# OpenVINO NPU reranker service spec

Status: proposed localhost prototype; not live RAG integration.
Target port: `127.0.0.1:18818`.
Safety posture: foreground smoke first, no persistent enablement, no Atlas/Hermes/RAG routing changes without Will's explicit approval.

## Recommendation

Use `cross-encoder/ms-marco-MiniLM-L6-v2`, exported to OpenVINO IR as INT8, served by the local stdlib HTTP service in `server.py` on OpenVINO Runtime `NPU`.

Why this choice:

- It is a small BERT-family cross-encoder reranker intended for MS MARCO-style passage ranking, matching the second-stage RAG use case better than another embedding-only similarity pass.
- The model shape is simple pairwise text classification/scoring: `(query, document) -> score`, which maps cleanly to OpenVINO Runtime and avoids introducing a heavier LLM worker for reranking.
- INT8 OpenVINO IR keeps memory and compile/runtime cost low enough for a localhost sidecar and is already represented in the repo defaults:
  `/home/will/.cache/openvino-models/rerankers/ms-marco-MiniLM-L6-v2-int8-ov`.
- The service can fail closed on startup when `OPENVINO_RERANKER_DEVICE=NPU` but `/sys/class/accel/accel0/device/npu_busy_time_us` does not increase, preventing false "NPU-backed" claims.

Runtime default:

```text
OPENVINO_RERANKER_HOST=127.0.0.1
OPENVINO_RERANKER_PORT=18818
OPENVINO_RERANKER_DEVICE=NPU
OPENVINO_RERANKER_MODEL=cross-encoder/ms-marco-MiniLM-L6-v2
OPENVINO_RERANKER_MODEL_DIR=/home/will/.cache/openvino-models/rerankers/ms-marco-MiniLM-L6-v2-int8-ov
OPENVINO_RERANKER_MAX_LENGTH=512
OPENVINO_RERANKER_MAX_DOCUMENTS=100
OPENVINO_RERANKER_MAX_BODY_BYTES=5242880
```

## Endpoint contract

### Health and readiness

`GET /healthz` and `GET /readyz` return JSON.

`/readyz` must return HTTP 200 only when the model is loaded and startup smoke passed. For NPU mode, startup smoke must include a positive `npu_busy_delta_us`.

Representative ready response:

```json
{
  "status": "ok",
  "ok": true,
  "service": "openvino-reranker",
  "model": "cross-encoder/ms-marco-MiniLM-L6-v2",
  "model_dir": "/home/will/.cache/openvino-models/rerankers/ms-marco-MiniLM-L6-v2-int8-ov",
  "device": "NPU",
  "available_devices": ["CPU", "NPU"],
  "max_length": 512,
  "startup_smoke": {"ok": true, "duration_ms": 12.3, "npu_busy_delta_us": 1234},
  "last_inference": null,
  "ready_error": null
}
```

### Rerank

`POST /rerank` and compatibility alias `POST /v1/rerank` accept:

```json
{
  "query": "how do I verify OpenVINO NPU usage?",
  "documents": [
    {"id": "good", "text": "Check /sys/class/accel/accel0/device/npu_busy_time_us before and after inference.", "metadata": {"source": "synthetic"}},
    {"id": "bad", "text": "This note is about making sourdough starter."}
  ],
  "top_k": 2,
  "return_documents": false
}
```

Compatibility notes:

- `documents` may be strings or objects with `id`, `text`, and optional object `metadata`.
- `top_k` is preferred; `top_n` is accepted for common reranker-client compatibility.
- `return_documents=false` is recommended for RAG integration to avoid echoing private source text into logs or intermediate traces.
- The optional `model` field may be sent by clients but is not used for routing; this sidecar serves one configured model.

Successful response:

```json
{
  "ok": true,
  "model": "cross-encoder/ms-marco-MiniLM-L6-v2",
  "device": "NPU",
  "query": "how do I verify OpenVINO NPU usage?",
  "input_count": 2,
  "top_k": 2,
  "duration_ms": 10.5,
  "npu_busy_delta_us": 1234,
  "results": [
    {"index": 0, "id": "good", "score": 8.1, "raw_score": 8.1, "probability": 0.9997},
    {"index": 1, "id": "bad", "score": -4.2, "raw_score": -4.2, "probability": 0.0148}
  ]
}
```

Error response shape:

```json
{"ok": false, "error": "human-readable error", "results": []}
```

Status behavior:

- 400: invalid JSON schema, empty query, missing/empty documents, invalid document text, or non-positive/non-integer `top_k`/`top_n`.
- 413: request body above `OPENVINO_RERANKER_MAX_BODY_BYTES`.
- 503: model not ready.
- 500: unexpected inference/runtime failure.

## CLI contract

Foreground-only review start:

```bash
ss -ltnp | grep ':18818\b' || true
cat /sys/class/accel/accel0/device/npu_busy_time_us
source /home/will/.venvs/openvino-reranker/bin/activate
OPENVINO_RERANKER_HOST=127.0.0.1 \
OPENVINO_RERANKER_PORT=18818 \
OPENVINO_RERANKER_DEVICE=NPU \
OPENVINO_RERANKER_MODEL_DIR=/home/will/.cache/openvino-models/rerankers/ms-marco-MiniLM-L6-v2-int8-ov \
python /home/will/lab/swarm/openvino-reranker-npu/server.py
```

Client smoke:

```bash
source /home/will/.venvs/openvino-reranker/bin/activate
python /home/will/lab/swarm/openvino-reranker-npu/smoke.py --url http://127.0.0.1:18818
```

Optional user-systemd unit exists as `openvino-reranker.service`, but this spec does not approve copying, starting, enabling, or wiring it into live paths.

