Polygon PoS is not an L2. It is an Ethereum sidechain

/u/aminokCryptocurrency News & Discussion false

TL;DR: "Layer 2" is often used as a catch-all for "cheap and fast" chains connected to Ethereum, but this blurs definitions. A true L2 (rollup) blockchain allows Ethereum to reject invalid states. A sidechain (like Polygon PoS) only asks Ethereum to record them. This becomes all the more relevant with Polygon's recent pivot toward the "Gigagas" roadmap over the previously planned Validium transition.

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The Core Distinction: Judge vs. Notary

When people say "L2", they usually mean any blockchain that bridges to Ethereum and lowers fees. But strictly speaking, "L2" describes a specific security relationship: who has the final say if the chain lies?

• A Rollup (L2) treats Ethereum as a Judge. If the L2 sequencer posts an invalid state root, Ethereum's protocol logic can reject it (via fraud proofs or validity proofs).
• A Sidechain (Polygon PoS) treats Ethereum as a Notary. It uses Ethereum to timestamp and log events, but Ethereum does not have the power to overturn the sidechain's consensus.

So this isn't just semantics; it determines whether your funds are safe if the other chain's validators collude.

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How Polygon Checkpoints Actually Work (The "Notary" Model)

To understand why Polygon PoS is a sidechain, you have to look at its architecture, specifically the Heimdall and Bor layers.

• Bor Layer: This is where block production happens. Polygon validators execute transactions and build blocks.
• Heimdall Layer: This is the verification layer. Heimdall nodes validate the Bor blocks and aggregate signatures.
• The Checkpoint: Periodically, Heimdall publishes a "checkpoint" to the RootChain contract on Ethereum.

The critical security property is that when the RootChain contract on Ethereum receives this checkpoint, it checks signatures, not validity.

It asks: "Did 2/3rds of Polygon validators sign this?"
It does not ask: "Are the transactions inside this checkpoint valid?"

If 2/3rds of Polygon validators decided to seize all user funds and sign a checkpoint reflecting that, the Ethereum smart contract would accept it as canonical. Ethereum is acting as a notary (verifying the signature), not a judge (verifying the law).

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The "Escape Hatch" Test (The "Judge" Model)

The strongest evidence of the difference lies in censorship resistance.

On a true L2 (Rollup) blockchain (like Arbitrum or Optimism), there is a mechanism often called an "escape hatch" or "forced inclusion."

• Arbitrum: You can send a transaction to the DelayedInbox contract on Ethereum L1. If the L2 sequencer ignores it for ~24 hours, you can call forceInclusion, and the protocol forces the L2 blockchain to process it.
• Optimism: You can call depositTransaction on the OptimismPortal. The derivation rules dictate that if the sequencer doesn't include it, the canonical chain eventually halts or reorganizes to include it.

Polygon PoS does not have this. If Polygon validators decide to censor your transaction, you cannot bypass them by talking to Ethereum. You are beholden to the Polygon validator set, not Ethereum's permissionless security.

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The Roadmap Reality: Doubling Down on the Sidechain Architecture

For a while, the counter-argument was "Polygon 2.0". In 2023, the plan was to upgrade Polygon PoS into a zkEVM Validium, which would have given it Ethereum-enforced validity proofs.

However, as of late 2025 and early 2026, while numerous independent L1 blockchains announced they would transition to L2 blockchains to gain Ethereum-enforced validity, Polygon leadership communication regarding the Validium upgrade has gone quiet. Instead, the focus has pivoted to the "Gigagas" roadmap and the AggLayer.

The new goal is:

• Priotizing immediate scalability upgrades
• Optimizing for stablecoin payments and Real World Assets (RWAs).
• Using the AggLayer for interoperability rather than shared security.

By de-prioritizing the transition to a Validium (which requires generating zk-proofs for every state transition), Polygon PoS is choosing strategic flexibility over standardization and full inheritance of Ethereum security.

So in practice this means it will continue to anchor to Ethereum for coordination, while otherwise relying on its own consensus to handle the transactions of its global payment system.

Whether this trade-off pays off remains to be seen. Running a secure sidechain requires maintaining a massive, independent validator set — a heavy cost that L2 blockchains avoid by outsourcing security to Ethereum. However, Polygon PoS generates enough revenue to absorb this overhead, making the sidechain model viable for them where it might bankrupt a smaller chain.

In any case, the distinction between the security model of Polygon PoS and that of L2 blockchains validates the classification: Polygon PoS is not an L2 blockchain relying on Ethereum for validity. It is a sidechain relying on Ethereum only for finality.

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