Coinbase’s Base Layer‑2 network completed its Denim upgrade on Nov. 5, 2026, retiring the Flashblocks data feeds that many applications had relied on for near‑real‑time block previews. While the legacy WebSocket endpoints still accept connections, they no longer push block or transaction data, leaving apps with silent connections unless they migrate to the new canonical‑block subscription model.

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What changed with the Denim upgrade

According to CryptoSlate, the Denim upgrade introduced a deterministic block cadence of 200 milliseconds, each block containing a full hash, state root and receipt set. These blocks are now the primary source of data, replacing the previous Flashblocks “preview” blocks that showed pre‑confirmation information. The upgrade also altered subscription identifiers: the former newFlashblocks event maps to newHeads, which now follows canonical block headers, while newFlashblockTransactions also maps to newHeads, requiring developers to fetch transaction lists manually.

Impact on existing applications

Applications still pointing at the retired Flashblocks endpoints can maintain an open WebSocket connection, but no updates will be received. Re‑connecting to the same endpoint does not resolve the issue, as the underlying data feed is permanently disabled. Developers are advised to switch to a WebSocket‑capable RPC provider or run their own Base node, updating any code that relied on the pending‑block tag. The pending tag now resolves to the latest canonical block rather than a pre‑confirmation state, meaning balance queries, transaction counts, or simulated contract calls that expected a preview may now return finalized data.

Migration guidance and open issues

Base’s migration guide recommends inventorying all Flashblocks subscriptions, WebSocket connections, and any usage of the flashblock_index flag. Testing should be performed on Base Sepolia or Vibenet before production rollout. The Denim notice, still marked “Verifying” as of Nov. 9, also referenced ongoing investigations into periodic transaction‑inclusion delays and a higher‑than‑expected rate of empty blocks, though no resolution has been posted. Developers needing stronger guarantees of finality must still track batch posting to Ethereum and Ethereum’s finality signals, as the 200 ms block arrival does not equate to final settlement.

What to watch next

The Base team has not yet provided an update on the reported inclusion delays or empty‑block anomalies, so monitoring future status‑page notices will be essential. Additionally, the ecosystem will likely see a wave of migration tools and SDK updates aimed at simplifying the switch from Flashblocks to newHeads subscriptions. Observers should also track whether Coinbase introduces a replacement for the pre‑confirmation state, which many DeFi and gaming dApps currently depend on for fast user feedback.