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Every trade event on a chain is appended as a leaf to that chain’s Merkle Mountain Range (MMR), an append-only tree kept by the MerkleManager contract. Its root commits to every leaf ever added, and every cross-chain proof is built against it. Only the escrow contracts can append. The fundamentals are unchanged from v1 (append-only, Poseidon2-hashed); this page covers what v3 records.

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Leaves with a domain

A leaf commits to the order hash and a domain, the kind of event:
v2 recorded deposits only. v3 also records cancels, settled payouts and forfeits, so the other chain can act on how a trade ended. The full list is on Deposit proofs. The domain says where the leaf is used: a lock on the ad chain is used by the payout on the order chain, and a deposit on the order chain by the payout on the ad chain. A proof about one event can never be bent to serve another.

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Roots are what you co-sign

Both chains’ MMR roots are fields of the SettlementAuth each party signs. Your signature authorizes settlement against a specific, known state of both trees. The cancel and forfeit paths use roots published by the anchor service instead.

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The relayer’s copy is rebuildable

The relayer keeps a local copy of each chain’s tree to build proofs quickly. The copy is a projection of on-chain events, not a source of truth: it replays the chains’ append events and checks its root against the on-chain root at every step. Wiped, it rebuilds from the chains alone.

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Build on it

The proofbridge-mmr npm package produces byte-identical outputs to the on-chain contracts. Use it to rebuild a tree from chain events, generate inclusion proofs offline, or check settlement state independently.