Block Builders Explained: Flashbots, Beaverbuild, Titan & The 2026 Auction

Answer first — In 2026, three to five block builders win >85% of Ethereum L1 slots: Beaverbuild, Titan, Rsync, BuilderNet, and Flashbots Builder. Each runs a different ordering algorithm and pays validators different fees. As a searcher, you don't pick one builder — you submit your bundle to all major relays and let the auction decide. But understanding which builder favors which kind of flow lets you tune bid strategy and bundle structure for higher inclusion.

For PBS context, see Flashbots Bundles Explained and MEV-Share vs Private Relays.

The PBS Pipeline in 30 Seconds

[Searcher] → [Builder] → [Relay] → [Proposer]
   ↑            ↑          ↑           ↑
 bundles    builds full  validates   chooses
            block        and bids    highest
                         on behalf   bid
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A searcher submits a bundle (one or more ordered transactions). Builders compete to assemble the most valuable block by combining bundles + public mempool txs. Relays act as auction houses — they receive builder blocks, hold blinded headers, and forward the highest-bid one to the proposer.

In 2026 the dominant relays are Flashbots, Ultra Sound, BloXroute, and Aestus. Most builders submit to all of them.

The 2026 Builder Landscape

Approximate 2026 share of relayed blocks (rolling 30-day):

These shift week to week. Track current shares at the Flashbots transparency dashboard and our Performance benchmarks.

How Each Builder Differs

Beaverbuild

• Aggressive bid policy: returns more value to proposers, higher inclusion rate.
• Has direct retail order flow integrations (private RPC endpoints used by some wallets).
• Strict on bundle reverts — failed bundles get scored down.

Titan

• Co-located in Frankfurt and Northern Virginia.
• Excellent block construction algorithm; combines bundles tightly.
• Favored by sophisticated arb searchers.

Rsync

• US-focused infra; lowest latency from US-East searchers.
• Selective on bundle types; rejects sandwich-against-protected flow.

BuilderNet

• Decentralized — multiple operators run instances.
• Open-source codebase. Best for searchers who want auditable builder behavior.
• Slightly higher latency than centralized builders.

Flashbots Builder

• Reference implementation. OFAC-compliant (filters sanctioned addresses).
• Conservative ordering. Lower revenue but predictable inclusion.

What "Submit to All" Actually Means

Modern searchers send the same bundle to multiple builders simultaneously. The first to land it in a block wins. There's no penalty for parallel submission — builders are competing, not coordinating.

FRB Agent's bundle dispatcher submits to Flashbots, Beaverbuild, Titan, Rsync, and BuilderNet by default. Operator can disable specific builders for compliance (some users disable OFAC-compliant builders for jurisdictional consistency, others disable non-compliant ones for the opposite reason).

Bid Math: How Much to Pay Builders

Your bundle includes a coinbase_transfer (a tx paying ETH to block.coinbase). That's your bid to the builder. Builder takes a cut, validator gets the rest.

Typical bid splits in 2026:

• Builder retains: 5–15% of coinbase transfer
• Validator receives: 85–95%
• Effective MEV "tax" on searchers: ~10–20% across the stack

Bidding too low: bundle is rejected, gas wasted on simulation only. Bidding too high: profit eroded.

The sweet spot is the inclusion probability curve — bid the marginal dollar that flips your inclusion likelihood from "maybe" to "yes."

Latency to Relay: It Matters Less Than You Think

Relay deadlines are typically 1–2 seconds before slot completion. Most searchers are well within that window. The real bottleneck is bundle quality, not RTT.

Exception: top-of-block bundles for liquidations and arb against fresh oracle updates need to arrive in the first 200ms of the slot. For those, latency matters.

What Searchers Should Ask About a Builder

When tuning your strategy:

1. Does this builder accept reverting transactions in bundles?
2. What's the bundle simulation timeout?
3. Does the builder filter sanctioned addresses?
4. Are there minimum bundle bid requirements?
5. Is bundle privacy guaranteed pre-inclusion?

Each builder has documented answers. FRB Agent's defaults match the union of "yes" answers across major builders.

Builder Privacy: What Leaks

Even with private bundles, certain information can leak:

• Bundles that lose still consume builder simulation time, generating measurable timing signals.
• Some builders publish anonymized bundle stats post-block.
• Reverting txs in bundles are visible to relay operators if not filtered.

For maximum privacy, use MEV-Share which adds explicit hint-only flow.

Builders on L2s

Most L2s in 2026 don't have PBS at the same level as L1:

• Base, Optimism: Sequencer is the builder. No multi-builder auction.
• Arbitrum: Sequencer with Sequencer Fair Ordering.
• Linea: Sequencer with proof scheduling.
• zkSync Era: Sequencer with private mempool.

Some L2 ecosystems are exploring shared-sequencer + builder markets (Espresso, Astria), but these are early in 2026.

Builder Centralization: The Open Question

Three to five builders winning >85% of slots is a centralization concern. Mitigations being worked on:

• Open builder participation (BuilderNet)
• Trust-minimized builders (TEE-based attestation)
• Distributed builder protocols (research, not production)

Searchers should care because if a builder duopoly forms, it can extract more rent. Diversifying submission across builders helps preserve competition.

How to Verify Your Bundles Land Where You Think

Each major builder publishes bundle hashes for accepted bundles. Cross-check against the included block's transactions. FRB Agent does this automatically and surfaces inclusion-by-builder analytics in the dashboard.

FAQ

Do I need to apply to a builder?

No. All major builders accept bundle submissions from any searcher to public endpoints. No application needed.

Is OFAC compliance required?

Depends on jurisdiction. US-domiciled searchers should generally use OFAC-compliant builders. Read Are MEV Bots Legal for the broader compliance picture.

Do builders give better treatment to high-volume searchers?

Some do informally. There are no documented preferential tiers, but high-volume searchers report higher inclusion rates that aren't fully explained by bid alone.

Can I run my own builder?

Yes — open-source builder codebases are public. But running a competitive builder requires significant infra investment. Most searchers don't.

How does FRB Agent pick which builder gets which bundle?

By default, every bundle is submitted to all major builders in parallel. The first to include wins. You can override per-strategy in advanced settings.

Related Reading

• Flashbots Bundles Explained
• MEV-Share vs Private Relays
• Inclusion Probability 101
• Best MEV Relays by Chain
• Private Bundle Debugging

Builder market shares are estimates from public relay data and shift continuously. Verify current numbers before tuning bid strategy.