> ## Documentation Index > Fetch the complete documentation index at: https://docs.immutable.com/llms.txt > Use this file to discover all available pages before exploring further. # OP Stack Migration > What changes when Immutable Chain migrates to the OP Stack, and what to do about it On this page, you'll learn what changes when [Immutable Chain](https://docs.immutable.com/docs/products/immutable-chain/overview.md) migrates to the OP Stack, who is affected, and what to do. Accounts, balances, contracts, approvals, the chain ID, and IMX as the gas token all carry over. In almost all cases, no user or developer action is required; see [Who is affected](#who-is-affected) for exceptions. > [!WARNING] > > The current Immutable testnet is being deprecated for game usage by the end of 2026. Move any game testing to a local chain or mainnet. Infrastructure and integration partners keep access through the migration and trial period. See [Testnet deprecation](#testnet-deprecation). > [!NOTE] > > Timing is indicative for now. Mainnet migration is targeted for mid-2027, after a public testnet migration in Q1 2027. Immutable will give substantial advance notice once dates are confirmed. ## Why Immutable Chain is migrating Immutable Chain currently runs as a [custom fork of Geth](https://github.com/immutable/immutable-geth) with a single validator producing a block every 2 seconds. Its [native bridge](https://github.com/immutable/zkevm-bridge-contracts) relies on an external cross-chain messaging network (Axelar). Migrating to the OP Stack makes Immutable Chain an Ethereum rollup, which: * Replaces the existing bridge with withdrawals verified directly against state commitments posted to Ethereum * Starts with permissioned fault proofs, with zk-validity proofs to follow * Lets Immutable adopt protocol upgrades, EVM upgrades, and security patches more quickly ## What changes | Area | Today | After migration | | - | - | - | | **Architecture** | Sidechain running a Geth fork | OP Stack rollup settling on Ethereum | | **Block time** | 2 seconds | 1 second | | **Block production** | Single validator | Single sequencer | | **Native bridge** | Axelar-based bridge; some withdrawals delayed by up to 24 hours | OP Stack bridge; withdrawals take about 7 days (start, prove, finalize) | | **Gas token** | IMX | IMX | | **Transaction fees** | Execution | Execution plus the cost of posting data to Ethereum | | **Node software** | Immutable Geth | `op-reth` and `op-node` | ### Bridging Bridged tokens keep their addresses on Immutable Chain. The assets that back them on Ethereum move from the current bridge to the OP Stack native bridge, so they stay backed. Native withdrawals will take about 7 days, across three steps: start the withdrawal on Immutable Chain, prove it on Ethereum, then finalize it once the challenge period ends. The upcoming migration guide will cover which of these steps Immutable's tools handle. Deposits arrive somewhat faster. Liquidity bridges such as Layerswap and Squid remain available for faster transfers. See [Bridging](https://docs.immutable.com/docs/products/immutable-chain/bridging.md). ### Fees Fees stay in IMX and increase slightly, because they now cover posting data to Ethereum as well as execution. ## Who is affected | Audience | Impact | | - | - | | **Players and users** | Assets and balances carry over. Native withdrawals to Ethereum take about 7 days; Layerswap and Squid remain available for faster transfers. | | **Developers and integrators** | Check anything that assumes 2-second blocks, such as timeouts or schedules measured in blocks. | | **Games testing on the current testnet** | Move testing to a local chain or mainnet before the end of 2026. | | **Apps that bridge with Immutable's SDKs or widgets** | Watch for SDK and widget releases that support the OP Stack bridge, and upgrade before the mainnet migration. | | **Exchanges and custodians that call the bridge directly** | Move to the new bridge contracts. | | **Indexers, explorers, and tools that decode full blocks or receipts** | Handle OP Stack transaction formats, including deposit transactions (type `0x7E`), as on other OP Stack chains. | | **Infrastructure providers** | Move nodes from Immutable Geth to `op-reth` and `op-node`. See [below](#for-infrastructure-providers). | ## Migration day The cutover is the switch from Immutable Geth to the OP Stack. Bridging pauses first, then block production; the length of the pause will be confirmed closer to the date. When block production resumes, the chain runs on the OP Stack. ## For infrastructure providers If you run Immutable Chain nodes directly or provide RPC services, you need to replace your Geth-based node with the OP Stack node stack. This applies to both [public and private nodes](https://docs.immutable.com/docs/products/immutable-chain/running-nodes.md). ### What to change | | Before | After | | - | - | - | | **Execution client** | Immutable Geth | `op-reth` | | **Consensus client** | None (Clique proof-of-authority) | `op-node` | | **Ethereum connection** | Not required | Ethereum execution RPC and beacon API endpoint | | **Initial sync** | Sync from genesis | Start from Immutable's snapshot or your own migrated state | `op-reth` and `op-node` start from state exported at the cutover, so they can't run before it. Start them from Immutable's snapshot, or from your own node's state using migration tooling in an upcoming, optional Immutable Geth release, then switch your RPC traffic to them. > [!WARNING] > > Immutable Geth nodes stop receiving new blocks after the cutover. Node configuration, snapshot locations, and `op-reth` and `op-node` flags will be published before the testnet migration. ### Using RPC providers If you use an [ecosystem RPC partner](https://docs.immutable.com/docs/products/immutable-chain/ecosystem-partners.md), they handle the node migration. Check with your provider that they support the OP Stack migration timeline. ## Testnet deprecation The current Immutable testnet will be deprecated for game usage by the end of 2026. Gas on Immutable Chain is cheap and the testnet has few third-party integrations, so most games test against local chains or mainnet. Infrastructure providers and integration partners, such as exchanges, indexers, and apps that bridge with Immutable's SDKs or widgets, keep access to the testnet through its migration and the trial period that follows, so nodes, SDK upgrades, and the new bridge flow can be tested before the mainnet migration. ## Timeline | Milestone | Target | | - | - | | Confirmed testnet migration date, migration guide, and updated bridging docs | At least one month before testnet migration | | Testnet migration | Q1 2027 | | Testnet trial | At least two months | | Mainnet migration | Mid-2027 | Dates depend on testing and integrity checks at each stage and may move. Contact your Immutable account manager for support during the migration. ## Next steps > [card] [Running Nodes](https://docs.immutable.com/docs/products/immutable-chain/running-nodes.md) > > Current node setup > [card] [Bridging](https://docs.immutable.com/docs/products/immutable-chain/bridging.md) > > Bridge assets to and from Immutable Chain > [card] [Ecosystem Partners](https://docs.immutable.com/docs/products/immutable-chain/ecosystem-partners.md) > > RPC provider partners > [card] [Differences from Ethereum](https://docs.immutable.com/docs/products/immutable-chain/differences-from-ethereum.md) > > Review chain differences This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.