It's important to note that there are various minor discrepancies between the behavior of Optimism and Ethereum. You should be aware of these descrepancies when building apps on top of Optimism.

Opcode Differences

Modified Opcodes

Opcode	Solidity equivalent	Behavior
COINBASE	block.coinbase	Value is set by the sequencer. Currently returns the OVM_SequencerFeeVault address (0x420...011).
DIFFICULTY	block.difficulty	Always returns zero. You can use an oracle for randomness.
BASEFEE	block.basefee	Currently unsupported.
ORIGIN	tx.origin	If the transaction is an L1 ⇒ L2 transaction, then tx.origin is set to the aliased address of the address that triggered the L1 ⇒ L2 transaction. Otherwise, this opcode behaves normally.

Added Opcodes

Opcode	Behavior
L1BLOCKNUMBER	Returns the block number of the last L1 block known by the L2 system. Typically this block number will lag by up to 15 minutes behind the actual latest L1 block number. See section on Block Numbers and Timestamps for more information.

Block Numbers and Timestamps

Block production is not constant

On Ethereum, the NUMBER opcode (block.number in Solidity) corresponds to the current Ethereum block number. Similarly, in Optimism, block.number corresponds to the current L2 block number. However, as of the OVM 2.0 release of Optimism (Nov. 2021), each transaction on L2 is placed in a separate block and blocks are NOT produced at a constant rate.

This is important because it means that block.number is currently NOT a reliable source of timing information. If you want access to the current time, you should use block.timestamp (the TIMESTAMP opcode) instead.

Timestamps

The TIMESTAMP opcode (block.timestamp in Solidity) uses the timestamp of the transaction itself. It gets updated every fifteen seconds.

Accessing the latest L1 block number

NOTICE

The hex value that corresponds to the L1BLOCKNUMBER opcode (0x4B) may be changed in the future (pending further discussion). We strongly discourage direct use of this opcode within your contracts. Instead, if you want to access the latest L1 block number, please use the OVM_L1BlockNumber contract as described below.

The block number of the latest L1 block seen by the L2 system can be accessed via the L1BLOCKNUMBER opcode. Solidity doesn't make it easy to use non-standard opcodes, so we've created a simple contract located at 0x4200000000000000000000000000000000000013 (opens new window) that will allow you to trigger this opcode.

You can use this contract as follows:

import { iOVM_L1BlockNumber } from "@eth-optimism/contracts/L2/predeploys/iOVM_L1BlockNumber.sol";
import { Lib_PredeployAddresses } from "@eth-optimism/contracts/libraries/constants/Lib_PredeployAddresses.sol";

contract MyContract {
   function myFunction() public {
      // ... your code here ...

      uint256 l1BlockNumber = iOVM_L1BlockNumber(
         Lib_PredeployAddresses.L1_BLOCK_NUMBER // located at 0x4200000000000000000000000000000000000013
      ).getL1BlockNumber();

      // ... your code here ...
   }
}

Using ETH in Contracts

As of the OVM 2.0 update (Nov. 2021), the process of using ETH on L2 is identical to the process of using ETH in Ethereum. Please note that ETH was previously accessible as an ERC20 token, but this feature has been removed as part of OVM 2.0.

For tooling developers and infrastructure providers, please note that ETH is still represented internally as an ERC20 token at the address 0xDeadDeAddeAddEAddeadDEaDDEAdDeaDDeAD0000 (opens new window). As a result, user balances will always be zero inside the state trie and the user's actual balance will be stored in the aforementioned token's storage. It is NOT possible to call this contract directly, it will throw an error.

Address Aliasing

Because of the behavior of the CREATE opcode, it is possible for a user to create a contract on L1 and on L2 that share the same address but have different bytecode. This can break trust assumptions, because one contract may be trusted and another be untrusted (see below). To prevent this problem the behavior of the ORIGIN and CALLER opcodes (tx.origin and msg.sender) differs slightly between L1 and L2.

The value of tx.origin is determined as follows:

Call source	tx.origin
L2 user (Externally Owned Account)	The user's address (same as in Ethereum)
L1 user (Externally Owned Account)	The user's address (same as in Ethereum)
L1 contract (using CanonicalTransactionChain.enqueue)	L1_contract_address + 0x1111000000000000000000000000000000001111

The value of msg.sender at the top-level (the very first contract being called) is always equal to tx.origin. Therefore, if the value of tx.origin is affected by the rules defined above, the top-level value of msg.sender will also be impacted.

