Pre-State Tracer

Snapshot accessed account state before execution, with optional before/after diff mode.

The pre-state tracer captures the account state that was accessed during execution. In diff mode, it shows what changed — before and after values for each touched account and storage slot.

Usage

mega-evme run 0x... --trace --tracer pre-state

The tracer name accepts both pre-state and prestate.

Options

Flag	Default	Aliases	Description
--trace.prestate.diff-mode	false	--trace.pre-state.diff-mode	Show state diff (before/after) instead of just the pre-state
--trace.prestate.disable-code	false	--trace.pre-state.disable-code	Omit contract bytecode from the output
--trace.prestate.disable-storage	false	--trace.pre-state.disable-storage	Omit storage slots from the output

Output Format

Schema

Each account entry in the output contains:

Field	Type	Description
balance	hex number	Account balance in wei
nonce	number	Account nonce (only in diff mode)
code	hex bytes	Contract bytecode (omitted with --trace.prestate.disable-code)
storage	object	Storage slots as hex key-value pairs (omitted with --trace.prestate.disable-storage)

In default mode, the output is a flat map of addresses to account state. In diff mode, the output has two top-level keys: pre and post, each containing a map of addresses.

Default Mode (pre-state only)

Shows the state of every account accessed during execution, as it existed before the transaction.

Running mega-evme run 0x60016000526001601ff3 --trace --tracer pre-state produces:

{
  "0x0000000000000000000000000000000000000000": {
    "balance": "0x0",
    "code": "0x60016000526001601ff3"
  },
  "0x4200000000000000000000000000000000000019": {
    "balance": "0x0"
  },
  "0xf39fd6e51aad88f6f4ce6ab8827279cfffb92266": {
    "balance": "0x0"
  }
}

Only fields that were actually read appear — accounts with no code show only balance.

Diff Mode

Shows both the pre (before) and post (after) state for each touched account. Fields that didn't change are omitted from the diff.

Running WETH.deposit() with --trace.prestate.diff-mode shows how the deposit modified balances and storage:

{
  "post": {
    "0x4200000000000000000000000000000000000006": {
      "balance": "0x222f824a54e20d8d47b",
      "storage": {
        "0x723077b8a1b173adc35e5f0e7e3662fd1208212cb629f9c128551ea7168da722": "0x38d7ea4c68000"
      }
    },
    "0xf39fd6e51aad88f6f4ce6ab8827279cfffb92266": {
      "balance": "0xddd2934f05f16e7",
      "nonce": 1
    }
  },
  "pre": {
    "0x4200000000000000000000000000000000000006": {
      "balance": "0x222f82117cf7c12547b",
      "code": "0x6080604052..."
    },
    "0xf39fd6e51aad88f6f4ce6ab8827279cfffb92266": {
      "balance": "0xde0b6b3a7640000"
    }
  }
}

In this example:

• The sender's balance decreased (paid ETH + gas), and nonce incremented to 1.
• WETH's balance increased by the deposited amount, and a new storage slot was written (the sender's balance mapping).
• The code field appears in pre but not post — it didn't change, so diff mode omits it from post.

Examples

Capture pre-state:

# Capture pre-state for a WETH.balanceOf call
mega-evme tx \
  --fork --rpc https://mainnet.megaeth.com/rpc \
  --sender.balance 1ether \
  --receiver 0x4200000000000000000000000000000000000006 \
  --input 0x70a08231000000000000000000000000f39fd6e51aad88f6f4ce6ab8827279cfffb92266 \
  --trace --tracer pre-state \
  --trace.output prestate.json

Capture state diff (before/after):

# WETH.deposit() modifies balances — diff mode shows the change
mega-evme tx \
  --fork --rpc https://mainnet.megaeth.com/rpc \
  --sender.balance 1ether \
  --receiver 0x4200000000000000000000000000000000000006 \
  --input 0xd0e30db0 \
  --value 1000000000000000 \
  --trace --tracer pre-state \
  --trace.prestate.diff-mode \
  --trace.output diff.json

Compact diff without code or storage:

# Replace 0xabc123... with a real transaction hash from MegaETH mainnet
mega-evme replay 0xabc123... \
  --rpc https://mainnet.megaeth.com/rpc \
  --trace --tracer pre-state \
  --trace.prestate.diff-mode \
  --trace.prestate.disable-code \
  --trace.prestate.disable-storage