On both Arbitrum and Optimism, a single operator called the sequencer decides the order transactions are processed in before anything settles on Ethereum. Neither network’s documentation claims that sequencer can block a transaction forever: Arbitrum’s own decentralization page says its sequencer can delay inclusion by up to 24 hours and reorder transactions over short spans, while Optimism’s documentation says a censored deposit transaction can be forced through Ethereum directly, with inclusion of that forced transaction guaranteed within a 12-hour window. Above the sequencer, each network also has a separate override layer – a DAO and multisig on Arbitrum, a Guardian role on Optimism – that can pause parts of the system entirely.
What a sequencer actually controls
On Arbitrum, the sequencer orders incoming transactions and handles batching, compression and posting of that data to Ethereum, according to Arbitrum’s own documentation on the sequencer and censorship resistance. It publishes a real-time feed that gives users what the documentation calls “soft finality” – a provisional confirmation that depends on the sequencer behaving honestly, ahead of the transaction actually reaching finality on Ethereum.
Optimism’s architecture works on a similar principle. According to Optimism’s own documentation on forced transactions, the sequencer processes transactions and produces blocks regularly during normal operation. The OP Stack Specification’s architecture diagram shows how the resulting data reaches Ethereum: a Batcher publishes transaction batches to a Batch Inbox Address on the L1 chain, which L2 nodes then fetch to derive the canonical chain. The sequencer is currently operated by the Optimism Foundation, and decentralizing it is an active development priority, according to ChainClarity’s explainer of Optimism’s design; that claim is not stated in the primary Optimism documentation or OP Stack specification reviewed for this page, and is treated here as single-sourced.
Arbitrum’s bypass: the delayed inbox and the 24-hour ceiling
Arbitrum’s sequencer documentation describes a Delayed Inbox contract on Ethereum, which the sequencer monitors for finalized retryable transactions and messages submitted directly on the parent chain, folding them into the transaction feed once confirmed. The documentation does not describe the Delayed Inbox specifically as a general censorship-bypass path open to any user transaction; it names a separate feature, Censorship Timeout, described as intended “to limit the negative effects of prolonged sequencer censorship or unexpected sequencer outages,” without detailing how it is triggered or how long it takes, in the excerpt reviewed for this page. Separately, the Arbitrum Foundation’s own progressive-decentralization page states that the sequencer can delay the inclusion of a user’s transaction by up to 24 hours and reorder transactions over short time-horizons, and adds that the sequencer “cannot compromise the system’s safety” or block a transaction from eventually executing. The evidence reviewed here does not specify whether this 24-hour bound is enforced through the Delayed Inbox, the Censorship Timeout feature, or some other mechanism.
Optimism’s bypass: forced transactions and the 12-hour window
Optimism’s documentation on forced transactions lays out a more fully specified timeline. Under normal operation, deposit transactions are included within 30 minutes – the network’s “Max Time Drift.” If the sequencer goes down, a user can submit a transaction directly to the OptimismPortal contract on Ethereum, bypassing the sequencer entirely. What happens next depends on how long the outage lasts, according to Optimism’s docs:
- Under 30 minutes: the transaction can still land within the usual 30-minute window once the sequencer resumes.
- Between 30 minutes and 12 hours: the forced transaction sits on Ethereum until the sequencer comes back online, or until the 12-hour “Sequencing Window” expires, whichever comes first.
- Past 12 hours: nodes begin producing blocks deterministically, including only the transactions that were forced onto Ethereum, while ordinary sequencer transactions wait until the sequencer catches up.
Optimism’s documentation also describes a sequencer “catch-up phase” after an outage of more than 30 minutes: L2 blocks are normally produced at a 6:1 ratio against Ethereum blocks (12 seconds per L2 block versus 2 seconds per L1 block), and that ratio temporarily switches to 1:1 while the sequencer works through the backlog. The documentation frames the 12-hour ceiling as a deliberate trade-off – long enough to let a sequencer recover without triggering an L2 reorganization, but short enough to guarantee eventual inclusion.
Who can override the sequencer at the protocol level
Below the sequencer, each network has a separate layer of accounts that can act on the protocol itself – a distinct question from who orders individual transactions.
