Solana Block Building

How Jito, BAM, Harmonic and plain Agave order transactions, and what that means for bidding.

TL;DR#

• Plain Agave: continuous scheduler, priority fee per CU.
• Classic Jito: Agave scheduler + parallel bundle path (50ms tip auction).
• BAM: one verifiable sequence from BAM nodes; leader must follow it.
• Harmonic: validator bound to a builder running a chosen strategy (FBA / FIFO / MREV / custom).
• Latency is a threshold on batched stacks (ms, not µs) and direct on continuous ones. Price wins everywhere except FIFO.

Stake share#

Categories overlap (Frankendancer can run Jito/Harmonic overlays; FireBAM exists). Harmonic's strategy split is not public.

Classic Jito#

Components

• Bundles: ≤5 txs, sequential, atomic, all-or-nothing, single slot. One tx transfers SOL to one of 8 tip accounts (= the bid).
• Block Engine (off-chain, regional): simulates, runs auction every ~50ms, groups bundles by overlapping write locks, highest tip wins per group; non-conflicting bundles all win. Forwards to connected Jito leaders.
• Validator (jito-solana): bundle stage (atomic) runs in parallel with the normal banking stage (priority-fee scheduler) for the whole slot.
• Tips: leader sets itself as tip receiver → tips swept to its tip distribution account → epoch-end merkle root → validator commission + staker claims (+ Jito protocol cut).

Facts that matter

• No 200ms Relayer delay anymore — validators connect directly to the Block Engine.
• No public mempool (shut 2024).
• No reserved top-of-block. Blocks stream as entries; bundles interleave all slot. Only a small CU reservation for tip-program cranks (unverified).
• Priority is per contested account, not block position.

Losing an auction

• Pay nothing (tip is inside the bundle).
• May win a later tick if still valid; usually the winner changed state → re-sim fails → dropped.
• Can't cross slots; ~2 slot useful life (third-party docs).
• No fallback to non-Jito leaders.
• Status via getInflightBundleStatuses (5 min lookback): Pending / Failed / Landed / Invalid.
• Watch uncled blocks — confirm at confirmed, not processed.

Ordering

• Within tick: tip. Across ticks: earlier first.
• TPU txs: priority fee per CU from buffer.
• Cross-path race: bundle stage vs banking stage on the same account → whoever grabs the lock first. Arrival/timing, not unified price.

Jito BAM#

• BAM nodes (TEE-based) receive txs, sequence by published rules, forward to leader.
• Leader executes exactly in that order; BAM nodes verify; reorder/insert → disconnected.
• Single sequence → no bundle-vs-banking race.
• Within batch: scheduler rule (priority by default, unverified). Plugins can change it. Exact batch length unverified.
• Preconfirmations (live Sep 9, 2026): sequenced txs streamed early to subscribers (~5–10ms edge) via Helius / Triton. Creates an information race.
• Frankendancer support via FireBAM.

Harmonic#

Architecture

• Remote TPU: aggregates txs, forwards to all builders.
• Builders: construct blocks per strategy.
• Block Engine: routes between validators and builders.
• Validator: executes + broadcasts.
• Streaming mode replaced the former Full Block Auction → validator is bound to one builder; builders compete for bindings, not per slot.
• Builder failover + direct fallback path.
• Clients: Salsa (Agave), Samba (Firedancer). Strategy via --strategy (Salsa) or [tiles.bundle] strategy (Samba); default fba.
• Whitelisted onboarding (validators and searchers).

Strategies

• Tips = CU priority fees (no tip account). 100% to validator, no protocol fee.
• Red lines on all strategies: no sandwiching, no content-based censorship.
• Bundle control accounts for anti-frontrun.

Critique angle: Harmonic argues Jito is conflicted (infra + BAM + IBRL scoring). Harmonic is run by Temporal (also Nozomi, HumidiFi) — same critique applies.

Plain Agave#

• No bundles, no auction.
• Continuous banking stage: buffer drained by priority fee per CU + write-lock availability.
• Pre-leader: holds txs received shortly before its slots.
• Arrival matters to be in the buffer; ~first-come on quiet accounts.

Comparison#

Latency under batching#

• Arrival ~uniform vs batch boundary → Δ faster ≈ Δ/T more cutoffs won.
• 50ms batch: 1ms ≈ 2% more cutoffs; 1µs ≈ 0.002%.
• Batching kills the µs race, not the ms race.
• Race starts upstream: seeing state (shreds / ShredStream) + compute.
• Spam substitutes for latency (SWQoS drops, lock conflicts, skipped slots) → high failed-tx rate.

TPU path vs bundles#

TPU + priority fee

• Works on every leader (incl. Harmonic via Remote TPU).
• Pays on failure. No atomicity. Can lose contested accounts to Jito bundles in the same tick.
• Needs: staked QUIC (SWQoS), direct leader targeting (current + next few), tight CU limit.

Bundle

• Atomic, ordered, no pay on fail, anti-frontrun controls.
• Leader-dependent (Jito/BAM or Harmonic).

Dual-send: send both; ensure mutual exclusion (same signature or durable nonce).

Sender playbook#

1. Resolve leader client + strategy per slot.
2. Jito / BAM leader → bundle + tip-account transfer; optionally race a TPU copy.
3. Harmonic / plain Agave → TPU with priority fee; Harmonic bundle if atomicity needed.
4. FIFO leader → speed only; extra fee buys nothing.
5. Always: staked QUIC, leader targeting, minimal write locks, size tips off getTipFloor, multi-region Block Engine submit, resubmit per slot.

Sources#

• Harmonic docs: https://docs.harmonic.gg/ · scheduling strategies · FAQ
• Jito low-latency send docs: https://docs.jito.wtf/lowlatencytxnsend/
• Blockworks Q2 2026 Jito report (via Solana Compass)
• Solana Compass, BAM preconfirmations (Sep 11, 2026)
• SolanaFloor / Syndica client distribution (Mar 2026)
• Shoal Research, block building on Solana
• Figment, anatomy of a Solana validator