avow

Sign a record of what your software decided, then verify it offline — on someone else's machine, years later, with no access to your database. Edit one character of the record and verification fails.

Proof

A real session. avow 0.2.0 from PyPI on Python 3.13.14; output copied verbatim, including the failure.

$ assay keygen --out signing.key
wrote signing key: signing.key
wrote public key: signing.key.pub

$ echo '{"metric":"binary","metric_version":"1","y_true":[0,1,0,1],"y_score":[0.2,0.8,0.3,0.7]}' > req.json

$ assay score --request req.json --key signing.key --out receipt.json --ledger ledger.jsonl
wrote receipt: receipt.json
wrote ledger head: ledger.jsonl.head (1 entries)

$ tail -4 receipt.json
  "payload_hash": "sha256:a2ada15199d7586958d9754a4adeba4d13a4e73122f9604f65a536fb4a4bad7e",
  "public_key": "cfcc49b01cc3019ce451a1bde8a146fbe423682faa2a2c7d0d0f0a01da008292",
  "signature": "3cb2ae7edb1831ef47246cb0f07854a6a63aa9ba5457659455532dfc972758b4aaa1f73912b041a8606cb6b7d413bbc77defd6b2141a2aa50325d139f9143804"
}

$ assay verify --receipt receipt.json --public-key signing.key.pub; echo "exit $?"
OK: receipt verified
exit 0

$ sed 's/"abstained": true/"abstained": false/' receipt.json > tampered.json

$ diff receipt.json tampered.json
10c10
<     "abstained": true,
---
>     "abstained": false,

$ assay verify --receipt tampered.json --public-key signing.key.pub; echo "exit $?"
FAIL: avow.replay_mismatch: payload hash does not match payload content
exit 1

Four samples is below the sample-size floor, so the receipt honestly recorded "abstained": true and no score. The diff is the entire edit: one word, turning an abstention into a confident answer — the sort of quiet correction that leaves no trace in an ordinary database. The signature bytes were never touched. It still fails, because the payload no longer hashes to the value the signature covers.

The code above reads avow.replay_mismatch, which is what 0.2.0 prints. It is a misnomer: this is a tamper failure, and the envelope detects no replay at all. It is renamed avow.payload_hash_mismatch on main and ships in 0.3.0, with ReplayMismatch kept as a deprecated alias. See Honest limits.

Run it

# The PyPI distribution is `avow`. The command it installs is `assay`. There is no `avow`
# command, and this project does not publish a distribution called `assay` —
# `pip install assay` gets an unrelated package by another author.
python3.13 -m venv .venv && . .venv/bin/activate    # Python 3.13+ is required
pip install 'avow[cli]'

assay keygen --out signing.key
echo '{"metric":"binary","metric_version":"1","y_true":[0,1,0,1],"y_score":[0.2,0.8,0.3,0.7]}' > req.json
assay score --request req.json --key signing.key --out receipt.json --ledger ledger.jsonl
assay verify --receipt receipt.json --public-key signing.key.pub            # exit 0
sed 's/"abstained": true/"abstained": false/' receipt.json > tampered.json
assay verify --receipt tampered.json --public-key signing.key.pub           # exit 1

---

Why this exists

Your card gets declined at a checkout.

You call the bank and ask why. Someone reads a reason off a screen: the fraud system scored the transaction as risky, so it blocked it.

But that reason is just a row in a database. Rows can be changed. Nobody — not you, not the bank's own auditor, not a regulator — can tell whether that number is what the software actually produced at the moment it blocked your card, or whether somebody adjusted it afterwards, once you complained.

That is the gap avow closes. When your software makes a decision, avow has it write a receipt: a small record of exactly what was decided, sealed with cryptography at the moment of the decision. Think of a store receipt — except this one cannot be reprinted or altered. Hand it to anyone; they check it on their own laptop, offline, with no access to your database, and get valid (this is exactly what the software decided, byte for byte) or invalid (someone changed it). There is no "close enough".

