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Finality and chain reorganizations

This reference explains how finality works on Arbitrum chains, why and when a chain can reorganize (reorg), how deep a reorg can go, and which confirmation level an indexer should wait for before treating data as irreversible.

This is intended for developers building indexers, block explorers, bridges, and custodial integrations, and it applies to Arbitrum One, Nova, and Dedicated Blockchains (Arbitrum chains)—including AnyTrust chains that settle to another Arbitrum chain.

For a step-by-step deposit and withdrawal detection procedure, see the Exchange integration checklist. This page provides the finality and reorg model that the checklist builds on.

The three confirmation levels​

An Arbitrum node exposes three levels of confirmation, surfaced as the standard Ethereum JSON-RPC block tags. Each maps to a different point in the transaction lifecycle and carries a different reversibility guarantee.

LevelBlock tagWhat it meansCan it be reverted?
Soft confirmationlatestThe Sequencer has ordered and executed the transaction and emitted it on the Sequencer feed.Yes—this is a promise from the Sequencer, not yet backed by the parent chain.
Parent-chain safesafeThe batch containing the block has been posted to the parent chain and reached the parent chain's safe block.Only by a deep parent-chain reorg.
Parent-chain finalfinalizedThe batch containing the block has been posted to the parent chain and reached the parent chain's finalized block.Only if the parent chain itself reverts a finalized block (on Ethereum, economically infeasible).
The load-bearing rule

An Arbitrum block is only as final as the parent-chain block that carries its batch. Soft confirmation is fast (sub-second) but reversible. Hard finality—the safe and finalized tags—is inherited from the parent chain and is what makes a block reorg-proof.

What "seconds to finality" really means​

On an AnyTrust chain it is common to assume finality is achieved in seconds, as soon as the Data Availability Committee (DAC) signs the block's data. That describes soft confirmation and data availability, not hard finality:

  • The Sequencer executes and broadcasts the block immediately—this is your sub-second soft confirmation.
  • The DAC signing a Data Availability Certificate (DACert) guarantees the data is retrievable, so the batch can be posted compactly. It does not settle the block against the parent chain.
  • Hard finality still requires the batch (or DACert) to be posted to the parent chain's Sequencer Inbox and for that parent-chain block to finalize.

So on your chain, "seconds" is the soft-confirmation latency; the reorg-proof guarantee tracks Base finality, which in turn tracks Ethereum finality.

How the node derives safe and finalized​

A Nitro node is split into a consensus side (reads the parent chain) and an execution side (runs the EVM and serves RPC). Finality flows from the parent chain into the child chain's safe/finalized heads:

  1. The node reads the parent chain's own safe and finalized blocks over RPC. It does not compute parent-chain finality itself—it defers to the parent chain's block tags. (Finality data is only used when the parent chain supports it, for example Ethereum proof-of-stake or another Arbitrum chain.)
  2. For each parent-chain safe/finalized block, the node finds the newest sequencer batch that was posted at or before that parent-chain block.
  3. The last child-chain block in that batch becomes the child chain's safe (or finalized) head, which is what the child-chain RPC serves for those tags.

In other words, a child-chain block is finalized when the parent-chain block that carries its batch is itself finalized. Optionally, a chain can be configured to further hold safe/finalized back until a local validator has validated the block.

Block-tag availability on Dedicated Blockchains

The safe and finalized tags are always available on Arbitrum One and Nova. On a self-managed chain, they are available only when your parent chain exposes finality data and your node is configured to track it. Confirm the behavior on your own chain before relying on finalized, and document the expected confirmation latency for your integrators.

Per-chain-type guarantees​

The confirmation model is identical across chains; what differs is which parent chain finality is inherited from, and therefore the latency.

Chain typeParent chainfinalized inherits fromApproximate hard-finality latency
Arbitrum One / NovaEthereumEthereum finalized (~2 epochs, ~13 min)Time to post the batch, plus Ethereum finality.
Arbitrum chain atop Ethereum (L2)EthereumEthereum finalizedSame as above.
Arbitrum chain atop an L2 (L3, e.g. on Base)The L2 (e.g. Base)The L2's finalized, which itself tracks EthereumThe L2's finality latency, which is bounded below by Ethereum finality.

For an L3 on Base, finalized on your chain cannot be more final than Base's finalized, and Base's finalized ultimately tracks Ethereum. The DAC does not shorten this—it only affects how data is made available, not how the parent chain finalizes.

