The exit mechanism is non-negotiable. A rollup that cannot guarantee a user's ability to withdraw assets to L1 is a sidechain, not a scaling solution. This forces a permanent cost structure for data availability and proof verification.
zk-rollups-the-endgame-for-scaling
Blog
The Cost of Forced Inclusion: Guaranteeing User Exit in ZK-Rollups
Forced inclusion is the non-negotiable, trustless exit mechanism for ZK-Rollups. This deep dive analyzes the architectural overhead, economic costs, and security implications of implementing this critical safety feature, comparing approaches across Starknet, zkSync Era, and Scroll.
introduction
THE EXIT GUARANTEE
Introduction: The Non-Negotiable Tax on Trustlessness
ZK-Rollups impose a mandatory cost to maintain the user's sovereign right to exit, a fundamental trade-off for trustless scaling.
Forced inclusion is the core cost driver. Unlike optimistic rollups with a 7-day challenge window, ZK-Rollups like zkSync and StarkNet must constantly prove state validity, incurring recurring L1 calldata and proof submission fees for every batch.
This creates a predictable tax per transaction. The cost of ZK-proof generation (via provers like RISC Zero) and data availability (on Ethereum or alternatives like Celestia) becomes a direct, unavoidable line item in the rollup's economic model.
Evidence: StarkEx processes ~300 TPS, with ~70% of its cost per transaction attributed to Ethereum calldata for state diffs, a direct manifestation of this exit guarantee tax.
key-insights
THE COST OF FORCED INCLUSION
Executive Summary: The Three-Part Burden
ZK-Rollups must guarantee user exit to be credibly neutral, a requirement that imposes a three-part burden on infrastructure.
01
The Data Availability Tax
Forced inclusion requires publishing all transaction data on-chain, negating ZK-Rollups' primary scaling promise. This creates a permanent cost floor tied to L1 calldata prices.
- Cost: ~80% of current rollup fees are for L1 data posting.
- Consequence: Limits scalability to ~100-200 TPS, far below theoretical ZK-proving limits.
~80%
Fee Overhead
~150 TPS
Scalability Cap
02
The Liveness Assumption
Users can only force-include a transaction if at least one honest actor (sequencer/validator) is alive and uncensored. This reintroduces a weak form of the very trust assumption ZK proofs aimed to eliminate.
- Risk: Coordinated sequencer downtime becomes a denial-of-service attack vector.
- Mitigation: Requires decentralized sequencer sets or permissionless proving, adding protocol complexity.
1-of-N
Honest Actor Required
High
Orchestration Cost
03
The State Growth Dilemma
Guaranteeing exit for any state transition forces the rollup to maintain and prove the entire historical state. This creates unbounded proving overhead and limits state design innovation.
- Impact: Proving times and costs scale with total state size, not just incremental updates.
- Trade-off: Forces adoption of stateless or expiry-based architectures, complicating developer UX.
O(n)
Proof Complexity
Unbounded
Storage Burden
thesis-statement
THE GUARANTEE
Core Thesis: Forced Inclusion is a Systemic Risk Transfer, Not an Elimination
Forced inclusion guarantees user exit by shifting systemic risk from the sequencer to the L1, creating a new cost and complexity vector.
Forced inclusion is a risk transfer mechanism. It does not eliminate the risk of a malicious or failed sequencer; it transfers the final settlement and censorship-resistance burden to the underlying L1, like Ethereum.
The user exit guarantee has a cost. Every ZK-rollup must reserve L1 block space and gas for forced inclusion transactions, creating a perpetual overhead that scales with user adoption and L1 congestion.
This creates a systemic dependency. Protocols like zkSync Era and StarkNet are not independent; their user safety is a direct function of Ethereum's liveness and gas price volatility.
Evidence: A single forced inclusion transaction on Arbitrum during peak L1 congestion can cost over 0.5 ETH in gas, a cost the protocol or its users must ultimately bear.
