# M8.6 — Upgrade GET /memory/query to hybrid with fallback

| Field | Value |
|---|---|
| Phase | M8 — Hybrid Search |
| Size | M — 1–2 days |
| Status | ✅ COMPLETE |
| Flags | — |
| Spec | inlined below |
| Blocks | M8.8, M8.9 |
| Depends | M8.3, M8.4 (query optimizer, RRF fusion complete) |

## Goal

Replace the `QueryWorker` call in `GET /memory/query` with `HybridQueryWorker`. Add `?method=` parameter for explicit strategy override. Implement fallback chain: hybrid → semantic → error.

## Design

**Updated request:**
```
GET /memory/query?query=...&project=...&limit=10&method=hybrid
Authorization: Bearer <JWT>
```

New parameter:
- `method` (optional) — `hybrid` (default), `semantic`, `lexical`. If omitted, `QueryOptimizer` decides.

**Handler logic:**
```rust
async fn query_handler(...) -> HttpResponse {
    let (claims, token) = validate_auth(...)?;
    check_capability(&claims, "memory:read")?;
    check_rate_limit(&claims, &state, "/memory/query")?;

    let method_override = query.get("method").map(|m| match m.as_str() {
        "semantic" => SearchStrategy::SemanticOnly,
        "lexical" => SearchStrategy::LexicalOnly,
        _ => SearchStrategy::Hybrid,  // includes "hybrid" and unknown values
    });

    // Try hybrid worker first
    if let Some(ref hybrid) = state.hybrid_query_worker {
        match hybrid.query(&project, &question, limit, &token, method_override).await {
            Ok(response) => return HttpResponse::Ok().json(response),
            Err(e) => {
                tracing::warn!("hybrid query failed, falling back: {}", e);
            }
        }
    }

    // Fallback: existing semantic-only worker
    match state.query_worker.query(&project, &question, Some(limit)).await {
        Ok(results) => HttpResponse::Ok().json(json!({
            "query": question,
            "project": project,
            "search_strategy": "semantic_fallback",
            "results": results,
        })),
        Err(e) => HttpResponse::InternalServerError().json(json!({"error": "query_failed"})),
    }
}
```

**AppState change:**
```rust
pub struct AppState {
    // ... existing fields ...
    pub hybrid_query_worker: Option<Arc<HybridQueryWorker>>,  // None if OpenSearch not configured
}
```

Constructed in `start_server()`:
```rust
let hybrid = if std::env::var("OPENSEARCH_HOSTS").is_ok() {
    let os_client = OpenSearchClient::new(hosts);
    Some(Arc::new(HybridQueryWorker::new(vector_store, embeddings, Some(Arc::new(os_client)))))
} else {
    // No OpenSearch configured — hybrid worker without lexical
    Some(Arc::new(HybridQueryWorker::new(vector_store, embeddings, None)))
};
```

**Backward compatibility:** If `OPENSEARCH_HOSTS` is not set, the worker still works but always uses `SemanticOnly`. Existing clients see the same results with an added `search_strategy` field.

## Steps

1. Add `hybrid_query_worker` field to `AppState`.
2. Construct `HybridQueryWorker` in `start_server()`, gated on `OPENSEARCH_HOSTS` env.
3. Update `query_handler()` with method override + fallback chain.
4. Add `?method=` query parameter parsing.
5. Update response format to include `search_strategy` and `metrics`.
6. Write integration test: query with `method=hybrid`, verify response has `metrics`.
7. Write integration test: query without OpenSearch, verify fallback to semantic.

## Acceptance

1. `GET /memory/query?query=test&project=p` returns `"search_strategy": "Hybrid"` when OpenSearch configured.
2. `GET /memory/query?query=test&project=p&method=semantic` forces semantic-only, response says `"search_strategy": "SemanticOnly"`.
3. If OpenSearch is down, query still succeeds with `"search_strategy": "semantic_fallback"`.
4. Response includes `metrics` block with timing data.
5. Existing clients that don't send `method` param get the same results as before (backward compat).
6. JWT token is forwarded to OpenSearch (not a new token, not admin credentials).

## Verify

```bash
# With OpenSearch running
curl -H "Authorization: Bearer $TOKEN" \
  "http://localhost:8080/memory/query?query=kubernetes+port&project=poimen" | jq .search_strategy
# Should output: "Hybrid"

# Force semantic
curl -H "Authorization: Bearer $TOKEN" \
  "http://localhost:8080/memory/query?query=kubernetes+port&project=poimen&method=semantic" | jq .search_strategy
# Should output: "SemanticOnly"

# Kill OpenSearch, retry
curl -H "Authorization: Bearer $TOKEN" \
  "http://localhost:8080/memory/query?query=kubernetes+port&project=poimen" | jq .search_strategy
# Should output: "semantic_fallback"
```

**False pass:** Handler catches the hybrid error silently and always falls back to semantic — user never sees hybrid results even when OpenSearch is healthy. Assert that with OpenSearch up, `metrics.lexical_candidates` is `Some(n)` where `n > 0`.

## Artifacts

- Modified `crates/mem-cli/src/http_server.rs`