newFlashblockTransactions

Subscribe via eth_subscribe to receive each transaction as it is pre-confirmed into a Flashblock. Pass true as the second parameter to receive full transaction and log data.

This subscription needs a WebSocket-enabled endpoint. The public Base endpoints (mainnet.base.org, sepolia.base.org) are HTTP only, so connect through a node provider such as Alchemy or QuickNode that offers WebSocket support for Base.

Requires base/base minimum client version v0.8.0. When full is true, each notification carries the transaction object with its receipt fields (including logs) embedded directly rather than in a nested receipt.

Each subscription emits one item per WebSocket message, with events arriving roughly every 200ms. If your handler does heavy work per event, throttle or debounce it to avoid blocking.

Parameters

Name	Type	Required	Description
subscriptionType	string	Yes	Must be "newFlashblockTransactions".
full	boolean	No	If true, each notification includes the full transaction object and associated logs. Defaults to false (minimal data only).

Returns

Field	Type	Description
result	string	Hex-encoded subscription ID.

Example

Subscribe

{"jsonrpc": "2.0", "id": 1, "method": "eth_subscribe", "params": ["newFlashblockTransactions"]}

Subscribe (full data)

{"jsonrpc": "2.0", "id": 1, "method": "eth_subscribe", "params": ["newFlashblockTransactions", true]}

Subscription ID response

{"jsonrpc": "2.0", "id": 1, "result": "0x1887ec8b9589ccad00000000000532da"}

Notifications

Every update arrives as a standard eth_subscription message. What lands in params.result depends on the full flag.

With full: false, result is just the transaction hash:

{
  "jsonrpc": "2.0",
  "method": "eth_subscription",
  "params": {
    "subscription": "0x1887ec8b9589ccad00000000000532da",
    "result": "0xe26de91f9037e903eefe70b28f613019253da603e67e0dbfe2f656dce5444311"
  }
}

With full: true, result is a transaction object that folds its receipt fields in directly (no nested receipt object). The schema below was verified live against base/v0.9.0:

Field	Type	Description
type	string	Transaction type: 0x0 Legacy, 0x2 EIP-1559, 0x7e Deposit.
chainId	string	Chain ID (hex).
nonce	string	Sender nonce (hex).
gas	string	Gas limit (hex).
maxFeePerGas	string	EIP-1559 max fee per gas (hex).
maxPriorityFeePerGas	string	EIP-1559 max priority fee per gas (hex).
to	string	Recipient address.
value	string	ETH value transferred (hex).
accessList	array	EIP-2930 access list.
input	string	Transaction input data (hex).
r	string	Signature r component.
s	string	Signature s component.
yParity	string	Signature parity (hex).
v	string	Signature v value (hex).
hash	string	Transaction hash.
blockHash	null	Always null — the transaction is pre-confirmed, not yet in a finalized block.
blockNumber	string	Block number (hex) of the in-progress Flashblock.
transactionIndex	string	Index within the block (hex).
from	string	Sender address.
gasPrice	string	Effective gas price (hex).
gasUsed	string	Gas used by this transaction (hex). Changed from an integer to a hex string in v0.8.0.
status	string	0x1 for success, 0x0 for failure.
cumulativeGasUsed	string	Total gas used in the block up to and including this transaction (hex).
contractAddress	string | null	Address of the created contract, or null.
logsBloom	string	Bloom filter of logs (hex).
logs	array	Array of log objects emitted by this transaction.

Heads up: gasUsed is delivered as a hex string (for example "0x26132"), not a number. This changed in v0.8.0, so update any parser that still expects an integer.

{
  "jsonrpc": "2.0",
  "method": "eth_subscription",
  "params": {
    "subscription": "0x1887ec8b9589ccad00000000000532da",
    "result": {
      "type": "0x2",
      "chainId": "0x2105",
      "nonce": "0x34ed",
      "gas": "0x7a1200",
      "maxFeePerGas": "0x257ab3c",
      "maxPriorityFeePerGas": "0x419c7c",
      "to": "0x6211a3742cf9d3b6677ecc7fd9dd102ab101d8e2",
      "value": "0x0",
      "accessList": [],
      "input": "0x...",
      "r": "0xa7cd30d21c30d4d60d27073c8bbc3ef5778527cf98eae0433e9d1f18c929dd5d",
      "s": "0x08c75921e6bb75e19112300f80998f88a2b0f1adc52df2c3597b171d8c8de68d",
      "yParity": "0x1",
      "v": "0x1",
      "hash": "0x6a010a5ce041ff0ee5a926db65d1ef512836cae822d5f2d58b63981bfa40aa7f",
      "blockHash": null,
      "blockNumber": "0x2c679a1",
      "transactionIndex": "0x83",
      "from": "0x2ad149d3d3099532d7c25c47cce37db6c4677b3a",
      "gasPrice": "0x8de7bc",
      "gasUsed": "0x26132",
      "status": "0x1",
      "cumulativeGasUsed": "0x16cb406",
      "contractAddress": null,
      "logsBloom": "0x00...00",
      "logs": []
    }
  }
}