Core Denim Upgrade Brings 200ms Blocks to Mainnet on November 5

Basic blocks are about to become 10 times shorter. By his latest node versionThe Coinbase-incubated L2 will activate its Denim hard fork on Thursday, November 5 at 6:00 p.m. UTC, trading its 2-second block time for five full blocks per second.

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• Real blocks, not previews: Each 200 ms slot will be a complete block, with its own block number, hash and receipts. This makes Flashblocks, the 200ms preconfirmation layer that Base has overlaid on its 2-second blocks since July 2025, obsolete, so Denim is turning it off. Applications that are still reading Flashblocks streams or the “waiting” tag should first switch to standard block streams.

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• Pay attention to the clock: The sneakiest change is to the code that counts blocks as time. By Base specificationeverything from deadlines and acquisitions to voting windows for governance will work 10x faster, and immutable contracts “cannot be readjusted.” A one-hour cooldown set at 1,800 blocks, for example, would elapse in six minutes. Basic advice is to rely on block.timestamp instead.

• An L3 victim: This is why said its OP Stack L3, which installs on Base, cannot scale and will permanently stop accepting user transactions around 40 minutes after Denim is enabled. Return to base closes on Tuesday, November 4 at 1:00 p.m. UTC.

• Fourth Solo Upgrade: Running on Base Sepolia since October 21, Denim is Base’s fourth upgrade since its breakup with Optimism’s Superchain, following Azul, Beryl, and Cobalt. It also adds millisecond timestamps and adjustments to the Base B20 token standard.

• Big picture: For everyday users, the difference with Denim’s UX should seem minimal, since Flashblocks already offered near-instant confirmations. The thing to watch on November 5 is less the speed itself than any contract still quietly using block numbers as a clock.