diff --git a/cmd/acb-index-builder/main.go b/cmd/acb-index-builder/main.go
index f06ab37..7444034 100644
--- a/cmd/acb-index-builder/main.go
+++ b/cmd/acb-index-builder/main.go
@@ -10,6 +10,7 @@ import (
 	"syscall"
 	"time"
 
+	"github.com/aicodebattle/acb/metrics"
 	_ "github.com/lib/pq"
 )
 
@@ -47,6 +48,10 @@ func main() {
 		"database", cfg.PostgresDatabase,
 	)
 
+	// Start internal metrics server (separate port for Prometheus scraping)
+	metricsSrv := metrics.StartServer()
+	defer metricsSrv.Close()
+
 	// Create output directory
 	if err := os.MkdirAll(cfg.OutputDir, 0755); err != nil {
 		slog.Error("Failed to create output directory", "error", err, "path", cfg.OutputDir)
@@ -79,6 +84,7 @@ func main() {
 		buildCount++
 		slog.Info("Starting build cycle", "count", buildCount)
 
+		buildStart := time.Now()
 		buildCtx, buildCancel := context.WithTimeout(context.Background(), cfg.BuildTimeout)
 		if err := runBuildCycle(buildCtx, db, cfg); err != nil {
 			slog.Error("Build cycle failed", "error", err)
@@ -86,6 +92,7 @@ func main() {
 			slog.Info("Build cycle completed", "count", buildCount)
 		}
 		buildCancel()
+		metrics.IndexBuildDuration.Observe(time.Since(buildStart).Seconds())
 
 		// Deploy every N cycles
 		if buildCount%cfg.DeployInterval == 0 {
diff --git a/cmd/acb-index-builder/rivalry_test.go b/cmd/acb-index-builder/rivalry_test.go
index d49aad6..811c53b 100644
--- a/cmd/acb-index-builder/rivalry_test.go
+++ b/cmd/acb-index-builder/rivalry_test.go
@@ -308,32 +308,50 @@ func TestComputeRivalries_MultiPlayerSkipped(t *testing.T) {
 func TestComputeRivalries_RecencyBoost(t *testing.T) {
 	now := time.Now()
 
-	// Pair 1: all matches in the last week (high recency)
+	// Pair 1: all matches in the last week (high recency), balanced 5-5 split
 	var recentMatches []MatchData
-	for i := 0; i < 10; i++ {
+	for i := 0; i < 5; i++ {
 		recentMatches = append(recentMatches, MatchData{
-			ID:       fmt.Sprintf("recent_%d", i),
+			ID:       fmt.Sprintf("recent_a_%d", i),
 			WinnerID: "bot1",
-			PlayedAt: now.Add(-time.Duration(i*12) * time.Hour), // within last 5 days
+			PlayedAt: now.Add(-time.Duration(i*12) * time.Hour),
 			Participants: []ParticipantData{
 				{BotID: "bot1", Score: 3, Won: true},
 				{BotID: "bot2", Score: 2, Won: false},
 			},
 		})
+		recentMatches = append(recentMatches, MatchData{
+			ID:       fmt.Sprintf("recent_b_%d", i),
+			WinnerID: "bot2",
+			PlayedAt: now.Add(-time.Duration(i*12+6) * time.Hour),
+			Participants: []ParticipantData{
+				{BotID: "bot1", Score: 2, Won: false},
+				{BotID: "bot2", Score: 3, Won: true},
+			},
+		})
 	}
 
-	// Pair 2: all matches 6 months ago (low recency)
+	// Pair 2: all matches 6 months ago (low recency), balanced 5-5 split
 	var oldMatches []MatchData
-	for i := 0; i < 10; i++ {
+	for i := 0; i < 5; i++ {
 		oldMatches = append(oldMatches, MatchData{
-			ID:       fmt.Sprintf("old_%d", i),
+			ID:       fmt.Sprintf("old_a_%d", i),
 			WinnerID: "bot3",
-			PlayedAt: now.Add(-180*24*time.Hour - time.Duration(i)*time.Hour), // ~6 months ago
+			PlayedAt: now.Add(-180*24*time.Hour - time.Duration(i)*time.Hour),
 			Participants: []ParticipantData{
 				{BotID: "bot3", Score: 3, Won: true},
 				{BotID: "bot4", Score: 2, Won: false},
 			},
 		})
+		oldMatches = append(oldMatches, MatchData{
+			ID:       fmt.Sprintf("old_b_%d", i),
+			WinnerID: "bot4",
+			PlayedAt: now.Add(-180*24*time.Hour - time.Duration(i+5)*time.Hour),
+			Participants: []ParticipantData{
+				{BotID: "bot3", Score: 2, Won: false},
+				{BotID: "bot4", Score: 3, Won: true},
+			},
+		})
 	}
 
 	allMatches := append(recentMatches, oldMatches...)