Flashblocks is a block builder sidecar for Optimism Stack chains. This sidecar is called Rollup-Boost. It’s designed and built by Flashbots—the same team that originally productized MEV on Ethereum. Co-designed with Uniswap Labs and OP Labs. As Base is an L2 running on the OP stack, the Base team has implemented Flashblocks on the Base chain. Flashblocks went live on the Base Mainnet in July 2025. The Flashblocks implementation on the Base chain is two-part:
  • Sequencer side — The Rollup-Boost sidecar is installed next to the OP Stack sequencer by the Base team.
  • Node side — The Flashblocks module is implemented on the Base fork of the Reth node.
This two-side architecture implements the following Flashblocks flow:
  • Base sequencer creates regular 2-second blocks and additionally creates sub-blocks called Flashblocks every 200ms.
  • Base sequencer streams these Flashblocks every 200ms over WebSocket to the Base Reth nodes that have the Flashblocks module enabled.
  • The sequencer-streamed Flashblocks & Flashblock states can then be retrieved by apps over the standard Base RPC endpoints & methods:
    • eth_getBlockByNumber with pending tag
    • eth_getTransactionReceipt
    • eth_getBalance with pending tag
    • eth_getTransactionCount with pending tag
    • eth_getTransactionByHash with pending tag
    • eth_sendRawTransactionSync
  • The transaction ordering is done at the Flashblocks level instead of the block level.
Each 2-second block on the Base chain is formed after 10 different Flashblocks have been processed—each Flashblock with its own transaction ordering.

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Quick Flashblock vs. full block data comparison

Let’s call an eth_getBlockByNumber with the pending tag to get the latest block data on a Flashblocks-enabled and non-Flashblocks-enabled node at the same time as block 33228756 was being formed on the Base mainnet. See the results in the Chainstack Labs base-flashblocks-transaction-latency-test repository:
  • eth-get-block-by-number-pending-examples/preconfirmed-flashblock.log — the result of running "method":"eth_getBlockByNumber","params":["pending",true] on a Flashblocks-enabled node as block 33228756 was forming.
  • eth-get-block-by-number-pending-examples/confirmed-block.log — the result of running "method":"eth_getBlockByNumber","params":["pending",true] on a non-Flashblocks-enabled node as block 33228756 was formed.
You will see the key differences in the results:
  • Transaction count — 52 in the Flashblock vs. 167 in the fully formed block.
  • stateRoot — empty in the Flashblock vs. computed in the fully formed block. An empty stateRoot at this point means there’s no final chain state for the block, hence “preconfirmation” in the name.
  • blockHash — different in the Flashblock and the fully formed block. The finalized transactions will have the hash of the fully formed block attributed to them.
  • receiptsRoot — different in the Flashblock and the fully formed block.
A quick view: 
"stateRoot": "0x0000000000000000000000000000000000000000000000000000000000000000",
"receiptsRoot": "0x5f2dd9fbe32e2c22e47d84edbe6a300e75a004495191ff9f6228ce5de8f7c6f8",
"blockHash": "0x61792b40d824cb6e99ad07de112c823e0f86995063c00486e8dd3ef9667b40fc",

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Transaction latency test tool

See the Chainstack Labs base-flashblocks-transaction-latency-test repository for the latency test tool and the comprehensive README file.

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Performance expectations

Flashblocks are produced at 200ms intervals, but actual confirmation times will be higher due to network latency:
  • Standard Base endpoint — ~2000ms (2-second block time)
  • Flashblocks-enabled endpoint — ~300-500ms (200ms Flashblock interval + network travel time)
The actual confirmation time depends on:
  • Network latency to/from the Base node
  • Transaction processing time
  • Time until next Flashblock (up to 200ms)
  • Network travel time for confirmation response

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Additional resources

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About the author

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Ake

Director of Developer Experience @ Chainstack Talk to me all things Web320 years in technology | 8+ years in Web3 full time years experienceTrusted advisor helping developers navigate the complexities of blockchain infrastructure