FORCED INCLUSION MECHANISMS
Architectural Overhead: A Comparative Breakdown
Comparing the technical trade-offs and costs of different mechanisms that guarantee user exit in ZK-Rollups.
| Architectural Component | ZK-Rollup Native (e.g., zkSync, StarkNet) | Third-Party Escape Hatch (e.g., Across, LayerZero OFT) | Optimistic Rollup Fallback (e.g., Arbitrum, Optimism) |
|---|---|---|---|
Exit Latency (Time to Finality) | ~1-4 hours (L1 proof + challenge window) | < 5 minutes (via external liquidity pool) | ~7 days (standard challenge period) |
On-Chain Gas Cost per Exit | ~500k-1M gas (L1 proof verification) | ~200k-300k gas (bridge contract interaction) | ~70k-150k gas (initiate challenge) |
Protocol-Level Capital Lockup | None (security from L1) | Required for liquidity pools (e.g., $100M+ TVL) | Required for bond staking (e.g., $1M+ per validator) |
Trust Assumption for Liveness | Only L1 is live | External bridge operators & oracles are live | At least one honest validator is live |
Censorship Resistance Vector | Sequencer withholding tx | Bridge relayer withholding attestation | Sequencer & validators colluding |
Implementation Complexity | High (ZK circuit for exit logic) | Medium (integration with external messaging) | Low (inherits from base OR fraud proof system) |
User Experience (UX) Friction | Manual proof submission via wallet | Automatic via aggregator (e.g., UniswapX, CowSwap) | Manual challenge initiation via monitor |
deep-dive
THE EXIT GUARANTEE
The Deep Dive: Contract Complexity & Economic Attack Vectors
Forced inclusion is the non-negotiable security primitive that ensures users can always withdraw from a ZK-Rollup, but its implementation creates systemic risk.
Forced inclusion is a liveness guarantee, not a performance feature. It mandates that a sequencer must include a valid withdrawal request within a fixed time window, enforced by a smart contract on L1. This prevents censorship but creates a single point of failure in the system's state transition logic.
The escape hatch contract is the most complex component. It must verify ZK proofs, manage fraud proofs in optimistic rollups like Arbitrum, and handle direct state transitions. This complexity directly correlates with audit surface area and bug bounty costs, as seen in zkSync Era's and Starknet's extensive security reviews.
Economic attacks target the forced inclusion mechanism. A malicious actor can spam the L1 contract with withdrawal requests to force the sequencer into unprofitable L1 transactions, creating a denial-of-service vector. This mirrors the 'gas griefing' attacks possible on Optimism's dispute game.
Evidence: The cost of a forced inclusion transaction is the L1 gas fee, which during network congestion can exceed $100. A sustained spam attack could render the rollup's economic model unsustainable, forcing a protocol subsidy or shutdown.
risk-analysis
THE COST OF FORCED INCLUSION
Risk Analysis: The New Attack Surface
ZK-Rollups guarantee state validity, but a sequencer can still censor users. Forced inclusion is the escape hatch, but its economic and technical costs create a new attack vector.
01
The Censorship Doomsday Clock
A malicious sequencer can freeze user funds by refusing to include transactions. Forced inclusion is the only recourse, but its high latency (potentially 7 days on Ethereum) and cost make it a last resort. This creates a credible threat model where censorship is economically viable for the sequencer.
- Attack Vector: Sequencer extracts value via MEV or ransom.
- User Cost: Exit delays and high L1 gas fees.
~7 days
Exit Delay
$100+
Exit Cost
02
The L1 Gas Auction Problem
Forced inclusion transactions compete in the public L1 mempool, triggering gas auctions. A malicious actor can front-run or spam the queue, dramatically inflating the exit cost for legitimate users. This turns a security mechanism into a financial weapon.
- Economic Attack: Spam drives L1 gas costs to unsustainable levels.
- Weaponized Delay: Extends the effective censorship period.
10-100x
Cost Spike
Unbounded
Attack Scale
03
zkSync's Priority Queue & StarkNet's SHARP
Protocols are engineering solutions to mitigate the cost. zkSync's Priority Queue allows users to pay the sequencer a premium for guaranteed inclusion, internalizing the auction. StarkNet's SHARP batches proofs for many transactions, amortizing the high fixed cost of L1 verification across users.
- Trade-off: Premiums vs. public goods.
- Scalability: Batching reduces per-user L1 footprint.