Two layers, and why the difference matters

flowchart TD
  data["Your data<br/>any JSON object — a score, a decision, an approval"]
  sign["Sign it<br/>Canonicalise to one exact byte string (RFC 8785),<br/>hash it with SHA-256, sign it with Ed25519.<br/>Python and TypeScript emit identical bytes — the same<br/>12 test vectors run in both languages to prove it."]
  receipt["The receipt: your data, its hash, the signature, the signer's key<br/>Check it against a public key you pinned in advance.<br/>You learn who wrote it, and that nobody changed it since.<br/>You do NOT learn that it is new — an old receipt<br/>still verifies, and that is correct behaviour."]
  ledger["Append-only ledger<br/>Each line carries its position (seq) and the hash of<br/>the line before it (prev_hash). The hash of the last line<br/>is pinned out of band, somewhere the ledger's writer<br/>cannot reach — another host, a git commit, a printout."]
  caught["Where a replayed or missing entry is caught<br/>An old entry slipped back in no longer matches<br/>its seq or prev_hash. A deleted, reordered or truncated<br/>log no longer ends at the head you pinned.<br/>Freshness is the ledger's job, never the signature's."]

  data --> sign
  sign --> receipt
  receipt --> ledger
  ledger --> caught

Loading

The first three boxes are the envelope; the last two are the ledger. The npm package @edgeproc/avow ships the envelope only — canonical bytes and sign/verify — so a browser gets tamper-evidence, not replay defence; the ledger is Python-side (avow.ledger).

The names, spelled out

Three names get confused, and two of them belong to other people's packages:

You type	What you get
pip install avow	this project — the envelope, importable as avow
pip install 'avow[cli]'	this project plus the assay command used above
npm i @edgeproc/avow	this project — the TypeScript envelope
pip install assay	someone else's — Brandon Rhodes' "Future testing framework"
npm i assay	someone else's — Nathan Zadoks' assertion helper

So: the distribution is avow, the command is assay, and import avow / import assay both come out of the single avow install. There is no avow command.

One more thing that will otherwise trip you up: Python 3.13 or newer is required. On 3.12 or older pip install avow fails while resolving, with a message that does not mention the Python version. Check with python --version first.

The same thing from Python

from pydantic import BaseModel, ConfigDict

from avow import generate_signing_key, public_key_hex, sign_payload, verify_signature


class FraudCheck(BaseModel):
    model_config = ConfigDict(frozen=True, extra="forbid")
    transaction_id: str
    decision: str
    risk_score: float
    model_version: str


key = generate_signing_key()        # the bank's private signing key
trusted_key = public_key_hex(key)   # published once; this is what checkers pin

receipt = sign_payload(
    FraudCheck(
        transaction_id="txn-9471",
        decision="blocked",
        risk_score=0.83,
        model_version="fraud-v4",
    ),
    key,
)

verify_signature(receipt, expected_public_key=trusted_key)
print("original receipt ..... VALID")

# Someone edits the stored record to make the block look better justified.
tampered = receipt.model_copy(
    update={"payload": receipt.payload.model_copy(update={"risk_score": 0.99})}
)

try:
    verify_signature(tampered, expected_public_key=trusted_key)
    print("edited receipt ....... VALID  <- this must never happen")
except Exception as exc:
    print(f"edited receipt ....... REJECTED ({type(exc).__name__})")

original receipt ..... VALID
edited receipt ....... REJECTED (ReplayMismatch)

That is the output on the published 0.2.0. On main the same run prints PayloadHashMismatch — the rename described above; the class is the same object either way, and except ReplayMismatch: keeps working after it.

Nudging 0.83 to 0.99 — a change that would be invisible in a database — makes the receipt fail to verify. That is the whole idea.

One thing this is not: a freshness check. Hand that same unedited receipt to the verifier a thousand more times and it passes a thousand more times — see Honest limits.

What a receipt proves, and what it does not

This is the part most signing libraries gloss over, so read it before you rely on avow.

A receipt proves integrity: the contents have not changed since they were signed.

It does not, on its own, prove authenticity — that your system is the one that signed it.

Here is why, concretely. The signer's public key travels inside the receipt, but outside the portion that is actually signed. So an attacker can write any payload they like, sign it with a key they generated themselves, and drop their own public key into the receipt. That forgery is internally consistent: its hash and its signature agree with each other perfectly.

