# Icarus Threshold Bug — 2026-07-16 (v2)

## The Problem

`icarus/hooks.py` calls `_search_qdrant(query, top_k=2, threshold=0.55)` from `pre_llm_call()`.
This passes `score_threshold=0.55` to `search_with_fallback()` in `context_enhancer.py`.

RRF fusion (hybrid dense+sparse) returns scores 0.33–0.50 even for good matches.
At 0.55, every query is filtered out. The search cascade falls to `level="none"`,
then hits SQLite fallback (`[CE-FALLBACK] SQLite search failed: no such table: lineage`),
and finally returns an empty list. Icarus injects nothing.

This is fundamentally different from dense-only search, which returns COSINE scores
0.90+ for the same queries. The threshold bug was invisible because:
- `_search_qdrant()` is fail-open (returns `[]` on any error)
- The plugin loads and runs without crashing
- "Runs without crashing" ≠ "returns useful results"

## Root Cause: RRF vs Dense Score Regimes

RRF (Reciprocal Rank Fusion) normalises scores from two independent retrievers
(dense and sparse) into a shared 0–1 range via `1/(k + rank)`. This inherently
produces clustered scores around 0.33–0.50 regardless of the underlying semantic
similarity. This is **not a bug in RRF** — it's how RRF works.

Dense-only search returns raw COSINE similarity (0.90+ for good matches).

The same query at different thresholds:

```
Query: "XRay VPS VPN"

=== Hybrid (RRF), threshold 0.55 ===
  Level: none, Results: 0
  Falls through to SQLite → [CE-FALLBACK] All fallback levels exhausted.

=== Dense-only, threshold 0.35 ===
  Level: dense-only, Results: 3
  [0.9938] Пароль Юлии Зозули
  [0.9078] Nagios
  [0.9017] Клавиатуры

=== Hybrid (RRF), threshold 0.35 ===
  Level: hybrid, Results: 2
  [0.5000] Инструкция как безопасно расширить кластер Garage до 3+ нод (v2.1)
  [0.5000] Пароль Юлии Зозули

=== Hybrid (RRF), threshold 0.30 ===
  Level: hybrid, Results: 4
  [0.5000] Garage кластер
  [0.5000] Пароль Юлии Зозули
  [0.3333] Двухфакторная VPN
  [0.3333] Nagios
```

At 0.35, RRF still filters the 0.33 results (2 out of 4 are lost).
At 0.30, all 4 are returned.

## The Fix

Lower `threshold` in `_search_qdrant()` in `/opt/hermes/.hermes/plugins/icarus/hooks.py`
from 0.55 to 0.30.

```python
# Line 722 — NOW:
qdrant_results = _search_qdrant(user_message, top_k=2, threshold=0.30)
```

The comment above it was also updated to explain the RRF vs dense score gap.

## Verification

After fix, all queries return hybrid results within 6ms Qdrant time:

```
XRay VPS VPN              → hybrid, 4 results
hermes plugin icarus       → hybrid, 3 results
telegram                   → hybrid, 4 results
wireguard                  → hybrid, 4 results
```

No SQLite fallback noise. No `level="none"`.

## Key Insight

When debugging an empty `[qdrant]` block in Icarus context injection:
1. First check if `_search_qdrant()` even runs (no import error → PYTHONPATH fix)
2. Then check if it returns results (threshold too high for RRF)
3. These are SEPARATE bugs — the first was fixed 2026-07-14 (PYTHONPATH),
   the second on 2026-07-16 (threshold 0.55→0.30)