EIP-8390: Remove the Sync Committee

Remove the onchain Altair sync committee and its rewards, superseded by offchain ZK proofs

Abstract

Remove the sync committee from the consensus layer, and with it the Altair light client protocol, which depends on it entirely. SYNC_REWARD_WEIGHT goes too, so consensus issuance falls by 2/64.

The sync committee exists because a light client could not verify the whole validator set. ZK proving has since made that verification practical, so the sample is no longer needed.

Motivation

The sync committee is a sample of 512 validators, resampled every 256 epochs, about 27.3 hours. A light client believes a header when two thirds of that sample sign it. It exists because a light client in 2021 could not verify the whole validator set.

Three properties make it a poor foundation.

It is not slashable. Altair defines no slashing condition for sync committee messages. A corrupted committee can sign a header for a chain that does not exist and lose nothing. EIP-7657 proposes adding a slashing condition, which is the clearest evidence that none exists today.

It is a small sample. The active set is 901,505 validators holding 42,328,615 ETH. The committee is 512 of them, one seat per 1,761 validators. A light client that follows it is backed by 0.06% of the validator set, rotated daily, with no penalty for lying.

Issuance pays for it. SYNC_REWARD_WEIGHT is 2 against a WEIGHT_DENOMINATOR of 64, so the committee takes 1/32 of consensus issuance. At the current stake that is about 33,800 ETH a year, out of about 1,082,000 ETH.

The replacement is feasible today

A light client could not check a million BLS signatures an epoch in 2021, so it was given 512 to check. Succinct proofs remove that limit.

Proving Casper FFG finality over the full active validator set now fits within one epoch on a single GPU. Verifying such a proof takes milliseconds and needs no key material beyond a commitment to the verifying program. The same proof verifies on other chains, where the sync committee was never usable. Implementations exist today.

A light client backed by such a proof inherits the FFG guarantee. To reverse a finalised checkpoint, a two-thirds coalition must surround its own vote, and a third of the stake is slashed. The sync committee carries no comparable penalty.

Specification

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in RFC 2119 and RFC 8174.

Names used below without definition are defined in the ethereum/consensus-specs repository. EIP8390_FORK_EPOCH is the activation epoch.

Execution layer

This requires no changes to the execution layer.

Consensus layer

Removed constants

SYNC_COMMITTEE_SIZE, EPOCHS_PER_SYNC_COMMITTEE_PERIOD, SYNC_COMMITTEE_SUBNET_COUNT, TARGET_AGGREGATORS_PER_SYNC_SUBCOMMITTEE, SYNC_COMMITTEE_BRANCH_DEPTH, and the light client generalized indices.

SYNC_REWARD_WEIGHT. WEIGHT_DENOMINATOR stays at 64, and the weight is not redistributed.

DOMAIN_SYNC_COMMITTEE, DOMAIN_SYNC_COMMITTEE_SELECTION_PROOF and DOMAIN_CONTRIBUTION_AND_PROOF. Implementations MUST NOT reuse these DomainType values.

Removed containers

SyncAggregate, SyncCommittee, SyncCommitteeMessage, SyncCommitteeContribution, ContributionAndProof, SignedContributionAndProof, SyncAggregatorSelectionData, LightClientBootstrap, LightClientUpdate, LightClientFinalityUpdate, LightClientOptimisticUpdate and LightClientHeader.

Removed functions

process_sync_aggregate, process_sync_committee_updates, get_next_sync_committee, get_next_sync_committee_indices, and every light client helper built on them.

Modified BeaconBlockBody

Remove the sync_aggregate field. All other fields keep their order and type.

Modified BeaconState

Remove the current_sync_committee and next_sync_committee fields. All other fields keep their order and type.

