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Get started with a reliable Optimism RPC endpoint to use Flashblocks. Flashblocks is a block builder sidecar for OP Stack chains. The sidecar is called Rollup-Boost, designed and built by Flashbots and co-designed with Uniswap Labs and OP Labs. As Optimism Mainnet is the canonical OP Stack chain, the Optimism team has implemented Flashblocks on Optimism. Flashblocks went live on the Optimism Mainnet sequencer in 2026. The Flashblocks implementation on Optimism is two-part:
  • Sequencer side — the Rollup-Boost sidecar and the op-rbuilder Reth-based builder run next to the OP Stack sequencer.
  • Node side — the Flashblocks module is implemented on the Reth-based execution client running on Chainstack.
This two-side architecture implements the following flow:
  • The Optimism sequencer creates regular 2-second blocks and additionally creates sub-blocks called Flashblocks every 250ms.
  • The sequencer streams these Flashblocks every 250ms over WebSocket to the Reth nodes that have the Flashblocks module enabled.
  • The sequencer-streamed Flashblocks and the preconfirmation state can then be retrieved by apps over the standard Optimism RPC endpoints and methods with the pending block tag:
  • The transaction ordering is done at the Flashblock level instead of the block level.
Each 2-second block on Optimism is formed after approximately 8 Flashblocks have been processed, each with its own transaction ordering.
Unlike Base, Optimism does not expose Flashblocks as dedicated WebSocket subscriptions like eth_subscribe newFlashblocks. The customer-facing Flashblocks interface on Optimism is the standard JSON-RPC interface with the pending block tag, as defined in the Optimism Flashblocks specification.

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Quick Flashblock vs. full block data comparison

Call eth_getBlockByNumber with the pending tag on a Flashblocks-enabled Optimism endpoint while a 2-second canonical block is forming. The response evolves every 250ms as new Flashblocks arrive — the same block number, but the transaction list grows append-only and the block hash changes with each Flashblock. A quick view of the same canonical block number sampled 500ms apart while the block was being assembled on Chainstack: 
"number": "0x90b665c",
"hash": "0x0469f8a27172dbd4...",
"transactions": [ /* 15 transactions */ ]
The block number stays the same across all three samples — only the contents grow and the preconfirmation hash updates. Once the 2-second canonical block is sealed, the final block hash is computed and the transactions are attributed to it.

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Verify Flashblocks behavior

You can verify Flashblocks behavior on your Optimism endpoint by polling eth_getBlockByNumber with the pending tag and watching the response evolve within a single block number. Replace YOUR_OPTIMISM_ENDPOINT with your Chainstack Optimism node HTTPS endpoint: 
for i in 1 2 3 4 5 6 7 8 9 10; do
  curl -s "YOUR_OPTIMISM_ENDPOINT" \
    -H "Content-Type: application/json" \
    -d '{"jsonrpc":"2.0","id":1,"method":"eth_getBlockByNumber","params":["pending",false]}' \
    | python3 -c "import sys,json; b=json.load(sys.stdin)['result']; print(int(b['number'],16), b['hash'][:18], len(b['transactions']))"
  sleep 0.25
done
On a Flashblocks-enabled endpoint, you will see multiple distinct hashes and growing transaction counts for the same block number within a 2-second window. On a non-Flashblocks endpoint, you will see at most one or two distinct pending blocks per canonical block.

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Performance expectations

Flashblocks on Optimism are produced at 250ms intervals, but actual end-to-end latency will be higher due to network travel time:
  • Standard Optimism endpoint — ~2000ms (2-second block time).
  • Flashblocks-enabled Optimism endpoint — ~350-550ms (250ms Flashblock interval + network travel time).
The actual confirmation time depends on:
  • Network latency to and from the Optimism node.
  • Transaction processing time at the sequencer.
  • Time until the next Flashblock (up to 250ms).
  • Network travel time for the confirmation response.
Flashblock preconfirmations are not final. A preconfirmed receipt from eth_getTransactionReceipt, eth_sendRawTransactionSync, or a pending-tag call can be revoked if the Optimism sequencer drops or reorders the transaction before the canonical 2-second block is sealed. Do not treat preconfirmed receipts as settlement guarantees. Wait for the transaction to be included in a finalized canonical block before releasing funds or considering a payment settled.

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Optimism vs. Base Flashblocks at a glance

AspectOptimismBase
Flashblock interval250ms200ms
Canonical block time2s2s
Flashblocks per block~8~10
Customer-facing interfaceJSON-RPC with pending tagJSON-RPC with pending tag and dedicated WebSocket subscriptions
eth_subscribe newFlashblocks etc.Not exposedExposed

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Additional resources

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About the author

Ake

Ake Director of Developer Experience @ Chainstack
Talk to me all things Web3
20 years in technology | 8+ years in Web3 full time years experience
Last modified on May 18, 2026