Flashblocks API overview

Every public Base endpoint (mainnet.base.org / sepolia.base.org) is Flashblocks-enabled, serving the full set of standard Ethereum JSON-RPC methods plus a set of pre-confirmation-specific additions. Together they let you read state, simulate transactions, and stream events against sequencer-ordered data up to ~1.8 seconds before a block seals.

Every standard Ethereum JSON-RPC method accepts the "pending" block tag to resolve against pre-confirmed state instead of the transaction pool. See the RPC overview for endpoint URLs.

HTTP methods

Method	Description
`eth_simulateV1`	Simulate transaction bundles against pre-confirmed state.
`base_transactionStatus`	Check whether a transaction has been received by the node mempool.

WebSocket subscriptions

Over a WebSocket connection, eth_subscribe with newHeads emits a new event approximately every 200ms per Flashblock instead of every 2 seconds, and three additional subscription types become available. Note that the public Base endpoints are HTTP only — WebSocket subscriptions require a WebSocket-enabled node provider such as Alchemy or QuickNode.

Subscription	Description
`newFlashblockTransactions`	Stream individual transactions as they are pre-confirmed (~200ms each).
`pendingLogs`	Stream filtered event logs from pre-confirmed transactions.
`newFlashblocks`	Stream full Flashblock payload objects from the sequencer.

Infrastructure stream

The raw Flashblocks infrastructure stream is the upstream WebSocket feed consumed by Flashblocks-aware RPC nodes. It emits a new message approximately every 200ms as the sequencer pre-confirms transactions.

Applications should not connect directly to the infrastructure stream — these endpoints are reserved for node operators. App developers should use the WebSocket subscription methods above through a Flashblocks-aware RPC provider.

Network	Raw stream URL
Mainnet	`wss://mainnet.flashblocks.base.org/ws`
Sepolia	`wss://sepolia.flashblocks.base.org/ws`

Flashblock object

The root structure of each infrastructure stream message.

Field	Type	Description
`payload_id`	string	Unique identifier for the block being built. Remains consistent across all Flashblocks within a single full block.
`index`	number	Flashblock index within the current block. Starts at 0 (system transactions only). User transactions begin at index 1. Typically reaches 9–10 per block, but may exceed 10 during sequencer timing drift.
`base`	Base Object	Block header properties. Only present when `index` is `0`. See Base object.
`diff`	Diff Object	Incremental block state changes for this Flashblock. Present in every message. See Diff object.
`metadata`	Metadata Object	Supplemental data. Unstable — fields may change without notice. See Metadata object.

Base object

Contains full block header properties. Only present in the index: 0 message (the first Flashblock of each full block).

Field	Type	Description
`parent_hash`	string	Hash of the parent block.
`fee_recipient`	string	Address receiving transaction fees (coinbase).
`block_number`	string	Block number in hex.
`gas_limit`	string	Maximum gas allowed in this block (hex).
`timestamp`	string	Unix timestamp of block creation (hex).
`base_fee_per_gas`	string	EIP-1559 base fee per gas (hex).
`prev_randao`	string	Previous RANDAO value used for on-chain randomness.
`extra_data`	string	Arbitrary data field set by the sequencer.
`parent_beacon_block_root`	string	Root of the parent beacon block (EIP-4788).

Diff object

Contains the incremental block state changes for this specific Flashblock. Present in every message.

Field	Type	Description
`state_root`	string	Merkle root of the state trie after applying this Flashblock’s transactions.
`block_hash`	string	Hash of the partial block at this Flashblock index. Changes with each Flashblock as more transactions are pre-confirmed.
`gas_used`	string	Cumulative gas used up to and including this Flashblock (hex).
`blob_gas_used`	string	Cumulative blob gas used (EIP-4844, hex).
`transactions`	string[]	Array of RLP-encoded transactions included in this Flashblock.
`withdrawals`	array	Validator withdrawals (always empty on Base L2).
`receipts_root`	string	Merkle root of transaction receipts.
`logs_bloom`	string	Bloom filter for logs in this Flashblock.
`withdrawals_root`	string	Merkle root of withdrawals.

