diff --git a/pkg/server/service_health_test.go b/pkg/server/service_health_test.go
index 5f33e0aaf2..7716ca13bd 100644
--- a/pkg/server/service_health_test.go
+++ b/pkg/server/service_health_test.go
@@ -6,6 +6,7 @@ import (
 	"net"
 	"net/http"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 
@@ -118,12 +119,31 @@ func TestFrontendAssetPathsStayLocalAndBounded(t *testing.T) {
 	}
 }
 
+// syncBuffer is a goroutine-safe bytes.Buffer for capturing watchdog log output
+// written from another goroutine while the test polls it.
+type syncBuffer struct {
+	mu  sync.Mutex
+	buf bytes.Buffer
+}
+
+func (b *syncBuffer) Write(p []byte) (int, error) {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	return b.buf.Write(p)
+}
+
+func (b *syncBuffer) String() string {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	return b.buf.String()
+}
+
 // A bound-but-not-serving listener is otherwise undiagnosable: nothing accepts
 // connections and no startup log line is emitted while a synchronous step
 // stalls. The watchdog must name the last completed phase and dump the local
 // goroutine stacks so a recurrence can be read from the field log.
 func TestStartupWatchdogLogsPhaseAndStack(t *testing.T) {
-	var buf bytes.Buffer
+	var buf syncBuffer
 	phase := &startupPhase{}
 	phase.mark("config watcher started")