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Subblocks (formerly Flashblocks) stream pre-confirmations on OP Stack chains. Two changes are rolling out to OP Sepolia and OP Mainnet: the subblock interval drops from 250 ms to 200 ms, and four fields in the stream payload are set to their zero value. The stream keeps its existing wire format, and the payload type is still named ExecutionPayloadFlashblockDeltaV1.
The subblock stream sets state_root, block_hash, withdrawals_root, and withdrawals to their zero value. The stream remains wire compatible, so reads return a zero value rather than raising an error. Audit stream integrations before August 17, 2026.

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What this means

  • The interval drops from 250 ms to 200 ms. This brings OP Sepolia and OP Mainnet to FLASHBLOCKS_TIME = 200ms, the default in the Flashblocks specification.
  • Four payload fields are set to their zero value. All four remain present in ExecutionPayloadFlashblockDeltaV1. Nothing is removed from the wire format.
  • Every other field, including receipts_root and logs_bloom, continues to carry a real value.
An integration that reads any of these fields receives a zero value and continues, rather than failing loudly at the parse boundary. We recommend you audit your application or integration for these reads directly ahead of the migration.

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The rollout is gradual

This is a rolling change rather than a single switch, so you may see both the previous and the new behavior for a period.

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Action required

For teams consuming the subblock stream directly:
  • Audit integrations for reads of state_root, block_hash, withdrawals_root, and withdrawals.
  • Confirm that a zero value in any of them cannot propagate into downstream state, balances, or proofs.
  • RPC providers that forward stream payloads to their own consumers should pass this notice on to those consumers.

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Timeline

Both dates are targets and may move.