API Reference

Nitro exposes three layers builders interact with: the Ethereum JSON-RPC (which is stock go-ethereum), a small set of Arbitrum-specific RPC extensions, and the L2 precompiles — fixed-address contracts that expose L2-native primitives. On the L1 side there are the bridge contracts.

Ethereum JSON-RPC

• Standard namespace (eth_, net_, web3_) — identical semantics to go-ethereum.
• eth_getBlockByNumber includes an l1BlockNumber field per block.
• eth_maxPriorityFeePerGas returns an estimate that accounts for the L1 data component — the recommended way to price transactions on the L2.

Arbitrum-specific RPC

• arb_getBlockWithSize — block plus the exact byte-size of its transactions, useful for estimating L1 data fees.
• arb_estimateRetryableTicket — estimate the L2 gas cost of creating a retryable, without simulating it.
• arb_debugTracingCall — call tracing with Arbitrum-specific gas info (used by block explorers and debug tooling).

L2 precompiles (fixed addresses)

• 0x0000...064 ArbSys — arbBlockNumber, arbBlockHash, sendTxToL1, sendTxToL1CustomFee, withdrawEth, getStorageAt.
• 0x0000...06B ArbRetryableTx — createRetryableTicket, redeem, cancel, getBeneficiary.
• 0x0000...06C ArbGasInfo — getPricesInWei, getL1FeesApproximate, getL1GasUsed, getL1CurrentEstimate.
• 0x0000...06D ArbToken — depositERC20, withdrawERC20, getGateway.
• 0x0000...06E ArbOwner — setFeePool, setL1BaseFeeEstimate, withdrawEth, setChainConfig.

L1 contracts (reference)

• Bridge — enqueueDelayedMessage, executeCall, delayedInboxAccs, initialize.
• Inbox — createRetryableTicket, createEthDeposit, sendL1FundedUnsignedTransaction.
• Outbox — executeTransaction, processOutgoingMessage, calculateItemHash.
• SequencerInbox — addSequencerL2BatchFromOrigin, addSequencerL2BatchFromAnyAddress, addMessageToDelayedInbox.
• RollupCore / RollupUserLogic — newStakeOnNewAssertion, challengeAssertion, confirmNextAssertion.
• ChallengeManager — initializeChallengeBisection, oneStepProveExecution.

Edge cases

• Precompile addresses are L2-only — calling them on L1 hits empty accounts; gate any code that reads them behind a chain check.
• eth_maxPriorityFeePerGas on an Arbitrum node returns an L1-inclusive price; on an L1 node it returns an L1-only price. Don't pass the same value across chains.
• The l1BlockNumber block field is the L1 block the L2 block is anchored to — for L1-timestamp-based logic, prefer it over the L2 timestamp (which follows the sequencer's clock).