# Base's Denim Upgrade Brings 200ms Blocks to Mainnet on Nov. 5th

*Author: William M. Peaster*
*Published: Oct 30, 2026*
*Source: https://www.bankless.com/read/news/bases-denim-upgrade-brings-200ms-blocks-to-mainnet-on-nov-5th*

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Base’s blocks are about to get 10x shorter. Per its [latest node release](https://github.com/base/base/releases/tag/v1.5.1), the Coinbase-incubated L2 will activate its Denim hard fork on Thursday, Nov. 5th at 18:00 UTC, trading its 2-second block time for five full blocks each second. 

> [ — (@)
> ](https://twitter.com/buildonbase/status/2115897920834949514)

# **What’s the Scoop?**

- **Real blocks, not previews:** Each 200ms slot will be a full block, with a block number, hash, and receipts of its own. That makes Flashblocks, the 200ms preconfirmation layer Base has layered over its 2-second blocks since July 2025, obsolete, so Denim switches it off. Apps still reading Flashblocks streams or the “pending” tag need to move to standard block feeds first. 

- **Mind the clock:** The sneakiest change hits code that counts blocks as time. Per [Base’s spec](https://docs.base.org/upgrades/denim/200ms-blocks), anything from timelocks and vesting to governance voting windows will run 10x faster, and immutable contracts “cannot be retuned.” A one-hour cooldown set at 1,800 blocks, for instance, would lapse in six minutes. Base’s advice is to lean on block.timestamp instead. 

- **An L3 casualty:** Horizen [says](https://blog.horizen.io/changes-for-horizen) its OP Stack L3, which settles to Base, can’t adapt and will stop accepting user transactions for good roughly 40 minutes after Denim activates. Bridging back to Base closes Tuesday, Nov. 4th at 13:00 UTC. 

- **Fourth solo upgrade:** Running on Base Sepolia since Oct. 21st, Denim is Base’s fourth upgrade since it [broke from Optimism’s Superchain](https://www.bankless.com/read/optimisms-ugly-year), following Azul, Beryl, and Cobalt. It also adds millisecond timestamps and tweaks to Base’s B20 token standard. 

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