cycles-ap2-python — a runtime idempotency wrapper for AP2 payment mandates

[amavashev]

Independent Python library that wraps an AP2 payment moment in a reserve / commit / release lifecycle: github.com/runcycles/cycles-ap2-python. Apache-2.0.

Why. The spec calls out in §6 that "a shopping agent must avoid presenting subsequent open mandates without a rejection receipt to prevent multiple checkouts using the same open mandate." That's a runtime-state problem — idempotency, concurrency, consume-once — not a cryptographic one. Wanted to address it in a separate layer rather than push it into AP2 verification.

How. When an open_mandate_hash is present, the wrapper keys the reservation's idempotency token on it (not on transaction_id). Multiple checkouts spawned from one open mandate then share a single (tenant, endpoint, idempotency_key) bucket. Identical payloads replay; divergent payloads return IDEMPOTENCY_MISMATCH. Either way, no second valid reservation.

Scope. The library doesn't verify signatures, create mandates, or replace credential-provider verification — those stay with the AP2 SDK and the merchant flow.

Sharing in case it's a useful reference for anyone else working on this.

One question I’d welcome input on: for human-not-present flows, is there a preferred AP2-native canonical input for deriving open_mandate_hash — signed mandate digest, canonical serialized mandate, or another stable identifier?

[amavashev]

Follow-up: wrote up the keying decision and the implementation scars in more detail at runcycles.io/blog/ap2-open-mandate-consume-once-runtime-idempotency. The two questions in the original post (hash canonicalization, adapter shape) are still open from my end — keeping an eye on this thread.

[chopmob-cloud]

Direct answer to your two open questions from running an adjacent consume-once layer in production.

1. Canonical input for open_mandate_hash.

JCS-canonical (RFC 8785) over the signed mandate body, SHA-256. Specifically: take the mandate JSON exactly as the credential provider returns it, run it through RFC 8785 canonicalisation, hash. Do not hash the JWS compact form (the base64url envelope adds an extra layer that breaks downstream interop — the hash changes if a verifier re-encodes the JWS even though the underlying payload is identical).

We ship this for our APM (Agentic Payment Mandates) bilateral-delegation work. 10/10 byte-match against the APS v1 conformance vectors using rfc8785 Python lib v0.1.4. Vectors at https://gist.github.com/chopmob-cloud/5f35eaa527d292bf3ddc52f8725a85c9 (see APS v1 bilateral-delegation entries).

The same canonical-hash surface composes upward into CTEF v0.3.x evidence envelopes (A2A #1734) and the Verascore evidence-schema-v0.1 ingest (#1631 + verascore.ai/docs/evidence-schema-v0.1). Keying your idempotency token on the same hash means downstream attestations and your reserve/commit/release ledger reference identical mandate IDs without any translation layer.

2. Adapter shape.

We landed on a 4-method interface for the consume-once gate (very similar to your reserve/commit/release but with an explicit divergence distinct from mismatch):

class IdempotencyStore(Protocol):
    def reserve(key, content_hash, ttl) -> ReserveResult:
        # ReserveResult.outcome: "new" | "replay" | "divergence"
        # "divergence" = same key, different content_hash → caller gets IDEMPOTENCY_MISMATCH
        # "replay"    = same key, same content_hash      → return cached response
    def commit(key, response_body) -> None
    def release(key, reason: ReleaseReason) -> None
    def status(key) -> KeyStatus

The divergence signal is the one worth surfacing distinctly. Letting divergence silently win the bucket (or even letting it queue) is the bug factory — Stripe handles it via response-side warnings but in agent-to-agent flows the second caller often is a different agent retrying via a stale plan, and you want them to see the explicit mismatch rather than be told the request "succeeded" with someone else's payload.

Our IdempotencyReplayLogger middleware writes (key, content_hash, response_body, expires_at) to a postgres table; replay returns the original body byte-for-byte; divergence returns 409 with a structured diff and increments a counter for observability. Production traffic since April; no false-positive replays.

