Overview - Base Documentation

All Base public endpoints (mainnet.base.org / sepolia.base.org) are Flashblocks-enabled, exposing all standard Ethereum JSON-RPC methods plus a set of pre-confirmation-specific additions. These let you read state, simulate transactions, and stream events against sequencer-ordered data up to ~1.8 seconds before a block seals.

All standard Ethereum JSON-RPC methods support 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 if a transaction has been received by the node mempool

WebSocket Subscriptions

On a Flashblocks WSS endpoint, eth_subscribe with newHeads emits a new event approximately every 200ms per Flashblock instead of every 2 seconds. Three additional subscription types are also available that are exclusive to Flashblocks endpoints:

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 for node operators only. App developers should use the WebSocket subscription methods above via 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.

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).

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.

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 v0.8.0, new_account_balances and receipts are no longer present in the metadata object. Only block_number remains.

block_number

number

Block number as a decimal integer.

Receipt Object

metadata.receipts was removed in v0.8.0. This schema is preserved for reference for older node versions. On v0.8.0+, use eth_getTransactionReceipt for polling-based receipt data, or subscribe to newFlashblockTransactions with full: true for a real-time stream of pre-confirmed transaction data including logs.

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

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
  }
}

Documentation Index

​HTTP Methods

​WebSocket Subscriptions

​Infrastructure Stream

​Flashblock Object

​Base Object

​Diff Object

​Metadata Object

​Receipt Object

​Log Object

​Complete Examples

HTTP Methods

WebSocket Subscriptions

Infrastructure Stream

Flashblock Object

Base Object

Diff Object

Metadata Object

Receipt Object

Log Object

Complete Examples