Metadata object

The metadata object is not stable. Fields may be added, modified, or removed without prior notice. Do not build production dependencies on it — use the diff object or query finalized block data via standard RPC instead.

As of client version v0.8.0, new_account_balances and receipts are no longer carried in metadata; block_number is the only field that remains.

Field	Type	Description
`block_number`	number	Block number as a decimal integer.

Receipt object

metadata.receipts was dropped in v0.8.0; this schema is retained only as a reference for older node versions. On v0.8.0 and later, poll eth_getTransactionReceipt for receipt data, or subscribe to newFlashblockTransactions with full: true for a live stream of pre-confirmed transaction data including logs. Check the type field to determine the transaction type.

Field	Type	Description
`type`	string	Transaction type: `0x0` Legacy, `0x1` Access List, `0x2` EIP-1559, `0x7e` Deposit (L1→L2).
`status`	string	Transaction status: `0x1` for success, `0x0` for failure.
`cumulativeGasUsed`	string	Total gas used in the block up to and including this transaction (hex).
`logs`	Log[]	Array of event logs emitted by the transaction. See Log object.
`logsBloom`	string	Bloom filter for the logs in this receipt.
`transactionIndex`	string	Index of the transaction within the block (hex).

Log object

Field	Type	Description
`address`	string	Contract address that emitted the event.
`topics`	string[]	Array of indexed event parameters. Topic 0 is typically the event signature hash.
`data`	string	ABI-encoded non-indexed event parameters.
`blockHash`	string	Hash of the block containing this log.
`blockNumber`	string	Block number in hex.
`blockTimestamp`	string	Unix timestamp of the block as a hex string. OP Stack extension to the standard Ethereum log schema.
`transactionHash`	string	Hash of the transaction that emitted this log.
`transactionIndex`	string	Index of the transaction in the block (hex).
`logIndex`	string	Log’s index position within the block (hex).
`removed`	boolean	`true` if the log was removed due to a chain reorg.

Complete examples

Index 0 — includes the base object (block header):

{
  "payload_id": "0x03997352d799c31a",
  "index": 0,
  "base": {
    "parent_hash": "0x9edc29b8b0a1e31d28616e40c16132ad0d58faa8bb952595b557526bdb9a960a",
    "fee_recipient": "0x4200000000000000000000000000000000000011",
    "block_number": "0x158a0e9",
    "gas_limit": "0x3938700",
    "timestamp": "0x67bf8332",
    "base_fee_per_gas": "0xfa",
    "parent_beacon_block_root": "0x15b9e7c8ac4cbe92dafc849ed30a23e91624bbe5cbe199c0ccea3f7de7fc6d49"
  },
  "diff": {
    "state_root": "0x208fd63edc0681161105f27d03daf9f8c726d8c94e584a3c0696c98291c24333",
    "block_hash": "0x5c330e55a190f82ea486b61e5b12e27dfb4fb3cecfc5746886ef38ca1281bce8",
    "gas_used": "0xab3f",
    "transactions": ["0x7ef8f8a0b4afc0b7ce10e150801bbaf08ac33fecb0f38311793abccb022120d321c6d276..."],
    "withdrawals": []
  },
  "metadata": {
    "block_number": 22585577
  }
}

Index 1–N (diff only) — no base object:

{
  "payload_id": "0x03997352d799c31a",
  "index": 4,
  "diff": {
    "state_root": "0x7a8f45038665072f382730e689f4a1561835c9987fca8942fa95872fb9367eaa",
    "block_hash": "0x9b32f7a14cbd1efc8c2c5cad5eb718ec9e0c5da92c3ba7080f8d4c49d660c332",
    "gas_used": "0x1234f",
    "transactions": ["0x02f90133...", "0x02f90196..."],
    "withdrawals": []
  },
  "metadata": {
    "block_number": 22585577
  }
}