Developers maintaining Base applications that relied on Flashblocks need to switch to canonical block subscriptions and state reads. According to ChainCatcher’s November 9, 2026 report, Base’s Denim upgrade stopped Flashblocks production and replaced it with a complete canonical block—the network’s authoritative block—every 200 milliseconds.
Activation dates
Denim activated on:
- Base Sepolia: October 21, 2026, at 18:00 UTC.
- Base mainnet: November 5, 2026, at 18:00 UTC.
After activation, both networks stopped producing Flashblocks. Flashblocks subscriptions, WebSocket streams and pending-state interfaces no longer return data. According to the official documentation cited in the report, endpoints such as wss://mainnet.flashblocks.base.org/ws still accept connections but no longer push data. A successful connection therefore does not mean a stream is delivering updates.
Update subscriptions and state reads
The report lists these migration changes:
| Previous interface | Replacement |
|---|---|
eth_subscribe("newFlashblocks") |
eth_subscribe("newHeads") |
pendingLogs |
logs |
newFlashblockTransactions |
Subscribe to newHeads, then fetch transactions from the corresponding canonical blocks |
RPC calls using the "pending" block tag |
Read the latest canonical block and state instead |
These changes move applications away from Flashblocks and pending-state data toward canonical blocks and their associated state.
Update node configuration and transaction predicates
Mainnet node operators need base/base v1.5.1 or later; Sepolia node operators need v1.5.0 or later. The report also instructs operators to remove these configuration settings:
RETH_FB_WEBSOCKET_URL--flashblocks-url--max-pending-blocks-depth--flashblocks.ping-interval
Validity transactions containing a flashblock_index predicate—a condition governing transaction validity—will be rejected. The official recommendation cited by ChainCatcher is to use block_number to set the range instead.
Original report: ChainCatcher, published November 9, 2026, at 22:59:43 UTC+08:00.