Skip to main content
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

MethodDescription
eth_simulateV1Simulate transaction bundles against pre-confirmed state
base_transactionStatusCheck 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:
SubscriptionDescription
newFlashblockTransactionsStream individual transactions as they are pre-confirmed (~200ms each)
pendingLogsStream filtered event logs from pre-confirmed transactions
newFlashblocksStream 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.
NetworkRaw stream URL
Mainnetwss://mainnet.flashblocks.base.org/ws
Sepoliawss://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.

​
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.

​
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 transaction type.

​
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.

​
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"
      }
    }
  }
}