Base
This API exposes a low-latency WebSocket feed for protected pending transactions on Base and allows searchers to submit backrun bundles into a short, competitive auction window.
An API key is required. Keys authenticate the WebSocket connection and may be rate-limited or revoked if rules are violated.
WebSocket Connection
Connect using your API key:
wss://base.ofa.titanbuilder.xyz/ws/v1/<API_KEY>The protocol uses JSON-RPC 2.0 over WebSocket. If the connection drops, clients must reconnect and resubscribe. No state is preserved across reconnects.
Transaction Subscription
After connecting, you must explicitly subscribe to receive transactions:
{
"jsonrpc": "2.0",
"id": 1,
"method": "eth_subscribe",
"params": ["mevblocker_partialPendingTransactions"]
}The stream emits eligible pending transactions eligible for backruns. Messages are delivered in arrival order at the server. No ordering or completeness guarantees are provided beyond that.
Submitting Bids
Bids are submitted over the same WebSocket connection using eth_sendBundle:
The first element must be the target transaction hash. The second element must be your signed backrun transaction in raw hex form. Only one backrun transaction is supported per bundle.
Bundles are only valid for the current flashblock auction. Late bundles may be ignored or revert on-chain.
Auction Mechanics
Each eligible transaction opens a short auction window (up to ~200ms). Bundles are ranked by effective priority fee and gas as seen by the sequencer.
If no winning bundle is selected before the end of the flashblock, the auction degrades to FIFO based on server-side receipt time.
With flashblocks enabled, latency is critical. Colocation in AWS us-east-1 is strongly recommended.
Refund Rules
Searchers must return at least 75% of realized profit to the refund address:
An example of this can be seen here.
This refund must be an explicit ETH transfer included in the backrun transaction itself. Profit is evaluated per bundle based on on-chain execution.
Searchers may send transactions directly to the sequencer, but any backrun against provided flow must also be submitted through this WebSocket. This is actively monitored.
Any form of sandwiching against protected transactions results in an immediate ban.
Monitoring and Enforcement
The system continuously monitors:
Sandwich behavior
Profit share compliance (≥ 75%)
Revert rates, especially during competitive auctions
Latency and submission patterns
Violations may result in reduced flow, artificial websocket delays, or a permanent ban, depending on severity and persistence.
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