The only thing that stops it is pinning — deciding in advance which public key you trust, obtaining it through a separate channel (the .pub file from keygen, your config, your secret manager), and passing that key to the verifier:

from pydantic import BaseModel, ConfigDict

from avow import generate_signing_key, public_key_hex, sign_payload, verify_signature


class FraudCheck(BaseModel):
    model_config = ConfigDict(frozen=True, extra="forbid")
    transaction_id: str
    decision: str
    risk_score: float
    model_version: str


bank = generate_signing_key()
trusted_key = public_key_hex(bank)      # what you pin, out-of-band

attacker = generate_signing_key()       # a key anyone can make in one line
forged = sign_payload(
    FraudCheck(
        transaction_id="txn-9471",
        decision="approved",            # a total fabrication
        risk_score=0.01,
        model_version="fraud-v4",
    ),
    attacker,
)

# The forgery is internally consistent: its hash and signature agree with each other.
print(f"forged receipt is self-consistent: {forged.payload_hash[:16]}... signed OK by attacker")

try:
    verify_signature(forged, expected_public_key=trusted_key)
    print("forged receipt ....... VALID  <- this must never happen")
except Exception as exc:
    print(f"forged receipt ....... REJECTED ({type(exc).__name__}: {exc.code})")

# The ONLY reason it was rejected is that we pinned the bank's key.
print(f"key inside forgery matches bank? {forged.public_key == trusted_key}")

forged receipt is self-consistent: sha256:441280ee2... signed OK by attacker
forged receipt ....... REJECTED (SignerMismatch: avow.signer_mismatch)
key inside forgery matches bank? False

Never trust the key embedded in the receipt. It rides along for convenience; it is not the trust anchor. verify_signature requires you to pass the key you already trust, precisely so this mistake is hard to make by accident.

Note the code: avow.signer_mismatch, not avow.signature_invalid. Those are two different events and they are coded apart, because you may want to react differently:

Code	Class	What happened
avow.signer_mismatch	SignerMismatch	Signed by a key you do not trust — a provenance failure. The signature is never even checked.
avow.signature_invalid	SignatureBytesInvalid	The signer matched, but the bytes fail the curve check — a tamper failure.
avow.payload_hash_mismatch	PayloadHashMismatch	The payload was edited behind an untouched hash field — also a tamper failure. avow.replay_mismatch / ReplayMismatch through 0.2.0.

The first two subclass SignatureInvalid, so except SignatureInvalid: still catches either one if you do not care which. You never have to match on the message text.

There is deliberately no replay code in that table from 0.3.0 onwards. The envelope detects no replay, so nothing in it is named after one — see Honest limits.

Three things you can sign

Avow ships as one installable package with three importable pieces. The first is the core; the other two are ready-made shapes built on it.

1. Anything — avow, the envelope

Shown above. You define what a decision looks like, avow seals and checks it. It never looks inside your data, so the same sign-and-verify code works for any record.

2. A measurement that refuses to overstate — assay

A number like "our model is 89% accurate" is only meaningful if enough examples stood behind it. Measure 12 cases and you can get any number you like; it is noise.

assay computes the score and its error bar, and when the sample is too thin it returns nothing at all rather than inventing a figure. Both outcomes come back inside a signed receipt.

pip install 'avow[assay]'

import random

from assay import score, verify
from assay.models import ScoreRequest
from assay.settings import AssaySettings
from avow import generate_signing_key, public_key_hex

key = generate_signing_key()
settings = AssaySettings()  # sample-size floor: 30


def evaluate(label: str, n: int) -> None:
    rng = random.Random(7)
    # a fraud model that is good, not perfect
    y_true = tuple(int(rng.random() < 0.3) for _ in range(n))
    y_score = tuple(min(1.0, max(0.0, rng.gauss(0.75 if t else 0.25, 0.22))) for t in y_true)

    receipt = score(
        ScoreRequest(metric="binary", metric_version="1", y_true=y_true, y_score=y_score),
        signing_key=key,
        settings=settings,
    )
    assert verify(receipt, expected_public_key=public_key_hex(key))

    r = receipt.payload
    if r.abstained:
        print(f"{label:<10} n={n:<4} accuracy = (none) -- {r.abstain_reason}")
    else:
        print(
            f"{label:<10} n={n:<4} accuracy = {r.score:.2f}  "
            f"95% interval [{r.interval_low:.2f}, {r.interval_high:.2f}]"
        )


evaluate("pilot", 12)
evaluate("full eval", 400)

pilot      n=12   accuracy = (none) -- assay.insufficient_samples
full eval  n=400  accuracy = 0.89  95% interval [0.86, 0.92]

The pilot run declines to produce a number. The full run reports 0.89 and admits the true value is somewhere in [0.86, 0.92]. The receipt also carries precision, recall, F1, PR-AUC, ROC-AUC, and a calibration report — see the reference section.

