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Overview

The update_deposit_limit instruction allows the program authority to modify the maximum deposit amount for the SOL merkle tree. For SPL tokens, use update_deposit_limit_for_spl_token instead.
Only the program authority can call this instruction.

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Function Signature

pub fn update_deposit_limit(
    ctx: Context<UpdateDepositLimit>,
    new_limit: u64
) -> Result<()>

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Parameters

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new_limit
u64
required
New maximum deposit amount in lamports. Examples:
  • 1 SOL: 1_000_000_000
  • 100 SOL: 100_000_000_000
  • 1,000 SOL: 1_000_000_000_000 (default)
  • 10,000 SOL: 10_000_000_000_000

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Accounts

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tree_account
AccountLoader<MerkleTreeAccount>
required
The merkle tree account to update. PDA: ["merkle_tree"].
  • Mutable: Yes
  • Authority check: Must be owned by the signer
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authority
Signer
required
The program authority. Must match the tree’s authority field.
  • Signer: Yes

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Code Example

import * as anchor from "@coral-xyz/anchor";
import { Program } from "@coral-xyz/anchor";
import { PublicKey, LAMPORTS_PER_SOL } from "@solana/web3.js";

const program = anchor.workspace.Zkcash as Program<Zkcash>;
const authority = anchor.web3.Keypair.generate();

// Derive tree PDA
const [treeAccountPDA] = PublicKey.findProgramAddressSync(
  [Buffer.from("merkle_tree")],
  program.programId
);

// Update to 5,000 SOL maximum
const newLimit = new anchor.BN(5000).mul(new anchor.BN(LAMPORTS_PER_SOL));

await program.methods
  .updateDepositLimit(newLimit)
  .accounts({
    treeAccount: treeAccountPDA,
    authority: authority.publicKey
  })
  .signers([authority])
  .rpc();

console.log(`Deposit limit updated to ${newLimit.toString()} lamports`);

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Update SPL Token Deposit Limit

For SPL tokens, use the separate instruction with the mint-specific PDA: 
// For SPL tokens
const mint = new PublicKey("EPjFWdd5AufqSSqeM2qN1xzybapC8G4wEGGkZwyTDt1v"); // USDC

const [splTreePDA] = PublicKey.findProgramAddressSync(
  [Buffer.from("merkle_tree"), mint.toBuffer()],
  program.programId
);

// Update to 10 million USDC (with 6 decimals)
const newUsdcLimit = new anchor.BN(10_000_000_000_000);

await program.methods
  .updateDepositLimitForSplToken(newUsdcLimit)
  .accounts({
    treeAccount: splTreePDA,
    mint: mint,
    authority: authority.publicKey
  })
  .signers([authority])
  .rpc();

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Common Use Cases

When transaction volume increases, raise the limit to accommodate larger deposits:
// Increase from 1,000 SOL to 10,000 SOL
const newLimit = new anchor.BN(10_000_000_000_000);
await updateDepositLimit(newLimit);
During security incidents or maintenance, lower the limit to reduce risk:
// Decrease from 1,000 SOL to 100 SOL
const newLimit = new anchor.BN(100_000_000_000);
await updateDepositLimit(newLimit);
Set limit to 0 to prevent all new deposits:
// Temporarily disable deposits
const newLimit = new anchor.BN(0);
await updateDepositLimit(newLimit);

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State Changes

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MerkleTreeAccount

  • max_deposit_amount: Updated to new_limit
All other fields remain unchanged.

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Behavior

  1. Loads the tree account and verifies authority
  2. Updates the max_deposit_amount field
  3. Emits a program log message
  4. The new limit applies immediately to all subsequent deposits
Existing deposits are not affected. Only new deposits will be subject to the new limit.

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Authorization

The signer must be the tree’s authority. This is verified through Anchor’s has_one constraint: 
#[account(
    mut,
    seeds = [b"merkle_tree"],
    bump = tree_account.load()?.bump,
    has_one = authority @ ErrorCode::Unauthorized
)]
pub tree_account: AccountLoader<'info, MerkleTreeAccount>,

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Errors

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Unauthorized
Error
Thrown when the signer is not the tree’s authority.

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Program Log Output

When successful, the instruction logs: 
Deposit limit updated to: {new_limit} lamports
For SPL tokens: 
Deposit limit updated to: {new_limit} for mint: {mint_address}

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Monitoring Limit Changes

You can monitor limit changes by:
  1. Parsing transaction logs for “Deposit limit updated” messages
  2. Periodically fetching the tree account and checking max_deposit_amount
  3. Setting up event listeners for authority transactions
// Check current limit
const treeAccount = await program.account.merkleTreeAccount.fetch(treeAccountPDA);
console.log(`Current deposit limit: ${treeAccount.maxDepositAmount.toString()} lamports`);

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See Also

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