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Base API method that streams individual transactions as they are pre-confirmed into a Flashblock. Events arrive approximately every 200ms—one item per WebSocket message. This subscription type is exclusive to Flashblocks-enabled endpoints. See Flashblocks on Base for background on how Flashblocks work.

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Parameters

  • string — the subscription type, newFlashblockTransactions in this case.
  • boolean — (optional) when true, includes complete transaction objects and associated logs in each notification. Defaults to false (minimal data only).

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Response

  • subscription — the subscription ID.
Each notification delivers pre-confirmed transaction data. When full is false (default), the result is the transaction hash as a hex string. When full is true, the result is a complete transaction object with the following fields:
  • type — the transaction type (0x0 Legacy, 0x1 Access List, 0x2 EIP-1559, 0x7e Deposit).
  • chainId — the chain ID (hex).
  • nonce — the sender’s nonce (hex).
  • from — the sender address.
  • to — the recipient address.
  • value — the ether value transferred (hex).
  • input — the call data.
  • gas — the gas limit (hex).
  • maxFeePerGas — the maximum fee per gas, EIP-1559 (hex).
  • maxPriorityFeePerGas — the maximum priority fee per gas, EIP-1559 (hex).
  • gasPrice — the effective gas price (hex).
  • hash — the transaction hash.
  • blockHash — null because the transaction is pre-confirmed (not yet in a finalized block).
  • blockNumber — the pending block number (hex).
  • transactionIndex — the transaction’s index position in the block (hex).
  • accessList — the EIP-2930 access list (if applicable).
  • r, s, v, yParity — signature values.
  • logs — array of event logs emitted by the transaction. Each log includes address, topics, data, blockHash, blockNumber, blockTimestamp, transactionHash, transactionIndex, logIndex, and removed.

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eth_subscribe("newFlashblockTransactions") code examples

Note that subscriptions require a WebSocket connection and WebSocket cat for you to use this method in the console.Install WebSocket cat with:npm install -g wscat

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Minimal data (default)

$ wscat -c YOUR_CHAINSTACK_WEBSOCKET_ENDPOINT
# Wait for the connection to be established

Connected (press CTRL+C to quit)

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

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Full transaction objects and logs

$ wscat -c YOUR_CHAINSTACK_WEBSOCKET_ENDPOINT
# Wait for the connection to be established

Connected (press CTRL+C to quit)

> {"jsonrpc":"2.0","id":1,"method":"eth_subscribe","params":["newFlashblockTransactions",true]}
This will generate a continuous stream of data displaying pre-confirmed transactions as they are included in Flashblocks.
If your handler performs heavy processing per event, throttle or debounce it to avoid blocking the WebSocket connection.
Use eth_unsubscribe | Base to remove the subscription.

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Use case

The eth_subscribe("newFlashblockTransactions") method is useful in scenarios where sub-second transaction visibility matters:
  • Latency-sensitive trading. DEX aggregators and MEV searchers can observe individual transactions as soon as the Base sequencer pre-confirms them—up to ~1.8 seconds before the full block seals—enabling faster reaction to on-chain events.
  • Real-time transaction monitoring. Wallet providers and block explorers can show users their transaction status within ~200ms of submission instead of waiting for the next 2-second block, significantly improving perceived confirmation speed.
  • Event-driven automation. Applications that trigger downstream actions on specific transactions—such as bridge relayers or liquidation bots—can act on pre-confirmed data instead of polling eth_getBlockByNumber with pending.
Last modified on April 1, 2026