pendingLogs

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, 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 "pendingLogs".
filterOptions	object	No	Optional log filter.

The filterOptions object accepts:

Field	Type	Description
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

Field	Type	Description
result	string	Hex-encoded subscription ID.

Example

Subscribe (with filter)

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

Subscription ID response

{"jsonrpc": "2.0", "id": 1, "result": "0x2a7bc8d4e3f5a6b1c2d3e4f5a6b7c8d9"}