# Excititor Advisory-AI Evidence Contract (v1)

Updated: 2025-11-18 · Scope: EXCITITOR-AIAI-31-004 (Phase 119)

This note defines the deterministic, aggregation-only contract that Excititor exposes to Advisory AI and Lens consumers. It covers the `/v1/vex/evidence/chunks` NDJSON stream plus the projection rules for observation IDs, signatures, and provenance metadata.

## Goals
- **Deterministic & replayable**: stable ordering, no implicit clocks, fixed schemas.
- **Aggregation-only**: no consensus/inference; raw supplier statements plus signatures and AOC (Aggregation-Only Contract) guardrails.
- **Offline-friendly**: chunked NDJSON; no cross-tenant lookups; portable enough for mirror/air-gap bundles.

## Endpoint
- `GET /v1/vex/evidence/chunks`
  - **Query**:
    - `tenant` (required)
    - `vulnerabilityId` (optional, repeatable) — CVE, GHSA, etc.
    - `productKey` (optional, repeatable) — PURLish key used by Advisory AI.
    - `cursor` (optional) — stable pagination token.
    - `limit` (optional) — max records per stream chunk (default 500, max 2000).
  - **Response**: `Content-Type: application/x-ndjson`
    - Each line is a single evidence record (see schema below).
    - Ordered by `(tenant, vulnerabilityId, productKey, observationId, statementId)` to stay deterministic.

## Evidence record schema (NDJSON)
```json
{
  "tenant": "acme",
  "vulnerabilityId": "CVE-2024-1234",
  "productKey": "pkg:pypi/django@3.2.24",
  "observationId": "obs-3cf9d6e4-…",
  "statementId": "stmt-9c1d…",
  "source": {
    "supplier": "upstream:osv",
    "documentId": "osv:GHSA-xxxx-yyyy",
    "retrievedAt": "2025-11-10T12:34:56Z",
    "signatureStatus": "missing|unverified|verified"
  },
  "aoc": {
    "violations": [
      { "code": "EVIDENCE_SIGNATURE_MISSING", "surface": "ingest" }
    ]
  },
  "evidence": {
    "type": "vex.statement",
    "payload": { "...supplier-normalized-fields..." }
  },
  "provenance": {
    "hash": "sha256:...",
    "canonicalUri": "https://mirror.example/bundles/…",
    "bundleId": "mirror-bundle-001"
  }
}
```

### Field notes
- `observationId` is stable and maps 1:1 to internal storage; Advisory AI must cite it when emitting narratives.
- `statementId` remains unique within an observation.
- `signatureStatus` is pass-through from ingest; no interpretation beyond `missing|unverified|verified`.
- `aoc.violations` enumerates guardrail violations without blocking delivery.
- `evidence.payload` is supplier-shaped; we **do not** merge or rank.
- `provenance.hash` is the SHA-256 of the supplier document bytes; `canonicalUri` points to the mirror bundle when available.

## Determinism rules
- Ordering: fixed sort above; pagination cursor is derived from the last emitted `(tenant, vulnerabilityId, productKey, observationId, statementId)`.
- Clocks: All timestamps are UTC ISO-8601 with `Z`.
- No server-generated randomness; record content is idempotent for identical upstream inputs.

## AOC guardrails
- Enforced surfaces: ingest, `/v1/vex/aoc/verify`, and chunk emission.
- Violations are reported via `aoc.violations` and metric `excititor.vex.aoc.guard_violations`.
- No statements are dropped due to AOC; consumers decide how to act.

## Telemetry (counters/logs-only until span sink arrives)
- `excititor.vex.chunks.requests` — by `tenant`, `outcome`, `truncated`.
- `excititor.vex.chunks.bytes` — histogram of NDJSON stream sizes.
- `excititor.vex.chunks.records` — histogram of records per stream.
- Existing observation metrics (`excititor.vex.observation.*`) remain unchanged.

## Error handling
- 400 for invalid tenant or mutually exclusive filters.
- 429 with `Retry-After` when throttle budgets exceeded.
- 503 on upstream store/transient failures; responses remain NDJSON-free on error.

## Offline / mirror readiness
- When mirror bundles are configured, `provenance.canonicalUri` points to the local bundle path; otherwise it is omitted.
- All payloads are side-effect free; no remote fetches occur while streaming.

## Samples
- NDJSON sample: `docs/samples/excititor/chunks-sample.ndjson` (hashes in `.sha256`) aligned to the schema above.

## Versioning
- Contract version: `v1` (this document). Changes must be additive; breaking changes require `v2` path and updated doc.