On Arbitrum, chain ownership – the power to upgrade core contracts, change system parameters, or pause the system – is governed by the Arbitrum DAO, made up of ARB token-holders and delegates, according to the Arbitrum Foundation’s decentralization documentation. A Security Council can also exercise these powers: a “9 of 12 multisig wallet” that can act quickly in emergencies and more slowly for routine upgrades, with its members elected by the DAO every six months. The same documentation states that if a malicious DAO proposal passes, or if nine Security Council members are compromised, users would have several weeks to withdraw funds to Ethereum before the change takes effect. Validation on Arbitrum One now runs on the permissionless BoLD protocol, per the Foundation; Arbitrum Nova remains permissioned, secured instead by a six-member Data Availability Committee whose named members, per the same documentation, include ConsenSys Software Inc., QuickNode Inc., P2P.org, Google Cloud, Offchain Labs Inc. and Opensea Innovation Labs Private Limited. The Foundation’s risk note for Nova says the system’s safety could be compromised only if five of those six DAC members colluded with the sequencer.
On Optimism, the equivalent override sits inside the OP Stack’s Ethereum-side contracts. The OP Stack Specification lists a Guardian role with the power to pause and unpause the SuperchainConfig contract and to take what it calls “safety net actions” on the OptimismPortal, the DisputeGameFactory, and the DelayedWETH contract. The specification documents that this role exists and what it can touch; it does not say who currently holds it, how many parties are involved, or what threshold is required to act – a gap this page cannot fill from the evidence available.
A worked comparison
Put the two published figures side by side: the Arbitrum Foundation’s decentralization documentation states that a sequencer can delay a user’s transaction by up to 24 hours before it must be included, though the same documentation does not specify the mechanism through which that eventual inclusion happens. Optimism’s forced-transaction guide documents a shorter, and more fully specified, path: a user can submit a deposit directly to the OptimismPortal contract on Ethereum, with inclusion guaranteed within 12 hours. The two figures describe different things – Arbitrum’s is an outer bound on delay with the underlying mechanism unstated in the evidence gathered here, Optimism’s is a documented end-to-end bypass route – so the 12-versus-24-hour comparison is this page’s own reading of numbers published separately by each project, not a claim that the two networks offer an equivalent guarantee.
The common misreading
A common assumption is that an L2 transaction, once it shows up as confirmed in a wallet, is settled and immune from being reordered or delayed. Both projects’ own documentation says otherwise: Arbitrum’s sequencer feed produces only “soft finality,” which the Arbitrum docs describe as dependent on the sequencer behaving honestly, and Optimism’s deposit flow is explicitly designed around the possibility that the sequencer stops working or starts censoring. The bypass routes on both chains exist because the underlying centralization – one operator ordering transactions – has not been removed, only bounded.
What this page does not tell you
The documents cited here describe designed capabilities, not incident history. Neither Arbitrum’s nor Optimism’s own documentation, as reviewed for this page, states whether the 24-hour delay window, the 12-hour sequencing window, the Security Council’s emergency multisig, or Optimism’s Guardian pause power has ever actually been exercised on a production network, or what happened when it was.
The identities behind two of the key override roles are not disclosed in this evidence: the OP Stack Specification names the Guardian role and what it can do, but not who holds the key or how many parties are involved. Arbitrum’s Security Council composition is described by seat count and election cadence, not by the individual signers.
This page covers Arbitrum One and Arbitrum Nova, and OP Mainnet specifically. Other chains built on the OP Stack, such as Base, or on Arbitrum’s Orbit framework, can configure their own sequencer ownership and pause powers differently even while sharing the underlying codebase; the evidence gathered here does not extend to those chains.
Several of the source documents – including the Arbitrum Foundation’s decentralization page, marked as a “public preview document” subject to change, and Optimism’s own docs and specification pages – carry no visible publication date. Figures describing these fixed protocol parameters are current as of the versions of the documentation reviewed for this page; a reader checking them later should verify against the live pages rather than assume nothing has changed.
Fee and token-distribution figures from ChainClarity’s third-party explainer of Optimism, including the 40% gas-fee reduction attributed to the Bedrock upgrade and Optimism’s token-allocation percentages, are self-reported research by a site that is not Optimism itself, and are noted here as single-sourced background rather than treated as confirmed by the protocol.
Sources
Every fact above is attributed to one of these reports. Where they disagree, the article says so.