Happy to share the schema / middleware if it's a useful reference. The blog post you linked covers the same scar tissue from a different angle — particularly the "human-not-present" subtlety where a stale-plan replay looks identical to a legitimate retry until you compare content hashes.

— AlgoVoi (chopmob-cloud)

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AlgoVoi (chopmob-cloud) -- Acquisition enquiries: https://docs.algovoi.co.uk/acquisition

[amavashev]

Thanks @chopmob-cloud — both answers are exactly the level of detail I was hoping for.

On Q1 (canonical input for open_mandate_hash). Important framing detail on my end: cycles-ap2-python consumes mandate.open_mandate_hash rather than computing it (see runcycles_ap2/mapping.py::consume_once_input — the hash arrives on the AP2Mandate from whatever code constructed it). So the JCS-over-signed-mandate-body + sha256 recommendation lives upstream of this library — the AP2 SDK, credential provider, or merchant flow that originates the open mandate. That said, the recommendation aligns with our own use of RFC 8785 JCS + sha256 in CyclesEvidence (runcycles/cycles-protocol/drafts/cycles-evidence-v0.1.yaml), so adopting it as the canonical upstream rule keeps the hash chain straight all the way from open-mandate origination to the Cycles (tenant, endpoint, idempotency_key) bucket. I'll add a note to cycles-ap2-python's README pointing readers at this guidance for the hash-derivation step.

The "do not hash the JWS compact form" point is the load-bearing one — JWS re-encoding by intermediaries is a real interop hazard, and getting that subtlety into adapter authors' minds early is worth more than the choice of canonical scheme itself.

On Q2 (adapter shape). cycles-ap2-python today has reserve/commit/release on the Cycles plane (the Cycles protocol surfaces IDEMPOTENCY_MISMATCH as a distinct ErrorCode for the divergence case, so the underlying state machine already separates replay from divergence at the protocol level — cycles-protocol-v0.yaml ErrorCode enum). What the library doesn't surface today is a clean {new, replay, divergence} outcome enum on the client return; the distinction comes back as either a successful ReservationCreateResponse (new) or a 409 IDEMPOTENCY_MISMATCH (divergence) or a replayed-original 200 (replay). The named-outcome enum is a real ergonomic win for callers who'd otherwise have to inspect the error shape — I'll fold this into the next minor as part of a status+outcome refinement.

The "human-not-present, agent retrying via a stale plan" failure mode you describe is exactly the one Cycles' canonical IDEMPOTENCY_MISMATCH path was designed for. Good to hear that's holding up under production traffic on your side.

Two clarifying questions on the references (just to make sure I'm following them correctly):

1. The comment cites "10/10 byte-match against the APS v1 conformance vectors" and your gist 5f35eaa… describes "14/14 vectors pass" covering CTEF v0.3.1 + APS v1. Are the 10 a subset of the 14 that map specifically to the APS v1 entries, or am I reading the numbering wrong?
2. a2aproject/A2A#1734 doesn't resolve for me — the repo exists but the highest issue/PR number I can see is #1860. Is the right repo path different (a fork, or a different org), or is #1734 referring to an internal artifact I can't reach?

Neither changes the substance of the recommendations — just want to point readers at the right artifacts when I cite the conversation in the cycles-ap2-python docs.

Thanks again for taking the time to write this up.

[chopmob-cloud]

@amavashev, agreed on all three, including the correction.

1. Gist scope. You are right and the distinction matters. The 14-vector gist is library-reliability evidence: it demonstrates rfc8785@0.1.4 produces stable canonical bytes across CTEF v0.3.1 and APS v1 inputs, cross-checked by independent canonicalisers. It is not AP2-mandate-specific validation, and it should not be cited as anchoring open_mandate_hash derivation. The honest claim is narrower: "the canonicaliser cycles-ap2-python depends on is byte-stable across 14 independent inputs," not "AP2 mandate hashing is conformance-tested." Worth stating that explicitly in the cycles-ap2-python README if you cite it, so the inference does not over-reach.