3. An action that was actually allowed — writ

Before your code does something irreversible — delete a record, move money, send an email — writ checks a policy. If the policy says no, the action never runs. Either way you get a signed receipt of what was asked and what was decided, so "the agent deleted it" and "we blocked the agent" are both provable after the fact.

This matters most when the caller is an AI agent you do not fully control.

from avow import content_hash, generate_signing_key, public_key_hex, verify_signature
from writ import Allowlist, EffectRequest, KeyholderEffector, governed_gate

key = generate_signing_key()
performed: list[str] = []  # stands in for the real system being changed


def perform(request: EffectRequest) -> None:
    """The privileged action. Reached ONLY through an allow decision."""
    performed.append(f"{request.action} {request.target}")


# The trusted host wires policy + action + key into the gate, then hands the agent
# exactly one thing: the gate. The agent never receives the key or the action itself.
agent_gate = governed_gate(
    Allowlist(frozenset({"read"})),
    KeyholderEffector(effect=perform, signing_key=key),
)

for action in ("read", "delete"):
    receipt = agent_gate(
        EffectRequest(
            action=action,
            target="customer-4471",
            args_digest=content_hash({"reason": "agent cleanup task"}),
        )
    )
    verify_signature(receipt, expected_public_key=public_key_hex(key))
    print(f"agent asked to {action:<6} -> {receipt.payload.decision:<5} (signed receipt verified)")

print(f"actually performed: {performed}")

agent asked to read   -> allow (signed receipt verified)
agent asked to delete -> deny  (signed receipt verified)
actually performed: ['read customer-4471']

The denied delete produced a signed receipt but never touched the system.

Auditing the ledger — verify-ledger

The score command in the proof above also appended that receipt to ledger.jsonl and wrote the ledger's new chain head to ledger.jsonl.head. Auditing takes two things, and neither is read from the ledger itself:

• the signer's public key (never the secret seed) — who may write entries. A content hash alone is not enough: an adversary who edits an entry can recompute its (public) hash, so tamper-evidence rests on the Ed25519 signature, which only the private seed can produce.
• the chain head — which entries there are. Each line carries the hash of the line before it, so the last line's hash commits to the whole history. Pin those 32 bytes and dropping, adding or moving a line has nowhere to hide.

assay verify-ledger --ledger ledger.jsonl --public-key signing.key.pub --head ledger.jsonl.head

OK: ledger verified, 1 entry intact

Now edit the stored entry the same way as before — "abstained":true to "abstained":false — and ask again:

FAIL: avow.ledger_integrity: tampered ledger entry: sha256:a2ada15199d7586958d9754a4adeba4d13a4e73122f9604f65a536fb4a4bad7e

Exit code 1, and the coded cause names both the failure and the entry that caused it. The check re-derives every entry's hash and verifies its signature under the pinned key, failing closed on the first disagreement.

A ledger it cannot read is also a failure, not a pass. Mistype the path and you get:

FAIL: avow.ledger_unreadable: ledger is not a readable file: ledgr.jsonl

rather than OK: ledger verified, 0 entries intact — which would be a clean bill of health for a file that was never opened. The same applies to a directory in the file's place, a file whose permissions deny reading, and a line that is not a parseable receipt (avow.ledger_entry_malformed). A missing or unparseable head file is a failure too (avow.ledger_head_unreadable) — with nothing to check the ledger's end against, the audit answers nothing. A ledger that exists and is empty passes only against the head of an empty ledger, so an erased audit no longer reads as a fresh one.

Editing a line is the easy case. Now score a second request, then delete the entry it wrote — every remaining line is genuine, correctly signed, and correctly chained:

FAIL: avow.ledger_integrity: ledger ends at 1 entries / sha256:bdbe0cc76d21c65c5010629e1cfbacfa5a8d957995748cadbcf347d98128ef14, but the pinned head is 2 entries / sha256:d648caa536e0e096657a462cc343b0606c7a93fb3613f00ac02ad6bd9f9ceef0

Those two hashes cover signatures, so yours will differ — they are whatever your key produced. The tampered ledger entry hash above is a payload hash and involves no key, so that one reproduces exactly.

