diff --git a/internal/api/http_metrics_test.go b/internal/api/http_metrics_test.go
index 8a32be88b4..6fb06bdca9 100644
--- a/internal/api/http_metrics_test.go
+++ b/internal/api/http_metrics_test.go
@@ -1,6 +1,7 @@
 package api
 
 import (
+	"os"
 	"strconv"
 	"testing"
 	"time"
@@ -198,20 +199,38 @@ func TestNormalizeSegment(t *testing.T) {
 // could copy a short segment into a stack buffer and hide the allocation.
 var normalizeSegmentSink string
 
+// allocIsolationEnv names the test that a child process re-runs on its own.
+const allocIsolationEnv = "PULSE_TEST_ALLOC_ISOLATION"
+
+// measureAllocsInIsolation keeps the upstream test entry point while using the
+// bounded, measurement-marked child launcher shared with the background-fixture
+// control. The child flag and upstream environment marker both select the actual
+// measurement; the parent must observe that measurement, not just a passing test.
+func measureAllocsInIsolation(t *testing.T) bool {
+	t.Helper()
+	if *routeAllocationChild || os.Getenv(allocIsolationEnv) == t.Name() {
+		t.Cleanup(func() { t.Log("ROUTE_ALLOCATION_MEASURED " + t.Name()) })
+		return true
+	}
+	runRouteAllocationTest(t, t.Name())
+	return false
+}
+
 func TestNormalizeSegment_DoesNotAllocate(t *testing.T) {
-	isolatedRouteAllocationCheck(t, func() {
-		for _, seg := range []string{
-			"12345",
-			"550e8400-e29b-41d4-a716-446655440000",
-			"abcdefghijklmnopqrstuvwxyz1234567890abcdef",
-			"resources",
-			"metrics-store",
-		} {
-			if allocs := testing.AllocsPerRun(100, func() { normalizeSegmentSink = normalizeSegment(seg) }); allocs != 0 {
-				t.Errorf("normalizeSegment(%q) allocated %v times per call, want 0", seg, allocs)
-			}
+	if !measureAllocsInIsolation(t) {
+		return
+	}
+	for _, seg := range []string{
+		"12345",
+		"550e8400-e29b-41d4-a716-446655440000",
+		"abcdefghijklmnopqrstuvwxyz1234567890abcdef",
+		"resources",
+		"metrics-store",
+	} {
+		if allocs := testing.AllocsPerRun(100, func() { normalizeSegmentSink = normalizeSegment(seg) }); allocs != 0 {
+			t.Errorf("normalizeSegment(%q) allocated %v times per call, want 0", seg, allocs)
 		}
-	})
+	}
 }
 
 func TestNormalizeRoute(t *testing.T) {
@@ -273,13 +292,14 @@ func TestNormalizeRoute(t *testing.T) {
 var normalizeRouteSink string
 
 func TestNormalizeRoute_RootFastPathDoesNotAllocate(t *testing.T) {
-	isolatedRouteAllocationCheck(t, func() {
-		for _, path := range []string{"/", ""} {
-			if allocs := testing.AllocsPerRun(100, func() { normalizeRouteSink = normalizeRoute(path) }); allocs != 0 {
-				t.Errorf("normalizeRoute(%q) allocated %v times per call, want 0", path, allocs)
-			}
+	if !measureAllocsInIsolation(t) {
+		return
+	}
+	for _, path := range []string{"/", ""} {
+		if allocs := testing.AllocsPerRun(100, func() { normalizeRouteSink = normalizeRoute(path) }); allocs != 0 {
+			t.Errorf("normalizeRoute(%q) allocated %v times per call, want 0", path, allocs)
 		}
-	})
+	}
 }
 
 // ---------------------------------------------------------------------------