A recent blockhash does not stay recent

Why the message carries a hash at all

Every Solana transaction message includes a recent blockhash. It binds the message to a moment on the cluster and stops an old signed message from being replayed days later. When we read a transaction that already landed, the blockhash is a record of when it was allowed to land. When we are shown a signed message that never landed, the blockhash is the first thing we check.

The cluster accepts a blockhash for 151 slots. Slots are short, on the order of a few hundred milliseconds, so the practical window is about a minute. “Recent” is a strict word here.

Expired, unknown, or already used

If the blockhash is older than that window, the transaction will not be accepted. The usual reply is that the blockhash was not found. The fix, outside this room, is to rebuild the message with a new blockhash and sign again. We do not rebuild it in the sitting. We show the field and say why the old paper is stale.

A signature that did land is a different case. The blockhash served its purpose, the slot is closed, and rereading the field will not “refresh” anything. Visitors sometimes bring yesterday’s signature hoping to push it again. The walkthrough then becomes a reading of a finished transaction, which is still useful, and we say so before the printout is marked up.

How we use the field at the table

On the printout the blockhash sits near the header, and we read it before the instructions. If you are comparing two attempts of the “same” transfer, differing blockhashes mean they are different messages, even when the account list looks identical. Same accounts, new hash, new signature.

The clock on the reading-room wall is there for people, not for slots. We do not pretend we can time 151 slots with it. We use it so a visitor who arrived early can see the hour they booked.

If this is the field you want marked on your own signature, reserve a signature walkthrough or book a reading hour and name the guide.