AnyTrust and the DAC: what changes and what doesn't​

On an AnyTrust chain the batch poster asks the DAC to sign a DACert over the batch data. If enough committee members sign against the current keyset, the batch poster posts the compact DACert to the Sequencer Inbox instead of the full data. What this does and does not affect:

  • Does not change the finality model. Finality still tracks the parent chain. A DACert reaching parent-chain finality is what makes the block final; the DAC signature by itself is a data-availability guarantee.
  • Does not change reorg behavior. A DACert is posted to the Sequencer Inbox exactly like a full-data batch, so the reorg rules below apply unchanged.
  • Affects liveness under DAC failure. If the batch poster cannot collect enough signatures within a few minutes, it falls back to posting the full batch data directly to the parent chain as calldata, and the chain keeps producing and finalizing blocks. Fallback can be disabled (disable-dap-fallback-store-data-on-chain: true), in which case a DAC outage halts batch posting. See DAC and DAS operations for the full failure-mode table.

The reorg model​

Once a batch is posted to the Sequencer Inbox, the only thing that can reorganize an Arbitrum chain is a reorg of its underlying (parent) chain. Fraud proofs and dispute resolution never cause a reorg—a rejected assertion only prevents an invalid state from being confirmed for settlement; it never rewrites already-sequenced blocks.

When a parent-chain reorg does change the Sequencer Inbox or delayed inbox ordering, the child chain reorgs as follows:

  1. The node detects that the parent-chain-derived data has changed (the delayed-message accumulator or the sequencer-batch accumulator no longer matches what it stored).
  2. It walks back to the most recent block that still matches the canonical parent-chain-derived history—the common ancestor.
  3. It rolls the chain back to that block. State reverts to the state root at the common ancestor, and every transaction after that point is discarded. Block explorers and RPC will no longer return the discarded transactions.
  4. It re-derives the chain forward from the parent chain. Messages that were reorged out (within the re-sequencing window) may be re-sequenced, but block hashes, block numbers, and transaction ordering after the common ancestor can all differ from before.
Never key an index on block number alone across a reorg

After a reorg, the same block number can hold different transactions with a different block hash. Always store and compare block hashes and parentHash continuity, and rewind to the common ancestor rather than patching individual records. See Reorg detection patterns.

How deep can a reorg go?​

Child-chain reorg depth is bounded by parent-chain reorg depth, not by anything on the child chain and not by how often you post assertions.

A common misconception is to reason from the assertion (state-root) cadence: "we post state roots every hour, which at 250 ms blocks is ~14,400 blocks, so a reorg could be 14,400 blocks deep." That conflates two independent things:

  • Assertions / state roots are periodic settlement checkpoints posted to the Rollup contract for the dispute protocol. They do not cause or bound reorgs, and their cadence is irrelevant to reorg depth.
  • Batches (data) are posted continuously by the batch poster—not on the assertion cadence. Reorgs are driven by parent-chain reorgs of these batches, and only for the batches that are not yet final on the parent chain.

So the practical bound on a child-chain reorg is: the child blocks whose batches were posted within the parent chain's unfinalized window. For an indexer, this collapses to a simple rule:

  • If you index at finalized, you will never observe a reorg, because finalized parent-chain blocks do not revert.
  • If you index at safe, you are exposed only to a deep parent-chain reorg (rare on Ethereum-backed chains).
  • If you index at latest (soft), you are exposed to the full unfinalized window and must handle reorgs actively.

A 14,400-block child reorg would require the parent chain to reorg its entire corresponding unfinalized range at once—which for a Base → Ethereum-backed chain does not happen under normal finality assumptions. The correct defense is not to guess a maximum depth but to gate crediting on finalized (or to rewind to the common ancestor for anything you display before finality).

ArbitrumInternalTx behavior under a parent-chain reorg​

Every child-chain block begins with a system transaction of type ArbitrumInternalTx (0x6A), specifically an InternalTxStartBlock. It is guaranteed to be the first transaction in its block, and it records the parent-chain block number and parent-chain base fee that were in effect when the block's message was read. This is the mechanism that feeds parent-chain context (for example, the value returned by block.number and BLOCKHASH) into the EVM. See Geth at the core for details.

Because this transaction's contents are derived from the parent chain, a parent-chain reorg that changes a block's parent-chain context also changes that block's InternalTxStartBlock. There is no special handling that keeps it stable across a reorg—the affected blocks are reorged out and re-derived through the normal path above.

