Tamper-evident receipts for AI agent actions: a hash chain in 20 lines

Syen Systems — Wed, 14 Oct 2026 05:06:04 +0000

When an AI agent spends money, sends an email, or changes a record, someone eventually asks the same question: what exactly did it do, and can I trust this log? A normal log line can be edited after the fact. A receipt that is chained to the one before it cannot be edited quietly.

The idea

Each action gets a receipt. The receipt holds what was decided, the inputs the agent saw, who approved it, and a timestamp. It also holds the hash of the previous receipt. Change any old receipt and every hash after it stops matching.

import hashlib, json, time

def seal(prev_hash: str, action: dict) -> dict:
    body = {"ts": time.time(), "prev": prev_hash, "action": action}
    digest = hashlib.sha256(json.dumps(body, sort_keys=True).encode()).hexdigest()
    return {**body, "hash": digest}

def verify(chain: list[dict]) -> bool:
    prev = "genesis"
    for r in chain:
        body = {k: r[k] for k in ("ts", "prev", "action")}
        ok = r["prev"] == prev and r["hash"] == hashlib.sha256(
            json.dumps(body, sort_keys=True).encode()).hexdigest()
        if not ok:
            return False
        prev = r["hash"]
    return True

What to put in a receipt

The decision and the options that were considered, not only the winner.
The data the agent used (prices, documents, policy version) or a hash of it.
The human or policy that approved it.
The agent identity and the version of its instructions.

What it does not do

A hash chain shows that records were not changed after sealing. It does not prove the agent made a good decision, and it does not stop someone rewriting the whole chain from the start unless you anchor the latest hash somewhere you do not control (a timestamping service, a signed commit, a counterparty's copy). Keep that limit in mind when you describe it.

Where this shows up

Compliance rules for automated systems increasingly ask for logs of what a system did and why. Receipts like this are one way to keep those records in a form a third party can check without trusting you.

I write about agent audit trails and receipts for SYEN Systems (syensystems.com).

DEV Community: Syen Systems

Tamper-evident receipts for AI agent actions: a hash chain in 20 lines

The idea

What to put in a receipt

What it does not do

Where this shows up