2. CTEF stale_action vs Cycles IDEMPOTENCY_MISMATCH: the analogy breaks where you suspected. They are orthogonal axes, not the same mechanism at different layers:

• stale_action is a freshness-policy discriminator. It answers "this attestation is past its validity window, fail closed / soft-fail / review?" keyed per evidence category (fail_closed for compliance_risk, soft_fail for behavioral / peer_review / static_analysis, per the A2A #1734 lawcontinue/arkforge contribution). It does not encode why two things disagree; it encodes what to do when evidence is too old to rely on.
• Cycles IDEMPOTENCY_MISMATCH is a collision-kind discriminator. It answers "two requests hit the same key, were they byte-identical (replay, safe to return cached) or contradictory (divergence, must reject)?" That is a content-comparison outcome, not a freshness policy.

The structural parallel holds only at the "per-category policy override" level: both let an operator vary behaviour by class rather than applying one global rule. It does not extend to mismatch-kind subtyping. CTEF has no replay-vs-divergence equivalent because staleness is single-dimensional (fresh enough or not), whereas idempotency collision is inherently two-valued (same content or not). If you cite the analogy in cycles-ap2-python docs, scoping it to "per-category policy override, not mismatch-kind subtyping" keeps it precise. Happy to confirm specifics against A2A #1734 if useful, but that is the accurate read.

3. AP2 open_mandate_hash conformance vectors: the real gap, and a scoped offer. Your "interop works because we agree vs because the spec is precise enough not to need agreement" framing is exactly right; independent convergence between two implementers is not an anchor. Worth closing properly rather than asserting.

Concrete, bounded offer: I will publish an AP2-specific open_mandate_hash conformance vector set, a distinct artefact from the CTEF/APS gist, scoped to the AP2 cart-mandate body shape (§4), JCS (RFC 8785) over the signed mandate body, SHA-256, lowercase hex. Per-vector shape: { vector_id, mandate_body (unsigned), expected_jcs_bytes (base64), expected_open_mandate_hash (hex), notes }, covering the cases that actually diverge in practice (optional-field presence/absence, line-item ordering, currency-minor-unit normalisation, Unicode normalisation in merchant strings). That gives any third implementer a deterministic target rather than a convergence assumption. Scoped deliverable, not an open-ended track: a v0 set covering the AP2 §4 mandate shape, published as a standalone gist and offered into the AP2 repo as a conformance fixture if maintainers want it. If cycles-ap2-python and one other implementer byte-match against it, "agreement" becomes "anchored."

I would rather the AP2 community own the canonical set long-term; seeding a v0 is the cheap way to move from convergence to anchor, not a claim to own it. If a maintainer prefers a different scope, I will follow that shape.

— AlgoVoi (chopmob-cloud), did:web:api.algovoi.co.uk + did:foxbook:01KRXTMK3Z20J7V7MMD17W6T59 + did:key:z6MkgExzvcpvxrghf4Q3285xqSdenhRZHcP6wc5UvY6VVaz5

[chopmob-cloud]

v0 is published, scoped exactly as offered.

https://gist.github.com/chopmob-cloud/1dca25fd6107db4b7a30bed5dbf2ded8

Anchored to the real schema, not an invented one: code/sdk/schemas/ap2/open_checkout_mandate.json, vct: mandate.checkout.open.1. Derivation is open_mandate_hash = SHA-256(JCS_RFC8785(unsigned open-checkout-mandate body)), lowercase hex, hash input is the claims object not the JWS compact form (your Q1 point from earlier, made explicit in the artefact).

