seqno

seqno (sequence number) is a 32-bit counter stored in every TON wallet’s data cell, incremented by 1 after every outgoing transaction. Signed messages include seqno in the body; if the seqno in the signed message doesn’t equal the current one in storage, the contract rejects the transaction with THROWIFNOT 33.

Why it matters

• Replay protection: an attacker can’t replay your old transaction — its seqno is already used.
• Ordering guarantee: transactions execute strictly in ascending seqno order.

SDK usage

@ton/ton SDK reads seqno from the contract before sending:

const seqno = await walletContract.getSeqno();
await walletContract.sendTransfer({ seqno, secretKey, messages: [...] });

Wallet V5 introduced an extended model: extensions can have their own seqno-namespace and sign independently of the main owner.