diff --git a/src/stackone_defender/core/prompt_defense.py b/src/stackone_defender/core/prompt_defense.py
index 9018cb0..d036ce2 100644
--- a/src/stackone_defender/core/prompt_defense.py
+++ b/src/stackone_defender/core/prompt_defense.py
@@ -55,7 +55,7 @@ def __init__(
         if block_high_risk:
             self._config.block_high_risk = True
 
-        tool_rules = self._config.tool_rules if use_default_tool_rules else []
+        tool_rules = (config or {}).get("tool_rules") or (self._config.tool_rules if use_default_tool_rules else [])
 
         self._tool_sanitizer: ToolResultSanitizer = create_tool_result_sanitizer(
             risky_fields=self._config.risky_fields,
@@ -120,7 +120,20 @@ def defend_tool_result(self, value: Any, tool_name: str) -> DefenseResult:
         tier2_idx = _RISK_LEVELS.index(tier2_risk)
         risk_level = _RISK_LEVELS[max(tier1_idx, tier2_idx)]
 
-        allowed = not self._config.block_high_risk or risk_level not in ("high", "critical")
+        # Determine whether any threat signals were found (Tier 1 or Tier 2).
+        # fields_sanitized captures sanitization methods (role stripping, encoding detection, etc.)
+        # that may fire without adding named pattern detections, so we include it here.
+        has_threats = (
+            len(detections) > 0
+            or len(fields_sanitized) > 0
+            or (tier2_score is not None and tier2_score >= self._config.tier2.high_risk_threshold)
+        )
+
+        # Three cases for allowed:
+        # 1. block_high_risk is off -> always allow
+        # 2. No threat signals found -> allow (base risk from tool rules alone does not block)
+        # 3. Risk did not reach high/critical -> allow
+        allowed = not self._config.block_high_risk or not has_threats or risk_level not in ("high", "critical")
 
         return DefenseResult(
             allowed=allowed,
diff --git a/tests/test_integration.py b/tests/test_integration.py
index 29ab83f..50735e0 100644
--- a/tests/test_integration.py
+++ b/tests/test_integration.py
@@ -147,6 +147,44 @@ def test_returns_latency(self):
         assert result.latency_ms > 0
 
 
+class TestUseDefaultToolRules:
+    def test_does_not_apply_tool_rules_by_default(self):
+        defense = create_prompt_defense()
+        data = {"subject": "Weekly team update", "body": "Reminder about the meeting tomorrow at 10am.", "thread_id": "thread123"}
+        result = defense.defend_tool_result(data, "gmail_get_message")
+        # Without use_default_tool_rules, gmail tool rule should NOT seed risk_level to 'high'
+        assert result.risk_level not in ("high", "critical")
+
+    def test_does_not_apply_tool_rules_when_explicitly_false(self):
+        defense = create_prompt_defense(use_default_tool_rules=False)
+        data = {"subject": "Weekly team update", "body": "Reminder about the meeting tomorrow at 10am.", "thread_id": "thread123"}
+        result = defense.defend_tool_result(data, "gmail_get_message")
+        assert result.risk_level not in ("high", "critical")
+
+    def test_applies_tool_rules_when_true(self):
+        defense = create_prompt_defense(use_default_tool_rules=True, block_high_risk=True)
+        data = {"subject": "Weekly team update", "body": "Reminder about the meeting tomorrow at 10am.", "thread_id": "thread123"}
+        result = defense.defend_tool_result(data, "gmail_get_message")
+        # With use_default_tool_rules, gmail tool rule seeds risk_level: 'high' as base risk,
+        # but safe content with no detections should still be allowed through.
+        assert result.risk_level == "high"
+        assert result.allowed is True
+
+    def test_always_applies_custom_tool_rules_from_config(self):
+        from stackone_defender.types import ToolSanitizationRule
+        defense = create_prompt_defense(
+            use_default_tool_rules=False,
+            config={"tool_rules": [ToolSanitizationRule(tool_pattern="custom_*", sanitization_level="high")]},
+            block_high_risk=True,
+        )
+        data = {"name": "Safe content"}
+        result = defense.defend_tool_result(data, "custom_tool")
+        # Custom rules set base risk_level: 'high', but safe content with no detections
+        # should still be allowed through — base risk alone does not block.
+        assert result.risk_level == "high"
+        assert result.allowed is True
+
+
 class TestRealWorldScenarios:
     def setup_method(self):
         self.defense = create_prompt_defense()