Base’s retired Flashblocks endpoints still accept WebSocket connections but no longer stream data following the Denim upgrade, leaving applications that depend on those feeds without live updates unless developers migrate, according to Base’s migration guide.
Coinbase’s Ethereum Layer 2 activated Denim on Nov. 5 at 1 p.m. ET. The upgrade replaced one complete block every two seconds, with Flashblocks previews in between, with complete canonical blocks every 200 milliseconds, according to Base’s specification.
The migration guide explicitly warns that the retired mainnet and Sepolia testnet endpoints “still accept connections but no longer stream any data.” Reconnecting to the same endpoint therefore will not restore updates: developers must change the data source and subscriptions.
The requirement applies to applications and infrastructure that consumed Flashblocks. Base’s launch announcement directs app and wallet developers using those feeds to migrate, while saying end users need take no action. Its migration guide does not quantify how many integrations still use the retired feeds.
Flashblocks previously supplied early transaction feedback before a complete block arrived. Denim removes that separate preview stream; each 200-millisecond block now has its own number, hash, state root and transaction receipts.
What Builders Need to Change
Base tells developers to replace the retired connections with subscriptions through a WebSocket-capable RPC provider or their own node. The main subscription changes are:
Replace newFlashblocks with newHeads, which follows canonical block headers.
Replace pendingLogs with logs for contract events.
Replace newFlashblockTransactions with newHeads, then fetch the transactions in each canonical block.
Applications also need to review requests using the pending block tag. Base still accepts it, but it now resolves to the latest canonical block rather than a Flashblocks preview. Balance reads and contract simulations using that tag can no longer be treated as preconfirmation state. Transaction receipts become available once a transaction enters a canonical block; preconfirmed receipts are no longer available.
Faster blocks do not by themselves establish finality. Base says blocks become safe when their batch is posted to Ethereum and final when Ethereum finalizes that batch.
Separately, Base’s Denim status notice remained marked “Verifying” on Nov. 9. Its latest update, posted Nov. 5, said the team was investigating periodic transaction inclusion delays and a higher-than-expected rate of empty blocks.
Base recommends inventorying Flashblocks subscriptions, WebSocket connections and pending-tag requests, then testing migrated integrations on Base Sepolia or Vibenet.