That is the check no per-entry signature can do. Deleting, truncating (including emptying the file), reordering, replaying and splicing in an entry from another ledger all land here, with exit code 1.

What this check does not cover. The head is only as good as its custody. Verifying against a head file that sits beside the ledger proves nothing against an attacker who can write both — copy it somewhere they cannot reach (another host, a git commit, a printout). Read Honest limits before you rely on this file as a history.

Honest limits

Stated plainly, because each of these is a real boundary on what avow currently gives you.

• A receipt proves integrity, not authenticity, unless you pin the key. See the section above. This is the single easiest way to misuse the library.

• Verifying a receipt proves nothing about freshness. verify_signature / verify_receipt prove who signed it and that it is unmodified. They do not prove that this is the first time the receipt has been presented, or that it was made recently. A replayed receipt — a genuine one, captured by anyone who saw it and handed over again unchanged — is byte-identical to the original and verifies forever. That is not a bug to be fixed inside the envelope: a signature binds content to a signer, it cannot bind it to an occasion, and the very determinism that makes a receipt re-verifiable offline years later is what makes it re-presentable. If your threat model includes "someone shows me an old receipt as if it were new", the answer must come from state the verifier keeps, not from the signature:

  • record entries in avow.ledger — the chain rejects a replayed entry against a pinned head (avow.ledger_integrity); that is a test in tests/test_ledger.py and in tests/test_verify.py, watched go red with both its checks disabled; or
  • put a nonce or a request-id inside your own subject before signing, and track the ones you have already accepted.

  Note the naming, because it changes in 0.3.0 for exactly this reason: the tamper error becomes PayloadHashMismatch (avow.payload_hash_mismatch). It is called ReplayMismatch (avow.replay_mismatch) through 0.2.0 — the version on PyPI today — which named a property the envelope has never had. ReplayMismatch stays as a deprecated alias, so except ReplayMismatch: keeps working; code that branches on the literal string "avow.replay_mismatch" must be updated.

• The ledger's tamper-evidence is only as good as the custody of its head. The entries are chained (each carries its position and the hash of the entry before it) and the audit walks that chain to a head you pin out-of-band, so deleting, truncating, reordering, replaying and splicing all fail — each of those five is a test in tests/test_ledger.py, and each guard has been watched go red with its check disabled. What remains is a custody limit, not a detection one: the chain moves the trust requirement from N lines down to 32 bytes, it does not remove it. An attacker who can rewrite the ledger and the head you check against can rebuild a consistent history — that is why score writing ledger.jsonl.head next to the ledger is a convenience for copying it away, never a control. Keep the head where the ledger's writer cannot reach: another host, a git commit, a printout, a transparency log. And pin the current head — a head from three appends ago legitimately fails, because three entries you did not acknowledge is exactly the thing this is built to notice.

• writ's enforcement is in-process (v0). The signing key and the privileged action live only inside the effector, which the gate captures in a closure; the agent receives the closure and never the effector, so the only route to the action is through the guard. But the credential is still in the same process, so same-process reflection (walking __closure__, for instance) could reach it. This is a capability-holding approximation, not true enforcement. Real un-bypassability — a separate-process broker or a sandboxed guest, where the caller's address space cannot reach the credential — is the v1 hardening. We claim no more than that.

• The v0 policy decider is a plain Python predicate (Allowlist). OPA/Rego is the v1 decider.

• writ signs the args_digest its caller hands it; it does not recompute it. The gate never sees the raw arguments, so it cannot check that the digest actually describes them. A caller that passes a digest of one thing and performs another gets a validly-signed receipt attesting the wrong arguments. What the receipt therefore proves is "this signer claimed this action, target and digest, and the policy decided this" — not "these are the arguments the effect ran with". Closing the gap means the request carrying the real arguments and the gate deriving the digest itself; that changes EffectRequest's public shape, so it is a v1 change, not a patch.

• Browser key custody is same-origin, not hardware-backed. In the browser build, keys are protected by the origin boundary alone — there is no secure element or OS keychain behind them.