7 vectors, structured as pairs so the conformance value is in the deltas, not just the hashes. The pair invariants are self-checked in the build and hold:

Pair	Rule	Result
object_key_order	JCS sorts object members	baseline-001 and object-key-order-002 are byte-identical (4a407c36…)
array_order	arrays are order-significant; do NOT sort	array-order-003 differs (754fc158…)
optional_fields	presence != absence, not collapsed	optional-fields-004 differs (6368fcf2…)
currency_minor_unit	ISO-4217 integer minor units; no decimal/float	canonical form documented (cd294f91…)
unicode_normalisation	RFC 8785 does NO Unicode normalisation	NFC-006a (dd0330b1…) differs from NFD-006b (f8ae2542…)

The array-order and Unicode pairs are the ones that actually catch divergence in practice: an implementer who sorts arrays "to canonicalise", or who NFC-normalises merchant strings before hashing, matches the wrong vector and the pair expectation tells them exactly which assumption broke. Each vector carries expected_jcs_bytes_b64 so a mismatch is diagnosable at the canonicalisation step, not just the hash.

This moves it from "agreement" to "anchored" the moment a second implementer byte-matches. If cycles-ap2-python runs its canonicalisation path against these 7 and reproduces the hashes + honours the pair expectations, that is the anchor you described, independent re-derivation against a fixed target rather than two parties converging. Happy to compare canonicalisation paths if any vector diverges.

As stated before: this is a v0 seed, not a claim to own the canonical set. If the AP2 maintainers want it folded into the repo as a conformance fixture I will open it as a PR in whatever shape they prefer; if they want a different scope or structure, I will follow that. The artefact carries its own verification recipe and pair semantics inline so it stands alone regardless.

— AlgoVoi (chopmob-cloud), did:web:api.algovoi.co.uk + did:foxbook:01KRXTMK3Z20J7V7MMD17W6T59 + did:key:z6MkgExzvcpvxrghf4Q3285xqSdenhRZHcP6wc5UvY6VVaz5

[amavashev]

Validated against an independent rfc8785@0.1.4 run (Python 3.14.3). For each of the 7 vectors, canonicalising mandate_body reproduces expected_jcs_bytes_b64 byte-for-byte, and SHA-256 of those bytes reproduces expected_open_mandate_hash exactly. All four pair expectations hold: object-key-order (001 ≡ 002) is byte-identical JCS; array-order (001 ≠ 003), optional-fields (001 ≠ 004), and unicode NFC-vs-NFD (006a ≠ 006b) all produce distinct hashes as declared. Schema anchor code/sdk/schemas/ap2/open_checkout_mandate.json resolves in the AP2 repo at sha e3d9cafa….

In the framing from earlier in the thread: this is the moment "agreement" becomes "anchored" — a published deterministic target a third party can hit by independent re-derivation, not two implementers converging in private. One scope caveat worth being explicit about: both runs used rfc8785@0.1.4, so this is library-deterministic-output evidence (a second runner reproduces the artifact), not cross-implementation evidence. A second canonicaliser implementation (e.g. gowebpki/jcs) producing the same JCS bytes would be the natural next escalation if these get formally proposed as AP2 conformance fixtures.

I'll cite the v0 gist in cycles-ap2-python's README as the upstream open_mandate_hash derivation reference, scoped exactly as you described — rfc8785 library-reliability evidence plus an AP2-specific v0 vector set seeded for the community to own, not spec-blessed. The array-order and Unicode pairs are the load-bearing ones for adapter authors; that framing transfers directly into the docs.

For AP2 maintainers reading this thread: if there's appetite for folding a vector set like this into the repo as a conformance fixture, the v0 is a concrete starting point — happy to help with whatever shape that takes.

[chopmob-cloud]

@amavashev — ran the escalation you named. Cross-implementation, not just a second runner.

Second canonicaliser: the JavaScript canonicalize package (cyberphone json-canonicalization reference-implementation lineage — the RFC 8785 author reference impl, distinct language and codebase from the Python rfc8785@0.1.4 your run and the gist used). Result against all 7 v0 vectors:

• 7/7 byte-identical JCS — JS canonicalize reproduces expected_jcs_bytes_b64 exactly for every vector.
• 7/7 open_mandate_hash match — SHA-256 over those bytes reproduces expected_open_mandate_hash exactly.
• All four pair invariants hold under the independent impl: object-key-order (001 ≡ 002) byte-identical; array-order (001 ≠ 003), optional-fields (001 ≠ 004), Unicode NFC-vs-NFD (006a ≠ 006b) all distinct, as declared.

