pendingLogs

Subscribe to logs from pre-confirmed transactions matching an optional filter. Only available on Flashblocks WebSocket endpoints.

Subscribe via eth_subscribe to receive logs from pre-confirmed transactions matching an optional filter. Useful for monitoring contract events with sub-block latency.

Only available on Flashblocks WebSocket endpoints: wss://mainnet-preconf.base.org / wss://sepolia-preconf.base.org.

Requires base/base minimum client version v0.3.1.

Each subscription emits one item per WebSocket message. Events arrive approximately every 200ms. If your handler performs heavy processing per event, throttle or debounce it to avoid blocking.

Parameters

subscriptionType · string · required

Must be "pendingLogs".

filterOptions · object

Optional log filter.

Show Filter options

address · string | array

A single contract address or array of addresses to filter by.

topics · array

Array of topic filters in the same format as eth_getLogs.

Returns

The initial eth_subscribe response returns a hex-encoded subscription ID. After that, the node pushes one notification per matching log per WebSocket message.

Each notification’s result is a full log object:

result · object

A single log object matching the filter.

Show Log fields

address · string

Contract address that emitted the log.

topics · string[]

Indexed event parameters. Topic 0 is typically the event signature hash.

data · string

ABI-encoded non-indexed event parameters.

blockHash · string

Hash of the partial block at time of pre-confirmation. Always "0x0000...0000" for Flashblocks logs (block not yet sealed).

blockNumber · string

Block number (hex).

blockTimestamp · string

Unix timestamp of the block (hex).

transactionHash · string

Hash of the transaction that emitted this log.

transactionIndex · string

Index of the transaction within the block (hex).

logIndex · string

Index of this log within the block (hex).

removed · boolean

Always false for Flashblocks logs.

Notification Format

Notification

{
  "jsonrpc": "2.0",
  "method": "eth_subscription",
  "params": {
    "subscription": "0xe1f42d3d4c3e4a09c76c98857b01e50d",
    "result": {
      "address": "0x833589fcd6edb6e08f4c7c32d4f71b54bda02913",
      "topics": [
        "0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef",
        "0x000000000000000000000000828bbaee2935085bbb9bc6b72fb65e867205e1b5",
        "0x000000000000000000000000dbc6998296caa1652a810dc8d3baf4a8294330f1"
      ],
      "data": "0x0000000000000000000000000000000000000000000000000000000006bce5f9",
      "blockHash": "0x0000000000000000000000000000000000000000000000000000000000000000",
      "blockNumber": "0x27920d7",
      "blockTimestamp": "0x697c9e91",
      "transactionHash": "0xa98490d6c1b13a72e7753ba9217269a5059db631deab582b572c4d6c9399c240",
      "transactionIndex": "0xb9",
      "logIndex": "0x4ec",
      "removed": false
    }
  }
}

Example

Subscribe (with filter)

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "eth_subscribe",
  "params": [
    "pendingLogs",
    {
      "address": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
      "topics": ["0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef"]
    }
  ]
}