Modified process_block

Remove the call to process_sync_aggregate.

def process_block(state: BeaconState, block: BeaconBlock) -> None:
    process_block_header(state, block)
    process_withdrawals(state, block.body.execution_payload)
    process_execution_payload(state, block.body, EXECUTION_ENGINE)
    process_randao(state, block.body)
    process_eth1_data(state, block.body)
    process_operations(state, block.body)
    # process_sync_aggregate(state, block.body.sync_aggregate)  # [Removed in EIP-8390]

Modified process_epoch

Remove the call to process_sync_committee_updates.

Rewards

get_flag_index_deltas and get_proposer_reward do not change. SYNC_REWARD_WEIGHT was the only user of the remaining 2/64, so that share is not issued.

Networking

Remove the gossip topics sync_committee_{subnet_id}, sync_committee_contribution_and_proof, light_client_finality_update and light_client_optimistic_update.

Remove the syncnets ENR field. Remove the Req/Resp methods LightClientBootstrap, LightClientUpdatesByRange, LightClientFinalityUpdate and LightClientOptimisticUpdate.

For slots at or after EIP8390_FORK_EPOCH, a node MUST NOT publish on these topics. A node MUST ignore messages received on them.

Validator duties

Remove sync committee assignment, message production, subnet subscription, aggregator selection and contribution production.

Fork transition

The fork upgrade function copies every retained field and drops the two sync committee fields. For slots at or after EIP8390_FORK_EPOCH, a block that contains a sync_aggregate is invalid.

Rationale

Removal instead of repair

EIP-7657 makes sync committee messages slashable. That improves the current state and it is not enough. Slashing raises the price of corrupting the committee to the stake of 512 validators. It does not make the committee a claim about the validator set. A light client still trusts a sample, still assumes an honest majority inside it, and still pays 1/32 of issuance. A proof of what two thirds of the stake attested to leaves the sample no job.

The reward weight is not redistributed

Removing SYNC_REWARD_WEIGHT and leaving WEIGHT_DENOMINATOR at 64 cuts issuance by 1/32 and needs no other change. Lowering the denominator to 62 keeps issuance flat instead. Issuance policy is out of scope. Not paying for a removed mechanism is not.

The domain types are retired

DomainType values are cheap. Retiring three of them stops any pre-fork sync committee signature from being valid in a later context.

Backwards Compatibility

This is a consensus-layer breaking change and needs a coordinated fork.

It breaks every deployed Altair light client. A client that syncs through LightClientUpdate stops working at the fork.

Beacon API. These endpoints are removed: POST /eth/v1/beacon/pool/sync_committees, GET /eth/v1/beacon/states/{state_id}/sync_committees, POST /eth/v1/validator/duties/sync/{epoch}, GET /eth/v1/validator/sync_committee_contribution, POST /eth/v1/validator/contribution_and_proofs, POST /eth/v1/validator/sync_committee_subscriptions, and /eth/v1/beacon/light_client/*.

Encoding. BeaconState and BeaconBlockBody change shape. Tooling that decodes them per fork must be updated. States and blocks from before the fork stay decodable under their own fork schema.

Validators. Stakers need to do nothing. Sync committee duties stop being assigned. Staking yield falls with issuance, by 1/32 of the consensus component.

Security Considerations

Activating before a replacement

Removing the sync committee removes the only in-protocol way for a light client to follow the chain. Before this EIP activates, public proving infrastructure must exist that publishes finality proofs with characteristics its consumers can rely on. This EIP does not specify that infrastructure, and scheduling is decided by client teams rather than by this document.

Proof production is not incentivised

This EIP adds no in-protocol incentive to produce finality proofs, and does not propose one.

Signature replay through a reused domain

DOMAIN_SYNC_COMMITTEE (0x07000000) and DOMAIN_SYNC_COMMITTEE_SELECTION_PROOF (0x08000000) are retired. If a later message type takes either value, signatures made by pre-fork sync committees verify against it.

Copyright

Copyright and related rights waived via CC0.

Citation

Please cite this document as:

Lion (@dapplion), "EIP-8390: Remove the Sync Committee [DRAFT]," Ethereum Improvement Proposals, no. 8390, August 2026. Available: https://eips.ethereum.org/EIPS/eip-8390.