Pessimistic Proofs vs Optimistic Fraud Proofs: Securing Crosschain Interop

Interoperability is essential for a modular, horizontally scalable Web3. But it's also where the industry is most vulnerable. 

Third-party bridges have become a primary attack vector. Billions have been lost to core design vulnerabilities. Introducing additional trust assumptions for users who just want a clean, safe experience is bad from a UX perspective, and bad from a security perspective.

The alternative is native bridging. In optimistic (fraud proof) ecosystems, native bridges have problems: cryptoeconomic incentives and withdrawal delays that force users into fee-extracing third-party bridges if you want to execute swaps in less than a week.  

The problem is that fraud proofs, the optimistic approach, require a 7 day withdrawal delay.

Why optimistic approaches fall short

Optimistic ecosystems assume transactions are valid unless proven otherwise. Challenges to fraudulent state updates occur only after the fact. Fraud proof delays rely on cryptoeconomic incentives.

In an interoperable world, this means capital inefficiencies or workarounds that are bad for UX. Multichain ecosystems that want interoperability AND optimistic architectures have two choices. They can:

1. Lock users and developers into a single stack. Something like The Law of Chains limits each chain’s sovereignty, imposes a tax, while allowing for interoperability that’s faster-than-Ethereum latency. 
2. Force users into third-party bridges and interoperability solutions. This adds additional trust assumptions for sending assets across chains of different architectures. 

Optimistic systems work on the assumption that most participants are honest. When a chain submits a claim, like a token deposit or a contract call, the system accepts it as valid unless, unless the claim is disputed.

Fraud can be caught, but only with a dispute window. 

It’s the same thing as relying on a vehicle’s airbags and seatbelt, which protect passengers only after a car crashes. The model is lightweight, but the security depends on mechanisms that only kick in after something has gone wrong.

In native bridge design, this means value can move before proof is provided. If a chain lies and no one challenges it in time, funds can be drained or double-spent.

As a passenger, you’re constantly hoping the seatbelt and airbags work.

Pessimistic proofs: a safer model for crosschain trust

We don’t have to live in a world of fraud proofs. 

A special kind of zero-knowledge (ZK) proof, the pessimistic proof, takes the opposite stance: in a multichain world, where many different chains are connected, verify everything before execution.

Pessimistic proofs assume nothing is valid unless proven cryptographically. 

Every message or deposit moving from one chain to another must come with a validity proof, typically a zero-knowledge (ZK) proof, before it’s accepted by another chain.

Nothing moves unless the math checks out. 

In contrast to the seatbelts and airbags of optimistic systems, ZK proofs function more like advanced driver-assistance systems that prevent a crash from occurring. 

In Agglayer’s pessimistic proof model, even if a chain’s prover is unsound or malicious, the chain can’t drain more funds than are escrowed. 

No single chain can rug any other in an interoperable system secured by pessimistic logic. This model removes reliance on third-party relayers, watchtowers, or challenge windows. 

A pessimistic worldview doesn’t need to catch fraud because it prevents it.

Chains are unified but also secured from contagion; failure is contained to its own deposits, creating a cryptographic firewall for modular interoperability.

Agglayer CDK OP Stack Is Live

Agglayer CDK launched a new OP Stack configuration, introducing a hybrid approach: optimistic architecture secured by zero-knowledge fraud proofs.

This is optimism, but verified.

Chains built with the CDK OP Stack configuration (powered by Geth) gain native connectivity to shared Agglayer interop, but without rent-seeking or restrictive design. And with ZK validity proofs live in Agglayer v0.3, the architecture combines the best of ZK with the widely-adopted OP Stack infra.

The multistack CDK empowers builders to choose what works best: no lock-in, no tax, maximum sovereignty.

Why a Multichain World Needs Pessimistic Proofs

Most exploits in Web3 happen at the bridge level because traditional models move too fast and trust too much. 

When value is transferred based on unverified messages, or relies on off-chain parties to detect fraud, the system is only as strong as its weakest assumption.

Pessimistic proofs eliminate the need to trust. 

Chain-to-chain interop happens after the chains submit ZK proofs to validate claims like token deposits, execution results, or state changes. If the proof checks out, the receiving chain acts. If it doesn’t, nothing happens.

This provides safety across chains, even if one chain is compromised.

Agglayer: A Pessimistic Proof System for Crosschain Coordination

Agglayer applies the logic of pessimistic proofs using Succinct’s SP1, built with Polygon Plonky3, to secure crosschain messaging. In its model:

• Each connected chain must produce a ZK proof for outbound messages.
• Malicious or compromised chains are isolated by default and cannot trigger actions or drain funds on other chains.

For developers, this means building crosschain apps without inheriting bridge risk. For users, it means confidence that their funds are never exposed to unverified claims.

Agglayer optimizes for provable truth, without tax and at speed, which is a requirement for secure, scalable interoperability.

The Tradeoff Is Worth It

Yes, pessimistic proofs require more upfront computation. Yes, integrating ZK proof systems into bridge infrastructure took more engineering work. But the tradeoff is simple:

• Optimistic models are fast until they fail.
• Pessimistic models are safe even when everything else breaks.

In a world of adversarial environments, pseudonymous actors, and billions in value, the safer assumption is the one you can prove.

Building Toward Provable Interoperability

Web3 needs verifiability at every level of the stack.

The pessimistic proof shifts interoperability from a trust game to a proof game, from assuming honesty to enforcing it cryptographically. As the ecosystem grows more modular, this model offers a path to seamless, secure composability without compromise.

Interoperability at scale will be built on proof.

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About Polygon Labs:

Polygon Labs is a Web3 software company developing Polygon Proof-of-Stake network, the premiere blockchain for payments and RWAs, and Agglayer, a unified web of chains that feels like the Internet. Polygon is known as the low-cost, high velocity network, with billions secured in stablecoins, supporting a robust payments ecosystem to help grow Agglayer use cases in an interoperable Web3. Research from Polygon Labs has contributed to the development of widely-adopted zero-knowledge technology, with successful, independent projects incubated through the Agglayer Breakout Program, such as Katana, ZisK, Miden, PrivadoID, and more

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