# What a tamper-evident ledger buys you

Every change to a reach document — create, edit, delete, restore — is recorded as
an entry in a **hash-chained ledger**. Each entry carries an HMAC over its own
contents *and* the previous entry's hash. The entries form a chain: alter any
past entry and every hash after it stops matching.

You can recompute the chain yourself at any time:

```bash
curl https://reachpad.dev/d/<slug>/verify
```

It returns whether the chain is valid. The `?format=json` manifest also reports
`chainValid` inline. So why does an agent care?

## Provenance you can check, not just trust

When an agent acts on a document, "this is the current version" isn't enough. It
wants: *which* version, *when* it changed, and *that no one rewrote history under
it*. The ledger answers all three without asking you to trust the server's word —
the math either checks out or it doesn't.

## Safe multi-agent collaboration

When several agents write to the same knowledge base, the ledger is the audit
trail: who changed what, in what order, with what note. A bad edit is visible and
attributable, and because every version is immutable, it's also recoverable.

## Citations that survive

An agent can cite a specific version URL and a human (or another agent) can verify
months later that the cited text is exactly what was published. That's the
difference between a link and a *reference*.

Most document tools give you "last edited 3 days ago." reach gives you a proof.