Summary
Sub-slot execution (TOOL, by NuConstruct) divides each 12-second Ethereum slot into sub-slots of ~200ms–1 second. After each sub-slot, a partial state transition is shared, giving users early execution confirmation and enabling subsequent participants to build on updated state. Proposers opt in via a one-line MEV-Boost config change; no protocol fork is required. The primary benefit is increased trading volume (reduced forecast variance → more willing traders → more fees), not necessarily higher per-trade profit.
Problem
Ethereum’s 12-second slot time means traders must forecast price movements 12 seconds into the future. High variance across a 12-second window discourages participation:
- LP providers face 12 seconds of adverse price exposure per block
- Limit order traders must set wide tolerance bands to handle 12 seconds of uncertainty
- DeFi protocols with oracle dependencies are stale for up to 12 seconds
TOOL Architecture
TOOL (Trustless Orderflow Operations Layer) by NuConstruct:
- Proposers opt in by adding one line to their MEV-Boost config.
- The proposer’s slot is divided into sub-slots (~200ms to ~1s each).
- After each sub-slot, a partial state transition is computed and shared:
- Transactions executed in the sub-slot are committed
- The resulting state diff is broadcast
- Users receive “early execution confirmations”: their transaction executed in this sub-slot.
- Subsequent sub-slot participants (traders, searchers) build on the updated state.
Distinction from Pre-Confirmations
Sub-slot confirmations are:
- Backward-looking (“your tx just executed in this sub-slot”)
- Based on real execution — the state transition has happened
- Not a forward-looking guarantee (“your tx will be included”)
Pre-confirmations (like mev-commit) are:
- Forward-looking (“your tx will be in a future block”)
- Commitment from the proposer/relay before execution
- Require collateral/slashing if the commitment is violated
Research on CEX-DEX Arbitrage
Supporting research presented at the Cannes workshop:
Finding: Shorter sub-slots don’t necessarily increase individual trader profit per trade — the delta stays roughly the same. But trading volume increases significantly because shorter forecast windows reduce variance.
- Traders are more willing to participate when forecasting 1 second vs. 12 seconds
- Reduced uncertainty → more trades per block
- More trades → more total fees, even if per-trade profit is flat
- Simulation: multiple competing agents across different sub-slot configurations → aggregate block value increases with shorter sub-slots, primarily driven by volume
Implication: sub-slot execution benefits the network (more fees, more efficient markets) even though individual MEV per trade is unchanged.
Proposer Game Theory: Blinding
A creative design choice: proposers don’t see the real block value from sub-slotted blocks.
Why: if proposers could compare the sub-slot block value against a competing PBS block at the last moment, they could cherry-pick — accept the sub-slot blocks only when they’re more valuable, otherwise switch to PBS.
Blinding prevents this: proposers make a longer-term decision (opt in or out based on historical average rewards) rather than per-slot decisions. This makes last-minute switching unprofitable.
Interaction with PBS
The hardest unresolved question: how do sub-slots interact with the traditional PBS pipeline?
Four scenarios discussed at the workshop:
- Head-to-head competition: sub-slotted blocks compete against PBS blocks at slot end — winner takes all
- Reorg-able sub-slots: sub-slots can be “reorged” if total sub-slot value < a competing PBS block
- Merging: non-contentious transactions merged onto sub-slot execution (combining both approaches)
- Proposer constraint: proposers set a global rule that all blocks for their slot must be sub-slotted
No consensus emerged. This is an open design problem.
The Contentious/Non-Contentious Split
| State type | Sub-slot execution benefit | Relay merging applicability |
|---|---|---|
| Contentious (DEX arb, liquidations) | High (composability across sub-slots matters) | Low (contentious txs can’t be merged) |
| Non-contentious (transfers, independent calls) | Low (no composability benefit) | High (can be appended to winning block) |
Sub-slots and relay merging are complementary: sub-slots for contentious state, merging for non-contentious. Getting both to coexist in one pipeline is the central design challenge.
Deployment
- Opt-in for proposers via MEV-Boost config (one-line change)
- No Ethereum protocol change required
- NuConstruct is developing and testing the system
- Timeline: testnet phase in 2026; mainnet deployment dependent on validation
Related Pages
- Relay Block Merging — Complementary for non-contentious state
- PBS and MEV-Boost — Current PBS that sub-slots must coexist with
- Timing Games and Proof-of-Time — Sub-slots reduce timing game window
- Arbitrage: CEX-DEX and AMM Arb — CEX-DEX arb volume research supporting sub-slots
- Fast Confirmation Rule (FCR) — Alternative approach to fast confirmations
Key Sources
- What Emerged from the Blockspace Forum Workshop in Cannes (Apr 2026) — primary; sub-slot design; proposer blinding; CEX-DEX arb research; open questions on PBS interaction