Note that in general, tx.origin should not be used for authorization (opens new window). However, that is a separate issue from address aliasing because address aliasing also affects msg.sender.

Why is address aliasing an issue?

The problem with two identical source addresses (the L1 contract and the L2 contract) is that we extend trust based on the address. It is possible that we will want to trust one of the contracts, but not the other.

1. Helena Hacker forks Uniswap (opens new window) to create her own exchange (on L2), called Hackswap.

   Note: There are actually multiple contracts in Uniswap, so this explanation is a bit simplified. See here if you want additional details (opens new window).

2. Helena Hacker provides Hackswap with liquidity that appears to provide profitable arbitrage opportunities. For example, she can make it so that you can spend 1 DAI (opens new window)to buy 1.1 USDT (opens new window). Both of those coins are supposed to be worth exactly $1.

3. Nimrod Naive knows that if something looks too good to be true it probably is. However, he checks the Hackswap contract's bytecode and verifies it is 100% identical to Uniswap. He decides this means the contract can be trusted to behave exactly as Uniswap does.

4. Nimrod approves an allowance of 1000 DAI for the Hackswap contract. Nimrod expects to call the swap function on Hackswap and receive back nearly 1100 USDT.

5. Before Nimrod's swap transaction is sent to the blockchain, Helena Hacker sends a transaction from an L1 contract with the same address as Hackswap. This transaction transfers 1000 DAI from Nimrod's address to Helena Hacker's address. If this transaction were to come from the same address as Hackswap on L2, it would be able to transfer the 1000 DAI because of the allowance Nimrod had to give Hackswap in the previous step to swap tokens.

   Nimrod, despite his naivete, is protected because Optimism modified the transaction's tx.origin (which is also the initial msg.sender). That transaction comes from a different address, one that does not have the allowance.

Note: It is simple to create two different contracts on the same address in different chains. But it is nearly impossible to create two that are different by a specified amount, so Helena Hacker can't do that.

Network specifications

JSON-RPC differences

Optimism uses the same JSON-RPC API (opens new window) as Ethereum. Some additional Optimism specific methods have been introduced. See the full list of custom JSON-RPC methods for more information.

Pre-EIP-155 support

Pre-EIP-155 (opens new window) transactions do not have a chain ID, which means a transaction on one Ethereum blockchain can be replayed on others. This is a security risk, because transactions that are legitimate on one chain could be a security risk on another. For example, you might agree to send me 1 ETH on Goerli (chain ID 5) to help me test my contracts. If you submit the transaction as a pre-EIP-155 transaction, then I can wait until your address's nonce on mainnet (chain ID 1) is the same as the one you had when you submitted the Goerli transaction and send the transaction to mainnet. Mainnet would then interpret it as a legitimate transaction and transfer a real ETH from your account to mine (assuming your balance is high enough, of course)

Starting in November 2022, pre-EIP-155 transactions are no longer supported on Optimism using the public endpoint or through Alchemy.

Pre-EIP-155 transactions are dangerous

It is highly recommended not to use pre-eip-155 transaction. But if you absolutely must use them, Infura and QuickNode still support them. Just be careful.

Bedrock

In the Bedrock version there are even less differences between Optimism and L1 Ethereum.

Opcode Differences

Opcode	Solidity equivalent	Behavior
COINBASE	block.coinbase	Undefined
DIFFICULTY	block.difficulty	Random value. As this value is set by the sequencer, it is not as reliably random as the L1 equivalent. You can use an oracle for randomness.
NUMBER	block.number	L2 block number
TIMESTAMP	block.timestamp	Timestamp of the L2 block
ORIGIN	tx.origin	If the transaction is an L1 ⇒ L2 transaction, then tx.origin is set to the aliased address of the address that triggered the L1 ⇒ L2 transaction. Otherwise, this opcode behaves normally.
CALLER	msg.sender	If the transaction is an L1 ⇒ L2 transaction, and this is the initial call (rather than an internal transaction from one contract to another), the same address aliasing behavior applies.

`tx.origin == msg.sender`

On L1 Ethereum tx.origin is equal to msg.sender only when the smart contract was called directly from an externally owned account (EOA). However, on Optimism tx.origin is the origin on Optimism. It could be an EOA. However, in the case of messages from L1, it is possible for a message from a smart contract on L1 to appear on L2 with tx.origin == msg.sender. This is unlikely to make a significant difference, because an L1 smart contract cannot directly manipulate the L2 state. However, there could be edge cases we did not think about where this matters.