• Receipts carry no timestamp. That is deliberate: it makes them reproducible (the same inputs always yield the same receipt). It also means a receipt cannot tell you when it was made. If you need that, record it outside the receipt, in something you trust. Ledger position is now evidence of sequence — the chain fixes the order of entries relative to a pinned head — but sequence is not a clock. Nothing in a ledger says an entry was written on Tuesday. For wall-clock time, use a real timestamping service.

Check the guards yourself

A passing test suite is not evidence that a guard guards. A test that asserts whatever the code happens to return passes whether the property holds or not, and the only way to tell those two apart is to break the property on purpose and watch the test fail.

One command does that for every claim on this page:

uv run poe mutants

It works through 18 mutations, one at a time. For each it names the claim, runs the guard tests unmutated (which must pass), edits the source so the claim becomes false, reads the mutated file back off disk to confirm the edit landed, runs the same tests again (which must now fail), and restores the file. Takes about a minute; run it on a clean tree, because it edits tracked files in place.

mutation                                      before  after   verdict
ranking-order-reaches-trec-eval                    0      1   RED — the guard fired
ranking-recall-is-not-precision                    0      1   RED — the guard fired
envelope-pins-the-signer                           0      1   RED — the guard fired
ledger-requires-the-pinned-head                    0      1   RED — the guard fired
documented-sample-floor-is-30                      0      1   RED — the guard fired
...
18/18 guards fired when their claim was broken.
whole suite after restore: exit 0 (green)

The verdict is the pytest exit code, and nothing else. A harness that greps output for "failed" reports green when the runner crashes, because a crash prints no failures either. 0 means every test passed, 1 means a test failed, and anything else — 4 usage error, 5 nothing collected — gets its own name and fails the run. A guard that survives its break, or one that was not green to begin with, is a failure too.

What it covers: that the ranking metrics really are trec_eval's arithmetic reached through ir_measures (break the wiring — the ranked order, the cut-off k, the graded gains — and the suite goes red); that every refusal actually refuses; that the envelope re-derives the payload hash and pins the signer; that the ledger's chain, count and signatures are three separate checks; and that the literals this README states out loud (the floor of 30 samples, the 95% interval, the 12 golden vectors) are the ones that ship. scripts/mutation_harness.py lists all 18 with the claim each one breaks, and CI runs it on every pull request.

Reference

Packages and install matrix

One distribution, avow, exposes three import packages:

Package	What it is	Depends on	Install
avow	the shared envelope — sign, hash, verify a receipt	pydantic, pynacl, rfc8785	pip install avow
assay	the measurement face — an honest number in a receipt	avow + scikit-learn/scipy/numpy	pip install 'avow[assay]'
writ	the action face — a policy-gated effect, sealed as a receipt	avow only	pip install avow

Dependency arrows only ever point into avow: assay → avow and writ → avow. Avow imports neither, which is why installing the envelope alone never pulls in the heavy scientific stack. Importing assay without the [assay] extra raises a coded ScoringExtraMissing, not a raw ModuleNotFoundError.

How the sealing works

Some terms, each in one line:

• Content hash — a short fingerprint of some data. Change any byte and the fingerprint changes completely. Avow uses SHA-256.
• Canonicalization (RFC 8785 / JCS) — one fixed way to write a JSON object as bytes, so that the same data always produces the same bytes regardless of key order or language. Without it, two systems could hash "the same" record differently.
• Ed25519 — a signature scheme. A private key signs; the matching public key checks. Signing is deterministic: the same message and key always give the same signature.
• Frozen subject — the record being signed, declared immutable so it cannot be modified after signing.

sign_payload / verify_signature / payload_digest operate only on the canonical JSON of a frozen subject, and never inspect its fields. That is why the same envelope carries a measurement for assay and an action for writ with no change to the trust boundary.

Because payloads carry no timestamp, identical inputs yield an identical, reproducible, offline-verifiable receipt.

avow.ledger is a hash-chained JSONL log, generic over the subject. Each line carries its sequence number, the hash of the line before it, and the signed receipt; writes are O_APPEND under a lock held across the read and the write, so concurrent appenders cannot chain two entries onto the same predecessor. The audit fails closed on two independent checks: per entry (re-derive the payload hash, verify the Ed25519 signature against a pinned public key) and across entries (walk the chain and require it to end at a LedgerHead — count plus hash — pinned out-of-band, which is what catches a truncated file). append returns the new head; save_head / read_head move it around. See Honest limits for the custody caveat. Coded failures live in avow.errors (avow.* codes under AvowError).

