Migrate From Flashblocks - Base Documentation

200ms native blocks are live on Vibenet for experimental developer testing. This deployment may change, and you should not rely on it for production decisions.Cobalt is not active on Base Sepolia or Base Mainnet; activation times and required client versions remain undecided.

Summary

Use this guide if your application consumes Flashblocks. Cobalt replaces Flashblocks with canonical 200ms blocks, so Flashblocks subscriptions and pending state are unavailable after activation.

Recommended Action

Inventory every Flashblocks subscription and RPC request that uses the "pending" block tag before Cobalt. Replace each integration using the table below.

Replace Flashblocks Integrations

Flashblocks integration	Update for Cobalt
`eth_subscribe("newFlashblocks")`	Use `eth_subscribe("newHeads")`.
`eth_subscribe("pendingLogs")`	Use `eth_subscribe("logs")`.
`eth_subscribe("newFlashblockTransactions")`	Use `eth_subscribe("newHeads")`, then fetch transactions for each canonical block.
`eth_getBlockByNumber("pending", ...)`	Use normal RPC calls to read the latest canonical blocks.
`eth_getBalance(..., "pending")`	Use normal RPC calls to read the latest canonical state.
`eth_getTransactionCount(..., "pending", ...)`	Use normal RPC calls to read the latest canonical state.
`eth_call(..., "pending")`	Use normal RPC calls to read the latest canonical state.
`eth_estimateGas(..., "pending")`	Use normal RPC calls to read the latest canonical state.
`eth_simulateV1(..., "pending")`	Use normal RPC calls to read the latest canonical state.
`eth_getLogs` with a range ending at `"pending"`	Use normal RPC calls to query logs from canonical blocks.
`eth_getBlockTransactionCountByNumber("pending")`	Use normal RPC calls to read the latest canonical block.

The "pending" block tag and Flashblocks preconfirmation state have no direct equivalent after Cobalt. The replacements above return canonical data from 200ms blocks, not preconfirmed or pending data.

Test on Vibenet

Test your migrated integration on Vibenet. For the protocol design and RPC behavior, see 200ms Native Blocks.