A cancel produces a Groth16 proof that:

• Spends the maker’s existing leaf (advances the nullifier).
• Re-deposits the same balance under a fresh commitment.

The original order’s nonce is invalidated, so the relayer can’t match it after cancellation goes on-chain.

When to cancel

• The order has not yet been matched.
• You want to change side, price, or expiry.
• The relayer you submitted to has gone offline and can’t return your order.

If the order has already settled, there’s nothing to cancel — claim the payout instead.

Code

import { generateCancelProof, randomFieldElement } from "@zkscatter/sdk/zk";
import { callCancel } from "@zkscatter/sdk/contracts";
 
const cancelResult = await generateCancelProof(
  {
    note: storedNote.note,
    leafIndex: storedNote.leafIndex,
    merkleProof,
    eddsaPrivateKey: eddsa.privateKey,
    originalOrderHash,                // hash of the order being cancelled
    nonce: originalNonce,
    newSalt: randomFieldElement(),
  },
  { wasm: "/zk/cancel.wasm", zkey: "/zk/cancel_final.zkey" },
);
 
const tx = await callCancel(
  signer,
  network.contracts.privateSettlement,
  cancelResult,
);
await tx.wait();
 
// Replace the old note with the rolled-forward one.
await notes.put({
  ...storedNote,
  id: cancelResult.newCommitment.toString(16),
  commitment: cancelResult.newCommitment,
  leafIndex: -1,
  createdAt: Date.now(),
});
await notes.remove(storedNote.id);

Notify the relayer

The relayer doesn’t strictly need a hint — once the cancel is on-chain, the original nonce is unspendable. But notifying through the relayer API frees up the slot in their order book sooner:

// SDK signature: cancelOrder(address, nonce, signature, signal?).
// `nonce` must be the numeric nonce the relayer indexed.
await client.cancelOrder(account, Number(originalNonce), signature);

(Endpoint shape depends on the relayer’s HTTP API. Check getInfo() for capabilities.)

Race with matching

A cancel proof submitted at the same time as a settle is a race — only one can win because both consume the same nullifier. If your cancel reverts with nullifier reused, the order has already settled. Refresh the order list and treat it as filled.

Common errors