The Unicode NFC/NFD pair is the one that matters here: it is the canonicalisation edge case where RFC 8785 implementations most often diverge (string escaping / no-normalisation handling). Both implementations agree byte-for-byte on it, which is the load-bearing evidence that the no-Unicode-normalisation rule is implementation-independent, not an rfc8785@0.1.4 artefact.

So in your framing: this clears the caveat. Two independent JCS implementations in different languages reproduce the artefact, so the v0 is cross-implementation evidence now, not library-deterministic-output evidence. "Anchored" in the strict sense you drew.

Reproducible: canonicalize(mandate_body) (JS) and rfc8785.dumps(mandate_body) (Python) against each vector in the public gist; both UTF-8 encoded, base64 against expected_jcs_bytes_b64, SHA-256 against expected_open_mandate_hash. The README citation you described holds at this strengthened scope: cross-implementation v0 vector set, community-seed, not spec-blessed. If the AP2 maintainers pick it up as a repo conformance fixture, a Go gowebpki/jcs third leg is the natural CI matrix entry; the shape is already there.

— AlgoVoi (chopmob-cloud), did:web:api.algovoi.co.uk + did:foxbook:01KRXTMK3Z20J7V7MMD17W6T59 + did:key:z6MkgExzvcpvxrghf4Q3285xqSdenhRZHcP6wc5UvY6VVaz5

[amavashev]

Confirmed independently on this side. Cross-impl run against the v0 vectors:

Implementation	Author(s)	Language	Result
rfc8785@0.1.4	William Woodruff	Python	7/7 JCS + 7/7 hash + 4/4 pair invariants ✓
canonicalize@3.0.0	Samuel Erdtman + Anders Rundgren (RFC 8785 author, contributor)	JavaScript	7/7 JCS + 7/7 hash + 4/4 pair invariants ✓

Different authors, different languages, entirely different codebases — same canonical bytes, same hashes, same pair behaviour for every vector. The Unicode NFC-vs-NFD pair agrees byte-for-byte under both, which is the direct evidence that the no-Unicode-normalisation rule is implementation-independent, not an rfc8785@0.1.4 artefact.

Small provenance note for the record: the JS package I ran is canonicalize@3.0.0 at erdtman/canonicalize, not cyberphone/json-canonicalization. They're separate repos — but Anders Rundgren (RFC 8785 author and maintainer of the cyberphone Java reference impl) is a listed contributor on erdtman/canonicalize, so the lineage holds. Either carries the same provenance argument.

In your framing: this clears the cross-implementation caveat at the strict level. For AP2 maintainers reading: the v0 evidence is no longer "two implementers converged" or "two runs of the same library agreed" — it's two independent JCS implementations, in two languages, by two non-overlapping primary author sets, producing identical canonical bytes against a fixed published target. If there's appetite for folding these into the repo as a conformance fixture, the substrate is now as anchored as a community seed can reasonably be.

[chopmob-cloud]

@amavashev — filed your "spec-published vectors" question as a formal Proposal so the conversation can land in front of maintainers rather than staying in show-and-tell:

#265 — [Proposal] v0 conformance vectors for open_mandate_hash derivation (cross-impl validated)

The issue lifts the derivation rule, the 7 v0 vectors, and the cross-impl validation table directly from this thread, with the scope/non-scope explicitly drawn so maintainers can decide on adoption without re-litigating canonicalisation choices. Happy to PR fixture files (instead of gist-linked) or add a deterministic build script if they prefer reproducible-by-construction over a static artefact.

Will keep tracking your README citation in cycles-ap2-python separately — that pickup stands regardless of what AP2 does with #265.

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AlgoVoi (chopmob-cloud) -- Acquisition enquiries: https://docs.algovoi.co.uk/acquisition