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Base API method that streams event logs from pre-confirmed transactions matching an optional filter. 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, pendingLogs in this case.
  • object — (optional) the log filter options:
    • address — the contract address from which the logs should be fetched. It can be a single address or an array of addresses.
    • topics — an array of DATA topics. The event topics for which the logs should be fetched. It can be a single topic or an array of topics. Uses the same topic filter format as eth_getLogs.
      Read Tracking some Bored Apes: The Ethereum event logs tutorial to learn more about topics and event logs.

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Response

  • subscription — the subscription ID.
  • object — a log object matching the specified filter:
    • address — the contract address from which the event originated.
    • topics — an array of 32-byte data fields containing indexed event parameters.
    • data — the non-indexed data that was emitted along with the event.
    • blocknumber — the block number in which the event was included. null if it is pending.
    • transactionhash — the hash of the transaction that triggered the event. null if pending.
    • transactionindex — the integer index of the transaction within the block’s list of transactions. null if it is pending.
    • blockhash — the hash of the block in which the event was included. Set to 0x0000...0000 for pre-confirmed events.
    • blocktimestamp — the Unix timestamp of the block (hex).
    • logindex — the integer identifying the index of the event within the block’s list of events. null if pending.
    • removed — the boolean value indicating if the event was removed from the blockchain due to a chain reorganization. True if the log was removed. False if it is a valid log.

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eth_subscribe("pendingLogs") 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
In the following examples, we subscribe to USDC transfer events on Base. The address 0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913 is the Base USDC contract and 0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef is the Transfer(address,address,uint256) event signature. 
$ 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":["pendingLogs",{"address":"0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913","topics":["0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef"]}]}
This will generate a continuous stream of data displaying event logs from pre-confirmed transactions matching the filter. Use eth_unsubscribe | Base to remove the subscription.

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Use case

The eth_subscribe("pendingLogs") method is useful in scenarios where you need to react to specific smart contract events with sub-second latency:
  • DeFi monitoring. Track swap events on DEXs like Uniswap or Aerodrome as soon as they are pre-confirmed, giving you ~1.8 seconds of advance notice compared to waiting for the full 2-second block to seal. This is valuable for arbitrage detection, liquidity monitoring, and price feed updates.
  • Real-time token transfer tracking. Monitor ERC-20 transfers (such as USDC or WETH) for specific addresses or contracts as they happen, enabling faster notifications for wallets, payment processors, and compliance tools.
  • Smart contract event indexing. Event indexers can begin processing logs from pre-confirmed transactions instead of waiting for finalized blocks, reducing the end-to-end latency of on-chain data pipelines.
Last modified on April 1, 2026