Subblocks (formerly known as Flashblocks) which stream pre-confirmations on OP Stack chains have two changes that directly impact anyone consuming the stream. The rollout of these changes starts on OP Sepolia on *Monday, August 17, 2026* and on OP Mainnet on *Monday, August 31, 2026*.
This change is backwards compatible. The WebSocket stream URL and the RPC endpoints are not changing. Unless you rely on the four payload fields below, nothing changes for you and no action is needed.
What's changing - The subblock interval drops from 250ms to 200ms. Subblocks are produced more frequently. - Four payload fields are set to their zero value. `state_root`, `block_hash`, `withdrawals_root`, and `withdrawals` are zeroed in the subblock payload. The stream stays wire compatible, so this does not raise a parse error — an integration that reads any of them gets a zero value instead of failing loudly. The failure mode is silent, so audit rather than wait for an error. Every other field, including `receipts_root` and `logs_bloom`, still carries a real value.
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.
Action required If you consume the `subblock` stream, audit your integration for reads of `state_root`, `block_hash`, `withdrawals_root`, and `withdrawals`.
Timeline - Monday, August 17, 2026 — rollout begins on OP Sepolia - Monday, August 31, 2026 — rollout begins on OP Mainnet
Subblocks (formerly known as Flashblocks) which stream pre-confirmations on OP Stack chains have two changes that directly impact anyone consuming the stream. The rollout of these changes starts on OP Sepolia on *Monday, August 17, 2026* and on OP Mainnet on *Monday, August 31, 2026*.
This change is backwards compatible. The WebSocket stream URL and the RPC endpoints are not changing. Unless you rely on the four payload fields below, nothing changes for you and no action is needed.
What's changing - The subblock interval drops from 250ms to 200ms. Subblocks are produced more frequently. - Four payload fields are set to their zero value. `state_root`, `block_hash`, `withdrawals_root`, and `withdrawals` are zeroed in the subblock payload. The stream stays wire compatible, so this does not raise a parse error — an integration that reads any of them gets a zero value instead of failing loudly. The failure mode is silent, so audit rather than wait for an error. Every other field, including `receipts_root` and `logs_bloom`, still carries a real value.
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.
Action required If you consume the `subblock` stream, audit your integration for reads of `state_root`, `block_hash`, `withdrawals_root`, and `withdrawals`.
Timeline - Monday, August 17, 2026 — rollout begins on OP Sepolia - Monday, August 31, 2026 — rollout begins on OP Mainnet
Subblocks (formerly known as Flashblocks) which stream pre-confirmations on OP Stack chains have two changes that directly impact anyone consuming the stream. The rollout of these changes starts on OP Sepolia on *Monday, August 17, 2026* and on OP Mainnet on *Monday, August 31, 2026*.
This change is backwards compatible. The WebSocket stream URL and the RPC endpoints are not changing. Unless you rely on the four payload fields below, nothing changes for you and no action is needed.
What's changing - The subblock interval drops from 250ms to 200ms. Subblocks are produced more frequently. - Four payload fields are set to their zero value. `state_root`, `block_hash`, `withdrawals_root`, and `withdrawals` are zeroed in the subblock payload. The stream stays wire compatible, so this does not raise a parse error — an integration that reads any of them gets a zero value instead of failing loudly. The failure mode is silent, so audit rather than wait for an error. Every other field, including `receipts_root` and `logs_bloom`, still carries a real value.
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.
Action required If you consume the `subblock` stream, audit your integration for reads of `state_root`, `block_hash`, `withdrawals_root`, and `withdrawals`.
Timeline - Monday, August 17, 2026 — rollout begins on OP Sepolia - Monday, August 31, 2026 — rollout begins on OP Mainnet