~90%
Cost Reduction
Minutes
Exit Time
04
The Validator Dilemma: Decentralization vs. Liveness
True censorship resistance requires decentralized sequencer sets with slashing, like Ethereum's consensus. However, this introduces latency and complexity for ZK-proof generation and aggregation, threatening the rollup's performance edge. The core trade-off is decentralized liveness versus optimized throughput.
- Performance Hit: Proposer-Builder Separation for ZKPs is unsolved.
- Security Model: Moves from 1-of-N honest to N-of-N liveness assumption.
1-2s
Latency Add
N-of-N
Liveness Assumption
counter-argument
THE EXIT GUARANTEE
Counter-Argument: Isn't This Better Than Optimistic Rollup Challenges?
Forced inclusion is a superior liveness guarantee than the challenge period in optimistic rollups.
Forced inclusion is deterministic. The ZK-Rollup exit mechanism is a cryptographic guarantee, not a social one. A user's transaction is provably valid or invalid on-chain, eliminating the need for a 7-day fraud proof window like in Arbitrum or Optimism.
Optimistic rollups fail silently. If a sequencer censors you, you must wait the full challenge period to exit. In a ZK-Rollup, you submit a direct L1 transaction with a validity proof, forcing the sequencer to process it or prove it invalid. This is a hard liveness guarantee.
The cost is a feature. The high gas cost of forced inclusion on Ethereum is the price of this absolute guarantee. Protocols like zkSync Era and StarkNet implement this, making user exit censorship-resistant. Optimistic rollups trade this certainty for lower operational cost, creating a different risk profile.
Evidence: A forced inclusion transaction on zkSync Era costs ~$50-200 in L1 gas during peak congestion. The alternative—a 7-day locked exit via an Optimism bridge—carries opportunity cost and price risk exceeding that gas fee for any meaningful capital.
takeaways
FORCED INCLUSION ECONOMICS
Key Takeaways for Architects and Investors
The right to exit is the ultimate security guarantee for rollup users. Here's what it costs to enforce it.
01
The Problem: Sequencer Censorship is a Solvable Attack
A malicious or malfunctioning sequencer can block user transactions, freezing assets. Forced inclusion is the canonical defense, but its cost is often abstracted away.
- Attack Vector: Sequencer refuses to include a user's withdrawal.
- User Recourse: Submit a transaction directly to L1, bypassing the sequencer.
- Hidden Cost: This L1 transaction is ~100-1000x more expensive than a normal L2 tx.
100-1000x
Cost Spike
7 Days
Max Delay
02
The Solution: Prepay the Exit with Fee Markets & MEV
Architects must design economic systems that pre-fund forced inclusion, making it a predictable operational cost, not a user burden.
- Fee Pooling: Protocols like Arbitrum's surplus fees or Optimism's fee vaults can subsidize exit gas.
- MEV Redirection: Redirect a portion of sequencer MEV (e.g., from Uniswap, Curve arbitrage) to a dedicated security fund.
- Insurance Derivatives: Create a market for forced inclusion risk, allowing protocols to hedge exit costs.
$1M+
Typical Fund Size
Bonded
Sequencer Stake
03
The Architecture: Minimize Cost with ZK & State Proofs
The technical implementation of forced inclusion dictates its final cost. ZK-Rollups have a structural advantage.
- ZK Validity Proofs: A single, succinct proof on L1 can force inclusion of batches of transactions, amortizing cost.
- Optimistic Challenge Periods: In contrast, optimistic rollups like Arbitrum require a full fraud proof dispute, which is slower and more complex.
- Stateless Clients: Future designs using Verkle trees or zk-SNARKed state could allow trust-minimized exits without full L1 data.
~10k Gas/Tx
Amortized Cost
Minutes
Exit Time (ZK)
04
The Investment Lens: Security as a Sunk Cost
For investors, a rollup's forced inclusion mechanism is a critical due diligence item. It's non-negotiable infrastructure.
- Red Flag: No clear economic model for funding L1 exit transactions.
- Green Flag: Transparent treasury or fee model (e.g., Starknet's STRK staking for prover costs) dedicated to security.
- Valuation Impact: Protocols with cheaper, faster forced exit will attract higher-value TVL and institutional capital.
>10%
Fee Allocation
Non-negotiable
For Security
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