Accessing L1 information

If you need the equivalent information from the latest L1 block, you can get it from the L1Block contract (opens new window). This contract is a predeploy at address 0x4200000000000000000000000000000000000015 (opens new window). You can use the getter functions (opens new window) to get these parameters:

• number: The latest L1 block number known to L2
• timestamp: The timestamp of the latest L1 block
• basefee: The base fee of the latest L1 block
• hash: The hash of the latest L1 block
• sequenceNumber: The number of the L2 block within the epoch (the epoch changes when there is a new L1 block)

Address Aliasing

Because of the behavior of the CREATE opcode, it is possible for a user to create a contract on L1 and on L2 that share the same address but have different bytecode. This can break trust assumptions, because one contract may be trusted and another be untrusted (see below). To prevent this problem the behavior of the ORIGIN and CALLER opcodes (tx.origin and msg.sender) differs slightly between L1 and L2.

The value of tx.origin is determined as follows:

Call source	tx.origin
L2 user (Externally Owned Account)	The user's address (same as in Ethereum)
L1 user (Externally Owned Account)	The user's address (same as in Ethereum)
L1 contract (using CanonicalTransactionChain.enqueue)	L1_contract_address + 0x1111000000000000000000000000000000001111

The value of msg.sender at the top-level (the very first contract being called) is always equal to tx.origin. Therefore, if the value of tx.origin is affected by the rules defined above, the top-level value of msg.sender will also be impacted.

Note that in general, tx.origin should not be used for authorization (opens new window). However, that is a separate issue from address aliasing because address aliasing also affects msg.sender.

Why is address aliasing an issue?

The problem with two identical source addresses (the L1 contract and the L2 contract) is that we extend trust based on the address. It is possible that we will want to trust one of the contracts, but not the other.

1. Helena Hacker forks Uniswap (opens new window) to create her own exchange (on L2), called Hackswap.

   Note: There are actually multiple contracts in Uniswap, so this explanation is a bit simplified. See here if you want additional details (opens new window).

2. Helena Hacker provides Hackswap with liquidity that appears to allow for profitable arbitrage opportunities. For example, she can make it so that you can spend 1 DAI (opens new window)to buy 1.1 USDT (opens new window). Both of those coins are supposed to be worth exactly $1.

3. Nimrod Naive knows that if something looks too good to be true it probably is. However, he checks the Hackswap contract's bytecode and verifies it is 100% identical to Uniswap. He decides this means the contract can be trusted to behave exactly as Uniswap does.

4. Nimrod approves an allowance of 1000 DAI for the Hackswap contract. Nimrod expects to call the swap function on Hackswap and receive back nearly 1100 USDT.

5. Before Nimrod's swap transaction is sent to the blockchain, Helena Hacker sends a transaction from an L1 contract with the same address as Hackswap. This transaction transfers 1000 DAI from Nimrod's address to Helena Hacker's address. If this transaction were to come from the same address as Hackswap on L2, it would be able to transfer the 1000 DAI because of the allowance Nimrod had to give Hackswap in the previous step to swap tokens.

   Nimrod, despite his naivete, is protected because Optimism modified the transaction's tx.origin (which is also the initial msg.sender). That transaction comes from a different address, one that does not have the allowance.

Note: It is simple to create two different contracts on the same address in different chains. But it is nearly impossible to create two that are different by a specified amount, so Helena Hacker can't do that.

Blocks

There are several differences in the way blocks are produced between L1 Ethereum and Optimism Bedrock.

Parameter	L1 Ethereum	Optimism Bedrock
Time between blocks	12 seconds(1)	2 seconds
Block target size	15,000,000 gas	to be determined
Block maximum size	30,000,000 gas	to be determined

(1) This is the ideal. If any blocks are missed it could be an integer multiple such as 24 seconds, 36 seconds, etc.

Note: The L1 Ethereum parameter values are taken from ethereum.org (opens new window). The Optimism Bedrock values are taken from the Optimism specs (opens new window).

Network specifications

JSON-RPC differences

Optimism uses the same JSON-RPC API (opens new window) as Ethereum. Some additional Optimism specific methods have been introduced. See the full list of custom JSON-RPC methods for more information.

Transaction costs

Transaction costs on Optimism include an L2 execution fee and an L1 data fee.

Contract addresses

The addresses in which various infrastructure contracts are installed are different between L1 Ethereum and Optimism. For example, WETH9 (opens new window) is installed on L1 Ethereum on address 0xc02aaa39b223fe8d0a0e5c4f27ead9083c756cc2 (opens new window). On Optimism the same contract is installed on address 0x4200000000000000000000000000000000000006 (opens new window).