## Non-private smoke payload

Use only synthetic public-text fixtures. Do not query the Obsidian vault, private document directories, image folders, or live Chroma documents during smoke.

Minimum cases:

1. Query: `how do I verify OpenVINO NPU usage?`
   - Expected top document: `Check /sys/class/accel/accel0/device/npu_busy_time_us before and after inference.`
   - Distractor: `This note is about making sourdough starter.`
2. Query: `what port does the reranker service use?`
   - Expected top document: `The OpenVINO reranker prototype listens locally on port 18818.`
   - Distractor: `Whisper transcription accepts audio uploads.`
3. Query: `why should reranking not mutate vector collections?`
   - Expected top document: `Reranking is a read-only second-stage transformation after vector search.`
   - Distractor: `Boil pasta in salted water until al dente.`

Pass criteria:

- `/readyz` is HTTP 200 and reports `device=NPU`.
- Every case returns `ok=true` and a sorted `results` list with the expected top `id`.
- Response-level `npu_busy_delta_us` is positive for each case.
- External sysfs `after - before` is positive for each case or at least for the full smoke batch.
- Smoke script exits 0 and prints JSON with `ok: true`.

## NPU busy-time verification plan

HTTP 200 is not proof. Verification must capture both endpoint-reported and sysfs-observed deltas.

Procedure:

```bash
BUSY=/sys/class/accel/accel0/device/npu_busy_time_us
before=$(cat "$BUSY")
curl -fsS http://127.0.0.1:18818/rerank \
  -H 'Content-Type: application/json' \
  -d '{"query":"how do I verify OpenVINO NPU usage?","documents":[{"id":"good","text":"Check /sys/class/accel/accel0/device/npu_busy_time_us before and after inference."},{"id":"bad","text":"This note is about making sourdough starter."}],"top_k":2,"return_documents":false}' \
  | jq '{ok, device, npu_busy_delta_us, top_id:.results[0].id}'
after=$(cat "$BUSY")
echo "sysfs_npu_busy_delta_us=$((after-before))"
```

Acceptance:

- `device == "NPU"`.
- Response `npu_busy_delta_us > 0`.
- Shell-computed `sysfs_npu_busy_delta_us > 0`.
- If any value is zero/negative/missing, call the result CPU/unknown and do not claim NPU-backed reranking.

## Optional RAG second-stage integration plan (deferred)

This is a plan only. Do not enable it in live RAG without explicit approval.

Design:

1. Keep existing vector search and Chroma collection `obsidian_bge_npu` unchanged.
2. Retrieve more candidates from current vector search, e.g. `initial_k=20`.
3. Send only request-time candidate snippets/ids to `http://127.0.0.1:18818/rerank`.
4. Use reranker order to choose final `top_k`, e.g. `5`.
5. On timeout, connection error, invalid response, or non-positive NPU proof when proof is required, fall back to vector order and attach metadata like `rerank_error`; do not fail the whole RAG request unless explicitly configured.
6. Log counters and latency, but avoid logging raw private document text.

Disabled-by-default knobs:

```text
RAG_RERANK_ENABLED=false
RAG_RERANK_URL=http://127.0.0.1:18818/rerank
RAG_RERANK_INITIAL_K=20
RAG_RERANK_TOP_K=5
RAG_RERANK_TIMEOUT_MS=3000
RAG_RERANK_REQUIRE_NPU_PROOF=true
RAG_RERANK_RETURN_DOCUMENTS=false
```

Integration tests should use synthetic in-memory candidates first. Live-vault evaluation requires a separate approval and must not mutate or rebuild the vector collection.

## Docs and diagram implications

If this prototype advances beyond spec/review, update these surfaces while keeping live/prototype labels clear:

- `openvino-reranker-npu/README.md`: keep model/runtime, endpoint contract, smoke command, and approval gates synchronized with code.
- `swarm-common/obsidian-vault/will/will-shared-zap/Runbooks/OpenVINO NPU Services Runbook.md`: list `:18818` as prototype/not enabled, with foreground smoke and NPU sysfs proof.
- Service catalog / architecture notes: show live baseline `:18810`, `:18816`, `:18817`; show `:18818` as optional second-stage RAG prototype, not live routing.
- Diagrams: render `RAG :18810 -> optional reranker :18818` as dashed/disabled or "proposed"; do not imply Atlas/Hermes/gateway traffic is using it.
- Optional systemd unit: document as installable after approval, not enabled by default.

## No-go / defer criteria

Do not ship, enable, or integrate the reranker if any of these hold:

- Port `18818` is already owned by another live service.
- `NPU` is unavailable in `ov.Core().available_devices` or `/sys/class/accel/accel0/device/npu_busy_time_us` is missing.
- Foreground startup smoke fails or has non-positive NPU busy-time delta while configured for NPU.
- Synthetic smoke top-1 ranking fails or latency is unacceptable for the intended RAG timeout budget.
- Model export requires overwriting the existing model directory or touching Chroma/vector collections.
- The service must bind beyond `127.0.0.1` to be useful.
- Live RAG integration would require reindexing, collection mutation, private-doc smoke, or Atlas/Hermes/gateway routing changes without explicit approval.
- Logs or responses would persist raw private document text outside the existing RAG request path.

## Current local preflight observed during this spec pass

- `/sys/class/accel/accel0/device/npu_busy_time_us` is readable.
- `/home/will/.cache/openvino-models/rerankers/ms-marco-MiniLM-L6-v2-int8-ov` is present.
- `/home/will/.venvs/openvino-reranker/bin/python` is present.
- `:18818` was not listening during preflight.
- `server.py` and `smoke.py` pass `python -m py_compile`.

These observations are preflight only; they are not a live service/NPU smoke result.