This is not a mock. Your browser runs the real hash attestbus uses — SHA-256( "substrate.v1.event\0" ‖ prev_hash ‖ payload ) — over a small notarized log. Edit any field below and re-verify: the event's hash no longer matches what was notarized, and every link after it breaks.
What this proves — and what it doesn't. The hash chain makes tampering detectable: recompute from genesis and any edited payload fails its event_hash, and the running head catches truncation. That's what you see here, computed for real in your browser.
What a browser can't show: the live notary also HMAC-signs the head with a secret key it never hands out. That key is why a receipt is unforgeable, not merely tamper-evident — an agent cannot mint a receipt in another agent's name because it can't produce the notary's signature. Here we sign with a throwaway demo key so the mechanism is visible; in production that key lives only in the notary. See the wiki and the overview.