Overview

Flashblocks-specific RPC methods, WebSocket subscriptions, and the infrastructure stream schema for Base pre-confirmations.

Flashblocks endpoints expose 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 work identically on Flashblocks endpoints. Use the "pending" block tag to resolve against pre-confirmed state instead of the transaction pool. See the Overview for endpoint URLs and a full explanation of the two tiers.

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.

Azul simplifies the raw Flashblocks WebSocket payload. Account balances and receipts are not populated in the raw stream after Azul. Applications that need preconfirmed transaction or receipt data should use Flashblocks RPC behavior described in the RPC Overview.

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.

Flashblock fields

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 can exceed 10 during sequencer timing drift. Do not hardcode a final index; use payload_id changes or full block confirmation via standard RPC as the signal that the block has advanced.

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).

Base fields

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.

Diff Object

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

Diff fields

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, Flashblocks RPC methods, or finalized block data via standard RPC instead.

After Azul, the raw infrastructure stream does not populate account balances or transaction receipts in metadata. Use Flashblocks RPC by-hash lookups, pendingLogs, or newFlashblockTransactions when you need preconfirmed transaction, receipt, or log data.

Metadata fields

block_number · number

Block number as a decimal integer.

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"
  },
  "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
  }
}