diff --git a/adiuvare/core/scorer.py b/adiuvare/core/scorer.py
index 70304c5..3b80040 100644
--- a/adiuvare/core/scorer.py
+++ b/adiuvare/core/scorer.py
@@ -2,12 +2,16 @@
 
 _weights = {
     "payload": 0.40,
-    "behavior": 0.35,
-    "identity": 0.25,
+    "behavior": 0.30,
+    "identity": 0.15,
     "context": 0.10,
     "ip_rep": 0.05,
 }
 
+_total_w = sum(_weights.values())
+assert _total_w > 0, "default _weights must sum to a positive value"
+_weights = {k: v / _total_w for k, v in _weights.items()}
+
 
 def compute_score(sig_res: dict[str, SignalResult], snap=None) -> tuple[float, dict[str, float]]:
     breakdown: dict[str, float] = {}
@@ -16,9 +20,25 @@ def compute_score(sig_res: dict[str, SignalResult], snap=None) -> tuple[float, d
 
     weights = dict(_weights)
     if snap:
-        weights["payload"] = snap.payload_weight
-        weights["behavior"] = snap.behavior_weight
-        weights["identity"] = snap.identity_weight
+        snap_weights = {
+            "payload": snap.payload_weight,
+            "behavior": snap.behavior_weight,
+            "identity": snap.identity_weight,
+        }
+
+        # only snap-overridden keys are validated; context and ip_rep carry
+        # over from _weights which are already normalized and guaranteed positive
+        for k, v in snap_weights.items():
+            if v < 0:
+                raise ValueError(f"Weight for '{k}' must be non-negative, got {v}")
+
+        total_snap = sum(snap_weights.values())
+        if total_snap <= 0:
+            raise ValueError("Snap weights sum to zero or below - cannot normalize.")
+
+        weights.update(snap_weights)
+        total_w = sum(weights.values())
+        weights = {k: v / total_w for k, v in weights.items()}
 
     for name, res in sig_res.items():
         weight = weights.get(name, 0.0)
diff --git a/tests/test_command_probe.py b/tests/test_command_probe.py
index 43f0ce4..eeb4c10 100644
--- a/tests/test_command_probe.py
+++ b/tests/test_command_probe.py
@@ -11,7 +11,7 @@ def test_command_probe_semicolon_detected():
 
     assert gate.passed is True
     assert event is not None
-    assert event.score > 0.35
+    assert event.score > 0.30
     assert "payload" in event.breakdown
     assert event.breakdown["payload"] > 0.0
 
@@ -26,7 +26,7 @@ def test_command_probe_dollar_detected():
 
     assert gate.passed is True
     assert event is not None
-    assert event.score > 0.35
+    assert event.score > 0.30
     assert "payload" in event.breakdown
     assert event.breakdown["payload"] > 0.0
 
diff --git a/tests/test_scorer.py b/tests/test_scorer.py
index fe634c6..7b13f57 100644
--- a/tests/test_scorer.py
+++ b/tests/test_scorer.py
@@ -1,3 +1,4 @@
+import pytest
 from adiuvare.core.models import ConfigSnapshot, SignalResult
 from adiuvare.core.scorer import compute_score
 from adiuvare.core.verdict import compute_verdict
@@ -11,8 +12,8 @@ def test_score_uses_hardcoded_weights():
         }
     )
 
-    assert round(score, 3) == 0.395
-    assert round(breakdown["payload"], 3) == 0.28
+    assert score == pytest.approx(0.38, rel=1e-3)
+    assert breakdown["payload"] == pytest.approx(0.28, rel=1e-3)
 
 
 def test_verdict_maps_score_ranges():
@@ -39,8 +40,7 @@ def test_score_can_use_snapshot_weights():
         snap,
     )
 
-    assert round(score, 3) == 0.45
-    assert round(breakdown["payload"], 3) == 0.35
+    assert score == pytest.approx(0.393, rel=1e-3)
 
 
 def test_verdict_gets_identity_nudge_inline():
@@ -53,3 +53,29 @@ def test_verdict_gets_identity_nudge_inline():
         block_threshold=0.80,
     )
     assert compute_verdict(0.50, snap, identity_risk=0.70) == "throttle"
+
+def test_identity_heavy_detection():
+    score, breakdown = compute_score(
+        {
+            "identity": SignalResult(score=1.0, reason="known_bad_actor"),
+            "ip_rep": SignalResult(score=0.8, reason="blacklisted_ip"),
+        }
+    )
+    assert score == pytest.approx(0.20, rel=1e-3)
+    assert breakdown["identity"] == pytest.approx(0.15, rel=1e-3)
+
+def test_negative_weight_raises():
+    snap = ConfigSnapshot(
+        payload_weight=-0.5, behavior_weight=0.5, identity_weight=0.0,
+        flag_threshold=0.25, throttle_threshold=0.55, block_threshold=0.80,
+    )
+    with pytest.raises(ValueError, match="non-negative"):
+        compute_score({"payload": SignalResult(score=0.7, reason="sql_hit")}, snap)
+
+def test_all_zero_weights_raises():
+    snap = ConfigSnapshot(
+        payload_weight=0.0, behavior_weight=0.0, identity_weight=0.0,
+        flag_threshold=0.25, throttle_threshold=0.55, block_threshold=0.80,
+    )
+    with pytest.raises(ValueError, match="sum to zero"):
+        compute_score({"payload": SignalResult(score=0.7, reason="sql_hit")}, snap)
\ No newline at end of file