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Base API method that streams the accumulated pre-confirmed block state approximately every 200ms. Each notification delivers a standard Ethereum block object representing the current Flashblock snapshot—including all transactions pre-confirmed so far. This subscription type is exclusive to Flashblocks-enabled endpoints. See Flashblocks on Base for background on how Flashblocks work.

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Parameters

  • string — the subscription type, newFlashblocks in this case.

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Response

  • subscription — the subscription ID.
Each notification delivers a block object with the following fields:
  • hash — the block hash. Set to 0x0000...0000 because the block is still being formed (pre-confirmed).
  • parentHash — hash of the parent block.
  • sha3Uncles — hash of the list of uncles included in the block.
  • miner — the address of the fee recipient (coinbase), typically 0x4200000000000000000000000000000000000011.
  • stateRoot — root of the state trie. Set to 0x0000...0000 because the block is not yet finalized.
  • transactionsRoot — root of the transaction trie of the block.
  • receiptsRoot — root of the receipts trie of the block.
  • logsBloom — bloom filter for the logs in this Flashblock.
  • difficulty — always 0x0 on Base.
  • number — the block number, encoded as hexadecimal.
  • gasLimit — maximum gas allowed in this block (hex).
  • gasUsed — cumulative gas used up to and including this Flashblock (hex).
  • timestamp — Unix timestamp of block creation (hex).
  • extraData — arbitrary data field set by the sequencer.
  • mixHash — the previous RANDAO value.
  • nonce — always 0x0000000000000000 on Base.
  • baseFeePerGas — EIP-1559 base fee per gas (hex).
  • withdrawalsRoot — Merkle root of withdrawals.
  • blobGasUsed — cumulative blob gas used, EIP-4844 (hex).
  • excessBlobGas — excess blob gas (hex).
  • parentBeaconBlockRoot — parent beacon block root.
  • uncles — always empty on Base.
  • transactions — array of full transaction objects included in this Flashblock so far. Each object includes standard transaction fields (type, from, to, value, input, hash, etc.).
  • withdrawals — always empty on Base L2.
The hash and stateRoot fields are zeroed because the block is still being assembled. Once the full 2-second block seals, these values are computed and finalized. See Flashblocks on Base for a comparison.

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eth_subscribe("newFlashblocks") code examples

Note that subscriptions require a WebSocket connection and WebSocket cat for you to use this method in the console.Install WebSocket cat with:npm install -g wscat
$ wscat -c YOUR_CHAINSTACK_WEBSOCKET_ENDPOINT
# Wait for the connection to be established

Connected (press CTRL+C to quit)

> {"jsonrpc":"2.0","id":1,"method":"eth_subscribe","params":["newFlashblocks"]}
This will generate a continuous stream of block objects as new Flashblocks are produced by the sequencer (~every 200ms).
Each notification can be large (hundreds of KB) because it includes the full transactions array. If your handler performs heavy processing per event, throttle or debounce it to avoid blocking the WebSocket connection.

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Example notification

{
  "hash": "0x0000000000000000000000000000000000000000000000000000000000000000",
  "parentHash": "0x6d98f3e9368562605c063467030f8c6ab47ff606da890b879316a14f9841b3a7",
  "sha3Uncles": "0x1dcc4de8dec75d7aab85b567b6ccd41ad312451b948a7413f0a142fd40d49347",
  "miner": "0x4200000000000000000000000000000000000011",
  "stateRoot": "0x0000000000000000000000000000000000000000000000000000000000000000",
  "transactionsRoot": "0xf671d9e31238c090e32742144d05b5230b06b80af0a8611cbdac89cb79eae5b6",
  "receiptsRoot": "0x739875b7b5439ff233106142f0a74ca134ec51e4d4e61bed0884cbd60c83c7ac",
  "logsBloom": "0x00000000...",
  "difficulty": "0x0",
  "number": "0x2a13e81",
  "gasLimit": "0x17d78400",
  "gasUsed": "0x3f923ca",
  "timestamp": "0x69ccd9e5",
  "extraData": "0x01000000640000000500000000004c4b40",
  "mixHash": "0x54389ec106a631ccd44c4b3b59a5b01e1647d039ae6cf7893fb8286d451fdceb",
  "nonce": "0x0000000000000000",
  "baseFeePerGas": "0x4c4b40",
  "withdrawalsRoot": "0x7d152b4dc9c65e113c7498ce4c255d9d6740185dbd81dbcb6d7b8a4ac72ba13a",
  "blobGasUsed": "0xc82ab0",
  "excessBlobGas": "0x0",
  "parentBeaconBlockRoot": "0x77c576c8b150f4cc43c9df655988983a309b63b0339ca27a752f2811ad5cae53",
  "uncles": [],
  "transactions": ["...full transaction objects..."],
  "withdrawals": []
}
Use eth_unsubscribe | Base to remove the subscription.

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Use case

The eth_subscribe("newFlashblocks") method is useful when you need the complete pre-confirmed block state rather than individual transactions or logs:
  • Block building and state reconstruction. Applications that need to reconstruct the full pending block state—such as custom block explorers or analytics platforms—can consume the Flashblock stream directly instead of polling eth_getBlockByNumber with pending.
  • MEV strategy development. Searchers can observe the sequencer’s transaction ordering in real time across each ~200ms Flashblock, gaining insight into how the pending block is being constructed.
  • Infrastructure monitoring. Node operators and RPC providers can use the Flashblock stream to monitor sequencer health, block production cadence, and gas usage patterns with sub-second granularity.
Last modified on April 1, 2026