Flashblocks API overview

Flashblocks endpoints expose every standard Ethereum JSON-RPC method along with 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.

All standard Ethereum JSON-RPC methods work identically on Flashblocks endpoints. Pass the "pending" block tag to resolve against pre-confirmed state instead of the transaction pool. See the Overview for endpoint URLs and a full description of the two tiers.

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

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 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 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.

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.

Field	Type	Description
block_number	number	Block number as a decimal integer.
new_account_balances	object	Map of addresses to their updated ETH balances (hex). Only includes accounts whose balances changed in this Flashblock.
receipts	object	Map of transaction hashes to their Receipt objects.

Receipt object

The abbreviated transaction receipt included in metadata.receipts. 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.

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,
    "new_account_balances": {
      "0x000f3df6d732807ef1319fb7b8bb8522d0beac02": "0x0"
    },
    "receipts": {
      "0x07d7f06b06fea714c1d1d446efa2790c6970aa74ee006186a32b5b7dd8ca2d82": {
        "type": "0x7e",
        "status": "0x1",
        "cumulativeGasUsed": "0xab3f",
        "logs": [],
        "logsBloom": "0x00000000...",
        "transactionIndex": "0x0"
      }
    }
  }
}

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,
    "new_account_balances": {
      "0x4200000000000000000000000000000000000015": "0x1234"
    },
    "receipts": {
      "0x7c69632dc315f13a...": {
        "type": "0x2",
        "status": "0x1",
        "cumulativeGasUsed": "0x1234f",
        "logs": [],
        "logsBloom": "0x00000000...",
        "transactionIndex": "0x1"
      }
    }
  }
}