> For the complete documentation index, see [llms.txt](https://docs.syndromics.xyz/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.syndromics.xyz/risk/sequencer-and-chain.md).

# Sequencer and chain risk

Robinhood Chain is an Arbitrum Nitro rollup with a single sequencer operated by Robinhood. That is normal for an L2 at this stage, and it comes with specific failure modes that a lending protocol has to design for rather than hope around.

## What can go wrong

| Scenario                   | Effect on users                                                                                              |
| -------------------------- | ------------------------------------------------------------------------------------------------------------ |
| Sequencer outage           | No transactions are included on the L2 for the duration. Prices keep moving on the underlying market.        |
| Sequencer censorship       | Specific transactions (for example a repayment) are not included.                                            |
| Delayed L1 finality        | Withdrawals to Ethereum take the canonical 7 days; this does not affect on-L2 loan operations.               |
| Chain configuration change | DA mode, validator set, or the enabling of Timeboost or BoLD could change finality and ordering assumptions. |

## Mitigations

### Liquidation grace after an outage

`OracleRouter` reads the Chainlink L2 Sequencer Uptime Feed. When the feed reports that the sequencer has just come back, no liquidation auction can be started for a grace period of **one hour**. This gives borrowers time to top up or repay before anyone can act on a price that moved while nobody could transact.

### Everything is reachable through the L1 delayed inbox

Every state-changing function in Syndromics can be submitted through Arbitrum's delayed inbox on Ethereum. If the sequencer does not include a transaction within 24 hours, it can be force-included. In practice this means:

* a censoring sequencer cannot prevent a borrower from repaying,
* a censoring sequencer cannot prevent a lender from cancelling an offer,
* a borrower who anticipates an outage can queue a top-up on L1.

Combined with the grace period, the force-inclusion path is genuinely usable rather than theoretical.

### Timestamps, not block numbers

On Arbitrum-stack chains `block.number` returns an L1-derived value. Interest accrual, expiries and auction curves use `block.timestamp` only. If an L2 block height is ever needed, the protocol calls `ArbSys.arbBlockNumber()`.

### Calldata cost

The L1 data component of gas is priced on compressed calldata size. Offer structs are packed tightly and signatures are passed as compact 64-byte form where possible, so that a `settle` with twenty offers stays economical.

## Chain configuration watch

Whether Timeboost (express-lane ordering) or BoLD (permissionless validation) are enabled on Robinhood Chain affects two things: liquidator ordering during an auction, and how quickly a forced-inclusion transaction can be relied upon. Syndromics tracks both. If Timeboost is enabled, the auction design is reviewed so that express-lane access cannot systematically front-run keeper transactions, and any change is published in the governance log.


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