Fast Confirmation Rule on Mainnet
Question
Would the Fast Confirmation Rule work on Ethereum mainnet?
Background
The Fast Confirmation Rule (FCR) is a way for consensus clients to locally confirm blocks within seconds of the attestation deadline, rather than waiting ~13 minutes for FFG finality. No hard fork needed, it's a client-side check.
The full algorithm operates on the fork choice store: it starts from a safe checkpoint, computes LMD-GHOST scores using each validator's vote and the committee shuffling for the current and previous epochs, discounts proposer boost, and iteratively advances a confirmed_root through the canonical chain. A block is confirmed when its support is strong enough that no adversary (up to β fraction of stake) could cause a reorg. If a block doesn't get enough support from its own slot's committee, it can still confirm as attestations from subsequent slots accumulate.
Investigation
How many slots would confirm?
1,074,543 slots from Nov 7 2025 to Mar 17 2026 (131 days). Source: fct_attestation_correctness_canonical on xatu-cbt.
Per slot, we check what fraction of the committee voted for the head block (votes_head / votes_max). If it clears the threshold, that slot would've been fast confirmed.
View Query: fcr_head_vote_daily
At the 95% threshold, 96.9% of slots would fast confirm. The worst day was December 4 during Fusaka-related network disruption, but even then 80% of slots still cleared the bar. Drop to a 90% threshold and the overall rate hits 98.4%.
| Threshold | Overall | Worst Day (Dec 4) |
|---|---|---|
>= 95% (β = 0.25) | 96.9% | 80.4% |
>= 90% | 98.4% | 95.1% |
>= 85% | 98.8% | 97.9% |
>= 80% | 99.1% | 99.0% |
The ~3% that miss the 95% threshold are slots with late blocks, missed proposals, or low participation.
Would any reorged block be confirmed?
1,900 orphaned blocks from Nov 7 2025 to Mar 17 2026. Source: fct_block_proposer FINAL joined with int_attestation_attested_canonical on xatu-cbt.
For every orphaned block, we check how much of the committee actually voted for it by matching each validator's beacon_block_root vote against the orphaned block's root.
No. None came close:
| Metric | Value |
|---|---|
| Orphaned blocks checked | 1,900 |
Would confirm at >= 95% | 0 |
Would confirm at >= 80% | 0 |
Would confirm at >= 50% | 0 |
| Max support any orphan received | 47.8% |
| Average support | 2.1% |
| Median support | 0.4% |
| Not seen by committee at all | 20% |
Reorged blocks are blocks the committee never saw. The median orphan had 0.4% support. FCR wouldn't touch any of them.
How fast would blocks confirm?
372 slots sampled from Mar 3-17 2026, observed by 46 sentry nodes across 5 cities. Source: libp2p_gossipsub_beacon_attestation and libp2p_gossipsub_aggregate_and_proof on the xatu cluster.
For each slot, we decode aggregation bitfields from gossipsub messages and track cumulative unique validator support over time to find when the 95% threshold is crossed.
In this sample, 97% of slots confirm with a median time of 8,127ms after slot start, about 4.1 seconds after the attestation deadline. Geographic spread is tight: Helsinki confirms at ~8.1s, Sydney at ~8.25s, less than 200ms apart.
Note that this data only derives attestations from aggregates. By default, nodes would also observe 2/64 of the committee via unaggregated attestations, resulting in faster confirmations. Nodes that are manually subscribed to all subnets would most likely confirm even faster.
Takeaways
- 96.9% of slots would fast confirm at the 95% threshold across 131 days and ~1M slots, hitting 97%+ on most days
- Zero reorged blocks would be confirmed: the highest support any orphaned block received was 47.8%, with a median of 0.4%
- Blocks confirm ~4 seconds after the attestation deadline in a 372-slot sample, with under 200ms variation across 5 cities