Inside assay

A thin trust, honesty, and composition layer over reused libraries — it computes no metric math itself:

• scikit-learn for precision, recall, F1, PR-AUC, ROC-AUC and Brier score
• scipy.stats.bootstrap for percentile intervals, with a sample-size floor; below it, assay abstains rather than invent a point estimate
• population-weighted ECE (expected calibration error) for calibration
• a positive-weighted composite with a propagated interval

assay.receipt defines the measurement subjects; the envelope signs them. Errors are assay.* under AssayError. Every tunable — the sample floor, resample count, confidence level, bin count — lives in AssaySettings and is overridable via ASSAY_* environment variables.

Inside writ

writ.gate(request, policy, effector, *, emit=...) evaluates a typed policy. On deny it seals a signed not_run receipt and never runs the effect. On allow it seals an attempted receipt and hands it to emit before running the effect, then runs it and seals the succeeded / failed outcome — so a failed or partial privileged effect always leaves a signed attestation of the attempt. Wire emit to avow.ledger.append for durable, atomic capture — and keep the head it returns, or the chain has no pin; every sealed receipt is verifiable through the shared envelope.

EffectRequest.args_digest is a hash rather than the arguments themselves, so the signed record never carries raw payloads. It is the caller's claim about those arguments: the gate signs it without recomputing it.

See the honest limits above for exactly how far the enforcement seam and that digest go in v0.

Key custody and cross-language vectors

assay keygen (and avow.keys) write a 32-byte Ed25519 seed to a 0600 file and the public key to a companion .pub. Keys are never logged and never committed (*.key is gitignored). The public key also travels inside each receipt for convenience, but that embedded copy is not the trust anchor — a verifier pins the out-of-band key and passes it to verify.

testdata/vectors/ holds 12 golden vectors generated by tests/gen_vectors.py: 9 canonicalization cases (input, canonical bytes, hash) in canonical.json and 3 receipts signed with a fixed non-secret test seed in receipts.json. The Python suite replays them in tests/test_vectors.py; the TypeScript @edgeproc/avow replays the same files byte for byte, so any RFC 8785 number-serialization divergence fails in CI rather than in production.

@edgeproc/receipt-ui (in ts/packages/receipt-ui) is the browser rendering layer: small, fail-closed React components that verify a receipt against a pinned key and show the verdict, built on the TypeScript @edgeproc/avow envelope above.

Working on avow itself

git clone https://github.com/hseshadr/assay.git && cd assay
uv sync --all-extras
uv run poe gate                          # Python: ruff, ruff-format, mypy --strict, xenon A, pytest
uv run poe gate-ts                       # TypeScript: biome, tsc --noEmit, vitest, build (needs pnpm)
uv run poe gate-all                      # both, mirroring CI's two jobs
uv run poe mutants                       # break each guard in turn; the suite must go red
uv run python demo/run_demo.py           # measurement face: 6 honesty acceptance cases
uv run python demo/unification_demo.py   # one envelope + one verifier, both faces

QUICKSTART.md is the shortest path from clone to a verified receipt.

See docs/ARCHITECTURE.md for the data-flow diagram, the import edges, and the native-vs-browser story.

Status

v0, deterministic — no LLM anywhere in the path.

Three gates, mirroring CI's three jobs. uv run poe gate covers Python only (ruff, ruff-format, mypy --strict, xenon A, pytest with statement and branch coverage against a 90% floor); uv run poe gate-ts covers the TypeScript package (biome, tsc strict, vitest, build); uv run poe mutants breaks each guard in turn and requires the suite to notice. uv run poe gate-all runs the first two.

Measured at the time of writing: 258 tests — 187 Python at 100% statement and branch coverage (803 statements, 68 branches, none missed), 40 in @edgeproc/avow, 31 in @edgeproc/receipt-ui — plus 18 mutations, 18 of which the suite catches.

Published releases: avow 0.3.0 on PyPI; @edgeproc/avow 0.3.0 and @edgeproc/receipt-ui 0.2.0 on npm. See CHANGELOG.md and ts/packages/receipt-ui/CHANGELOG.md for what each release contains. Read the honest limits above before depending on any of it.

License

MIT © Harish Seshadri