newFlashblocks

Subscribe to receive full Flashblock payload stream as each pre-confirmed block is built. Only available on Flashblocks WebSocket endpoints.

Subscribe via eth_subscribe to receive the current pending block state as each Flashblock is built. Each message is the complete accumulated block — a standard Ethereum block object containing all transactions pre-confirmed so far.

Only available on Flashblocks WebSocket endpoints: wss://mainnet-preconf.base.org and wss://sepolia-preconf.base.org.

Requires base/base minimum client version v0.3.1.

Multiple notifications are emitted per block height. Each Flashblock (~200ms) triggers a new notification for the same block with more transactions added. This is accumulating state, not a diff — every message replaces the previous one. Your handler should treat each notification as the latest known state of the in-progress block.

Parameters

subscriptionType · string · required

Must be "newFlashblocks".

Returns

The initial eth_subscribe response returns a hex-encoded subscription ID. After that, the node pushes one notification per Flashblock (~200ms). Each notification’s result is a standard Ethereum block object identical in shape to eth_getBlockByNumber with hydrated=true, but representing the pending (not yet sealed) block.

Key differences from a sealed block:

• transactions is a full array of transaction objects (always hydrated — hashes are never returned)
• hash, stateRoot, receiptsRoot, transactionsRoot reflect the partial block state at time of emission and will differ from the final sealed block values
• The same block number will appear in multiple notifications as new Flashblocks accumulate

result · object

Standard Ethereum block object. Fields match eth_getBlockByNumber with hydrated=true.

Show Block fields

number · string

Block number (hex).

hash · string

Partial block hash at the time of this Flashblock. Changes with each emission until the block is sealed.

parentHash · string

Hash of the parent block.

timestamp · string

Unix block timestamp (hex).

gasLimit · string

Maximum gas allowed (hex).

gasUsed · string

Cumulative gas used so far (hex). Increases with each Flashblock.

baseFeePerGas · string

EIP-1559 base fee per gas (hex).

miner · string

Fee recipient address (coinbase).

stateRoot · string

State root at the time of this Flashblock (partial).

receiptsRoot · string

Receipts root (partial).

transactionsRoot · string

Transactions root (partial).

logsBloom · string

Bloom filter for logs in the block so far.

extraData · string

Sequencer extra data field.

transactions · array

Full transaction objects for all transactions pre-confirmed so far. Same shape as transactions returned by eth_getBlockByNumber with hydrated=true.

withdrawals · array

Validator withdrawals (always empty on Base L2).

uncles · array

Always empty on Base L2.

Example

Subscribe

{"jsonrpc": "2.0", "id": 1, "method": "eth_subscribe", "params": ["newFlashblocks"]}