Base Disables Flashblocks; Subscriptions and Pending States No Longer Return Data

ChainCatcher report: Base has discontinued Flashblocks in the Denim upgrade, replacing them with full canonical blocks generated every 200 milliseconds. The Denim upgrade was activated on Base Sepolia on October 21, 2026, at 18:00 UTC, and on Base Mainnet on November 5, 2026, at 18:00 UTC. After activation, Base ceased producing Flashblocks on both networks; Flashblocks subscriptions, WebSocket streams, and pending state no longer return any data. Official documentation states that endpoints such as wss://mainnet.flashblocks.base.org/ws will still accept connections but will no longer push any data. Developers must replace eth_subscribe("newFlashblocks") with eth_subscribe("newHeads"), replace pendingLogs with logs, and retrieve transactions via newHeads by fetching them from canonical blocks; RPC calls using the "pending" block tag should instead query the latest canonical block and state. For node configuration, Mainnet nodes must be upgraded to base/base v1.5.1 or later, and Sepolia nodes to v1.5.0 or later, with RETH_FB_WEBSOCKET_URL, --flashblocks-url, --max-pending-blocks-depth, and --flashblocks.ping-interval settings removed. Additionally, validity transactions with the flashblock_index predicate will be rejected; the official recommendation is to use block_number for range limitations instead.