fix: calculate conservative SLO percentiles

server/observability/slo.go

@@ -1,6 +1,7 @@
 package observability
 
 import (
+	"math"
 	"sort"
 	"time"
 )
@@ -52,7 +53,13 @@ func percentile(values []time.Duration, p float64) time.Duration {
 	}
 	ordered := append([]time.Duration(nil), values...)
 	sort.Slice(ordered, func(i, j int) bool { return ordered[i] < ordered[j] })
-	index := int(float64(len(ordered)-1) * p)
+	index := int(math.Ceil(p*float64(len(ordered)))) - 1
+	if index < 0 {
+		index = 0
+	}
+	if index >= len(ordered) {
+		index = len(ordered) - 1
+	}
 	return ordered[index]
 }
 

server/observability/slo_test.go

@@ -25,3 +25,9 @@ func TestEvaluateSLODoesNotInventFailureForEmptyOptionalWindows(t *testing.T) {
 		t.Fatalf("empty window violations = %+v", violations)
 	}
 }
+
+func TestPercentileUsesConservativeNearestRankForSmallWindows(t *testing.T) {
+	if got := percentile([]time.Duration{time.Millisecond, 101 * time.Millisecond}, .95); got != 101*time.Millisecond {
+		t.Fatalf("p95 underreported small window: %s", got)
+	}
+}