Program ID
  );

Lastly, build the instruction to initialize the rest of the Mint Account data. This is the same as with the original Token Program.

// Instruction to initialize Mint Account data
const initializeMintInstruction = createInitializeMintInstruction(
  mint, // Mint Account Address
  decimals, // Decimals of Mint
  mintAuthority, // Designated Mint Authority
  null, // Optional Freeze Authority
  TOKEN_2022_PROGRAM_ID, // Token Extension Program ID
);

Send Transaction

Next, let's add the instructions to a new transaction and send it to the network. This will create a Mint Account with the MintCloseAuthority extension enabled.

// Add instructions to new transaction
const transaction = new Transaction().add(
  createAccountInstruction,
  initializeMintCloseAuthorityInstruction,
  initializeMintInstruction,
);
 
// Send transaction
transactionSignature = await sendAndConfirmTransaction(
  connection,
  transaction,
  [payer, mintKeypair], // Signers
);
 
console.log(
  "\nCreate Mint Account:",
  `https://solana.fm/tx/${transactionSignature}?cluster=devnet-solana`,
);

Run the script by clicking the Run button. You can then inspect the transaction details on SolanaFM.

Close Mint Account

With the MintCloseAuthority extension enabled, the Close Authority can close the Mint Account to reclaim the lamports from the account.

// Send transaction to close Mint Account
transactionSignature = await closeAccount(
  connection,
  payer, // Transaction fee payer
  mint, // Mint Account address
  payer.publicKey, // Account to receive lamports from closed account
  closeAuthority, // Close Authority for Mint Account
  undefined, // Additional signers
  undefined, // Confirmation options
  TOKEN_2022_PROGRAM_ID, // Token Extension Program ID
);
 
console.log(
  "\nClose Mint Account:",
  `https://solana.fm/tx/${transactionSignature}?cluster=devnet-solana`,
);

Run the script by clicking the Run button. You can then inspect the transaction details on the SolanaFM.

Conclusion

The MintCloseAuthority extension enables developers to reclaim SOL that otherwise would have been permanently locked in a Mint Account. This feature is particularly useful for applications or games involving single-use NFTs that are meant to be burned. It ensures that the SOL allocated to Mint Accounts which are no longer used can be reclaimed and reused.

How to use the Mint Close Authority extension

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