fix: streaming e2e failures (vllm-project#1640)

Signed-off-by: xunzhuo <xunzhuo@vllm-semantic-router.ai>

Author: Xunzhuo

src/semantic-router/pkg/decision/engine.go

@@ -239,13 +239,16 @@ func signalConfidence(confidences map[string]float64, signalType string, name st
 	return 1.0
 }
 
-// evalAND returns true only when every child matches; confidence is the average.
+// evalAND returns true only when every child matches.
+// An empty conjunction acts as a catch-all/default route with zero confidence,
+// so it can serve as a fallback without outranking signal-backed decisions when
+// confidence-based selection is enabled.
 func (e *DecisionEngine) evalAND(
 	children []config.RuleNode,
 	signals *SignalMatches,
 ) (matched bool, confidence float64, matchedRules []string) {
 	if len(children) == 0 {
-		return false, 0, nil
+		return true, 0, nil
 	}
 	totalConf := 0.0
 	for _, child := range children {

src/semantic-router/pkg/decision/engine_empty_and_test.go

@@ -0,0 +1,103 @@
+package decision
+
+import (
+	"testing"
+
+	"github.com/vllm-project/semantic-router/src/semantic-router/pkg/config"
+)
+
+func TestDecisionEngine_EmptyANDActsAsCatchAll(t *testing.T) {
+	engine := NewDecisionEngine(
+		nil,
+		nil,
+		nil,
+		[]config.Decision{
+			{
+				Name:     "default-route",
+				Priority: 10,
+				Rules: config.RuleCombination{
+					Operator:   "AND",
+					Conditions: []config.RuleCondition{},
+				},
+			},
+		},
+		"priority",
+	)
+
+	result, err := engine.EvaluateDecisionsWithSignals(&SignalMatches{})
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+	if result == nil {
+		t.Fatal("Expected result but got nil")
+	}
+	if result.Decision.Name != "default-route" {
+		t.Fatalf("Expected default-route, got %s", result.Decision.Name)
+	}
+	if result.Confidence != 0.0 {
+		t.Fatalf("Expected zero confidence for catch-all route, got %f", result.Confidence)
+	}
+}
+
+func TestDecisionEngine_EmptyANDActsAsFallbackInConfidenceMode(t *testing.T) {
+	engine := NewDecisionEngine(
+		nil,
+		nil,
+		nil,
+		[]config.Decision{
+			{
+				Name:     "specific-route",
+				Priority: 200,
+				Rules: config.RuleCombination{
+					Operator: "AND",
+					Conditions: []config.RuleCondition{
+						{Type: "keyword", Name: "urgent"},
+					},
+				},
+			},
+			{
+				Name:     "default-route",
+				Priority: 100,
+				Rules: config.RuleCombination{
+					Operator:   "AND",
+					Conditions: []config.RuleCondition{},
+				},
+			},
+		},
+		"confidence",
+	)
+
+	t.Run("signal-backed route outranks catch-all", func(t *testing.T) {
+		result, err := engine.EvaluateDecisionsWithSignals(&SignalMatches{
+			KeywordRules: []string{"urgent"},
+			SignalConfidences: map[string]float64{
+				"keyword:urgent": 0.72,
+			},
+		})
+		if err != nil {
+			t.Fatalf("Unexpected error: %v", err)
+		}
+		if result == nil {
+			t.Fatal("Expected result but got nil")
+		}
+		if result.Decision.Name != "specific-route" {
+			t.Fatalf("Expected specific-route, got %s", result.Decision.Name)
+		}
+	})
+
+	t.Run("catch-all still matches without signals", func(t *testing.T) {
+		result, err := engine.EvaluateDecisionsWithSignals(&SignalMatches{})
+		if err != nil {
+			t.Fatalf("Unexpected error: %v", err)
+		}
+		if result == nil {
+			t.Fatal("Expected result but got nil")
+		}
+		if result.Decision.Name != "default-route" {
+			t.Fatalf("Expected default-route, got %s", result.Decision.Name)
+		}
+		if result.Confidence != 0.0 {
+			t.Fatalf("Expected zero confidence for catch-all route, got %f", result.Confidence)
+		}
+	})
+}