Micro‑Lab: Proposer Strategy with Builders, Relays, and MEV

You are the proposer for each slot. Builders send sealed bids via relays. Now you can also build a block locally by selecting and ordering transactions from the mempool, then compare your local block to the builders’ bids.

1InspectRead the mempool, bundles, bids, and per‑slot relay risk.
2Build (optional)Order txs in Local Builder to make your own block.
3SelectClick a builder bid or your Local Build in the Bids panel.
4Lock inPress “Lock in & reveal” to propose it, then Next Slot.
How this works — the pieces and how they connect

You are the proposer. You don't write transactions — you choose which block gets proposed for your slot. There are two ways to get a block, and this lab lets you do both and compare:

Comparing & locking in: your Local Build appears as a card in Builder Bids right next to the builders' bids, each showing its value to you (the highest is tagged best, and every card shows how it compares). Click the one you want, then press Lock in & reveal to propose it and settle the slot. That's the proposer's real decision: take the most valuable bid, or build it yourself — while watching censorship, inclusion rules, and relay reliability.

How do you beat the best bid? A builder pays you only a cut of the block it builds (each bid card shows builder keeps 8–16%); when you build locally you keep 100%. So to win, make your local block's total value rival a builder's: press Fill by tips to pack the highest-fee txs, then capture the MEV by ordering each bundle's transactions correctly (the Local Builder lists exactly what to add or reorder). Match their value and your 100% payout beats their discounted bid — just mind the censorship penalty if you grab toxic MEV.

Mempool

baseFee 20 gwei slot 1

Public pending transactions. You and every builder draw from here; each tx pays a tip to whoever includes it. Click add to move one into your local block.

txgasLimitmaxFeepriorityvaluetagincl?add
What do the tags mean?
user swapAn ordinary user trade (e.g. a DEX swap). Pays a tip; often the thing MEV is extracted around.
oracle updateA price-feed update. Its position in the block can enable or spoil MEV that depends on the price.
arb leg 1/2arb leg 2/2The two halves of an arbitrage. Both must be included with leg 1 before leg 2 to capture the arb (a safe bundle).
liquidation targetAn underwater position eligible for liquidation. Including it lets a liquidation bundle pay out (a safe bundle).
sandwich victimA user swap an attacker can sandwich. Profitable to exploit, but harms the user — toxic MEV.
sandwich frontrunsandwich backrunThe attacker's trades placed before and after the victim. Order frontrun → victim → backrun to capture the sandwich.
inclusionOn the inclusion list. When the inclusion mandate is on, this tx must be in your block or you pay a compliance penalty.
toxicsafeOn bundles: toxic = profit that harms a user (sandwiches) and raises your censorship score; safe = MEV that doesn't target a victim (arbitrage, liquidation).

Searcher Bundles

MEV opportunities, not blocks. Each pays extra ETH only if its listed txs are included in the shown order — by a builder, or by you in your local block.

Builder Bids (via Relays)

Complete blocks from competing builders, plus your own Local Build, each showing its value to you. Click one to select it, then lock in.

Click a bid or your Local Build above to select it.

Local Builder — be your own builder

gas budget 15,000,000 gas used 0 value 0.000 ETH bundles 0
Add & reorder txs under the gas budget. To capture a bundle's MEV, include its required txs in the order each bundle shows (arrows = required order). Your block shows up as “Local Build” in Builder Bids to compare and lock in.
#txgastagincl?updownremove

Policy & Scenario

Scenario = a preset situation to practice — picking one switches on the matching rules below. Policy = the rules actually in force this slot, which you can also set by hand.

🔒 Slot locked — you've already revealed this slot. Press Next Slot to play another.
Rules in force this slot:
↳ Transactions flagged inclusion must be in your block, or you pay a compliance penalty.
↳ One relay (marked at risk) may fail to reveal in time — selecting a bid via it can miss the slot.
40
↳ How hard the network penalizes high-censorship blocks (e.g. toxic MEV). Penalty grows with censorship score × this value.
12
↳ Your deadline to reveal. If a block's reveal latency exceeds this, the slot is missed (0 revenue).

Metrics Dashboard

Slots played
0
Cumulative revenue (ETH)
0.000
Missed slots
0
Inclusion compliance
0%
Censorship penalty (ETH)
0.000
Average latency (ms)
0
Relay diversity (last 5)
0/5
Radar chart of your trade‑offs.
Hotkeys: N = Next slot, R = Reveal. All values are simulated for teaching purposes.