Consequences for an indexer:

  • The ArbitrumInternalTx is ArbOS bookkeeping. It never moves user value and must never be credited or treated as a user transaction.
  • Do not assume the ordering, block number, or block hash of the surrounding transactions is preserved across a reorg. After the common ancestor, everything is recomputed: the internal tx stays first, but the transactions after it, their block assignment, and the block hashes can all change.
  • The only stable anchor across a reorg is the common ancestor's block hash. Rewind to it and re-scan.

On Ethereum, indexers often wait a fixed number of confirmations (for example, 12 blocks). That model does not translate to Arbitrum: child blocks are produced far faster than the parent chain finalizes, and reorg exposure is a function of parent-chain finality, not of a child-block count. Wait on the block tag, not on a fixed depth.

Use caseWait forRationale
Crediting deposits, releasing funds, any irreversible actionfinalizedFinalized parent-chain blocks do not revert, so a credited entry can never be reorged away.
Low-value or reversible UX (pending balances, activity feed)safe, or latest with active reorg handlingFaster, but you must be able to roll back if a reorg occurs.
Real-time display onlylatestSub-second, but explicitly provisional—label it as unconfirmed and expect it to change.

If your chain does not expose safe/finalized (see the note above), treat the Sequencer tip as soft and derive your own finality signal from the parent chain—for example, by tracking which batches have reached the parent chain's finalized block.

Reorg detection patterns​

Whichever level you index at below finalized, detect reorgs by tracking hash continuity and rewinding:

  1. Store each scanned block's hash, keyed by height.
  2. On each poll, confirm that each new block's parentHash matches the hash you stored for the height below it.
  3. On a mismatch, walk back to the most recent height whose stored hash still matches the canonical chain—the common ancestor.
  4. Discard every record and stored hash above the common ancestor, move your scan cursor back to it, and re-scan forward. Because a reorg can both remove and introduce transactions, re-checking only the records you already have is not enough.
  5. You never need to rewind below the finalized height, because finalized blocks cannot reorganize.

The Exchange integration checklist works through this algorithm in the context of a full deposit-detection pipeline.

The --node.bold.rpc-block-number flag​

You may encounter the node flag --node.bold.rpc-block-number. It is a validator-side setting: it selects which parent-chain block tag the BoLD validator reads onchain Rollup and assertion state from. It accepts latest, safe, or finalized, and defaults to finalized.

Not the tag your indexer reads

--node.bold.rpc-block-number controls how the BoLD validator reads the parent chain during dispute-protocol operations. It does not configure the latest/safe/finalized tags your indexer requests from the child-chain RPC. Do not set it expecting to change indexer-facing finality; use the block tags on your RPC queries instead.

The latest / safe / finalized names come from Ethereum's proof-of-stake finality: finalized is roughly two epochs (~64 slots, ~13 minutes) behind the head and safe roughly one epoch (~32 slots) behind. Nitro does not hard-code those distances — it reads the parent chain's own safe/finalized tags — but that is the origin of the "latest-64 / latest-32" shorthand you may have seen.

Corner cases​

The scenarios below are the ones integrators most often ask about. They follow directly from the model above.

A parent-chain reorg moves a block that held one of our state-root attestations​

No child-chain reorg results. State-root assertions are settlement checkpoints, not sequenced data; if the parent-chain block carrying an assertion is reorged, the node simply re-submits the assertion. A child-chain reorg happens only if the reorg changes the Sequencer Inbox or delayed-inbox ordering.

A reorg on Base's parent (Ethereum) affects a block holding some of our data​

Handled the same way. If it changes the derived batch/delayed-message ordering, the affected unfinalized child blocks are re-derived; anything already finalized is unaffected.

The DAC has an outage and cannot sign attestations​

With fallback enabled (the default), the batch poster stops waiting after a few minutes and posts the full batch data directly to the parent chain as calldata. The Sequencer keeps producing blocks and they keep finalizing—hard finality is not blocked by the DAC outage, though batches temporarily cost more to post. Finality is only delayed if you have disabled fallback.

The DAC is completely down and the batch poster cannot reach it​

This halts batch posting only if fallback is disabled (disable-dap-fallback-store-data-on-chain: true). With fallback enabled, the chain continues via direct calldata posting. Decide this trade-off deliberately: fallback favors liveness, disabling it favors keeping all data off the parent chain.

The Sequencer cannot post batches (or DACerts) to the parent chain​

The Sequencer keeps producing soft-confirmed blocks on the feed, and the batch poster retries until it can post. Until a batch reaches parent-chain finality, those blocks are soft only—treat them as reversible for indexing purposes, even though they appear immediately on the feed.