sm-feed 0.2.0

The Verifiable Agent Feed — a signed, hash-chained, cursor-based append-only feed a subscriber can prove is complete and untampered.

sm-feed

The Verifiable Agent Feed — a signed, hash-chained, cursor-based append-only feed a subscriber can prove is complete and untampered.

An agent that publishes an ongoing stream — claims, lifecycle events, aggregate signals — is served today by an RSS feed or a bespoke webhook. Both ask the subscriber to trust the server for completeness: a feed can silently drop or reorder an item and no one can tell. sm-feed closes that gap. Every entry is Ed25519-signed and hash-chained, so a subscriber that walks a contiguous run verifies authenticity, integrity, and completeness in one pass. It owns the feed — the wire shape and its verification — and nothing else: the transport, the storage, and the meaning of the payload are the consumer's. The payload is opaque (any JSON object with a type), so one format carries any agent's stream.

What this package secures (v0.1)

• Authenticity — each entry is Ed25519-signed by the feed's issuer DID.
• Tamper-evidence — entry_hash content-addresses the entry; any edit to any field breaks it and verification returns falsy (never raises on hostile input).
• Completeness — entries are hash-chained (prev_hash) with contiguous seq; a dropped or reordered entry breaks the chain from the subscriber's anchor.
• Head attestation — a signed head ({seq, entry_hash}) lets a subscriber pin "the feed as of here" and detect a later rewind. Rewind detection requires passing that pinned head back as expected_head; the check is inert without it (SPEC §5 rule 6).
• Adversarially tested. The conformance corpus includes a hostile vector per failure path: dropped entry, tampered payload, wrong anchor, forged head.

What this package does NOT (yet) do

• Actively surfacing forks across subscribers — equivocation detection from two conflicting signed heads ships (detect_equivocation, SPEC §7), but the gossip transport that brings a peer's head to you, and witness co-signing to catch a fork before two heads meet, are the consumer's / a later property.
• Merkle inclusion proofs — a deliberate non-goal, not deferred: the chain is strictly linear and completeness is proven by walking prev_hash (SPEC §8).
• Push delivery, retry, and subscription registration — the consumer's responsibility; this primitive fixes only the payload and its verification.
• Detecting a withheld or stale view — an issuer that freezes one subscriber at seq = 40 while serving others through seq = 90 has not tampered, rewound, or forked, and nothing here catches it. Completeness is a claim about the run between two anchors, never about being current (THREATMODEL.md).
• A trusted clock or key custody — issued_at is caller-asserted; key generation/rotation/revocation is out of scope. SPEC Appendix A reserves a feed/key-rotation payload convention, non-normatively.

Features

• One dependency (sm-arp, for the shared Ed25519 / JCS / did:key crypto).
• Pure, wall-clock-free core: the caller supplies timestamps.
• Pull (GET …/feed?since=<cursor>) and push (POST a page) deliver the same object; verify_page checks both.
• Bounded pages — a publisher may serve a prefix of a long backlog with the head running ahead of it, so a transport has a conformant way to cap a response body (SPEC §4.1).
• Opaque typed payload — the same wire format serves claims, events, or deltas.
• Compaction — a late subscriber adopts a signed state checkpoint as genesis and walks forward, instead of replaying from the start (adopt_checkpoint).
• Equivocation proofs — two conflicting signed heads yield a non-repudiable proof that an issuer forked its feed (detect_equivocation).

Installation

pip install sm-feed

The reference implementation is on PyPI; this repository is not yet public. The published sdist carries the specification, the threat model, and the conformance corpus, so a second implementation can be built from the package alone.

Quick start

from sm_arp import Identity
from sm_feed import FeedLog, verify_page

issuer = Identity.generate()
log = FeedLog(issuer)
log.append({"type": "example/claim", "title": "First"}, issued_at="2026-01-01T00:00:00+00:00")
log.append({"type": "example/claim", "title": "Second"}, issued_at="2026-01-01T01:00:00+00:00")

# Serve on pull (GET /feed?since=<cursor>) or POST on push — same object.
# limit= serves a bounded prefix of a long backlog (SPEC §4.1).
page = log.page(cursor=None, generated_at="2026-01-01T02:00:00+00:00")

# Subscriber: verify authenticity + completeness, keep the new cursor.
ok, reason, cursor = verify_page(page, expected_prev_hash=None, expected_head=None)
assert ok, reason

# Persist both, and pass them back on the next pull:
#   cursor["entry_hash"] -> expected_prev_hash
#   cursor["head"]       -> expected_head   (this is the rewind defence — pass it)
# cursor["complete_to_head"] is False while a bounded backlog is still draining.

A runnable end-to-end example (publish → verify → detect a dropped entry) is in examples/quick_start.py.

Reference fixtures

The golden vectors under conformance/vectors/ are regenerated by conformance/_vector_gen.py from a fixed, non-secret fixture seed ("11"×32 — regenerable, never a real key) and replayed by tests/test_conformance_vectors.py. They are the language-agnostic corpus a second implementation replays to prove interoperability.

Specification

• SPEC.md — normative wire shape and verification, working draft.

• WHITEPAPER.md — design rationale and axioms.

• THREATMODEL.md — what it defends, and what it doesn't. Worked payload profiles (non-normative), each with a runnable example:

• capability-announcement — an agent announcing what it can do, and withdrawing it. The first-person case: issuer and subject are the same DID. (example)

• registry-changelog — a registry exposing its mutations. The same shape in the third person; a registry that subscribes to announcement feeds aggregates them into one of these. (example)

Related packages

Package	Role
sm-arp	Agency Receipt Protocol — the signed, hash-chained record primitive sm-feed reuses for crypto
sm-authority	Common Authority Evidence — establishes who controls a subject
sm-bridge	AgentFacts / registry endpoints — a natural feed publisher and consumer

License

MIT

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First published: 2026-07-30 | Last modified: 2026-08-02

Personal research contributions aligned with Project NANDA standards. Stellarminds.ai