diff --git a/lib/gobuild/gobuild.go b/lib/gobuild/gobuild.go
index 5592019..17bf920 100644
--- a/lib/gobuild/gobuild.go
+++ b/lib/gobuild/gobuild.go
@@ -231,10 +231,10 @@ func (s *S3Cache) maybePutObject(ctx context.Context, outputID, diskPath, etag s
 		return fi.ModTime(), err
 	}
 	if written {
-		s.putS3Found.Add(1)
-		return fi.ModTime(), nil // already present and matching
+		s.putS3Object.Add(1)
+		return fi.ModTime(), nil
 	}
-	s.putS3Object.Add(1)
+	s.putS3Found.Add(1) // already present and matching
 	return fi.ModTime(), nil
 }
 
diff --git a/lib/gobuild/gobuild_test.go b/lib/gobuild/gobuild_test.go
new file mode 100644
index 0000000..d03cbad
--- /dev/null
+++ b/lib/gobuild/gobuild_test.go
@@ -0,0 +1,114 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package gobuild_test
+
+import (
+	"context"
+	"expvar"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"sync"
+	"testing"
+
+	"github.com/aws/aws-sdk-go-v2/aws"
+	"github.com/aws/aws-sdk-go-v2/credentials"
+	"github.com/aws/aws-sdk-go-v2/service/s3"
+	"github.com/creachadair/gocache"
+	"github.com/creachadair/gocache/cachedir"
+	"github.com/tailscale/go-cache-plugin/lib/gobuild"
+	"github.com/tailscale/go-cache-plugin/lib/s3util"
+)
+
+// fakeS3 is a minimal in-memory S3 endpoint supporting HEAD and PUT on
+// path-style object URLs.
+type fakeS3 struct {
+	mu   sync.Mutex
+	data map[string][]byte
+}
+
+func (f *fakeS3) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	switch r.Method {
+	case http.MethodHead:
+		if _, ok := f.data[r.URL.Path]; !ok {
+			w.WriteHeader(http.StatusNotFound)
+		}
+	case http.MethodPut:
+		body, _ := io.ReadAll(r.Body)
+		f.data[r.URL.Path] = body
+	default:
+		w.WriteHeader(http.StatusMethodNotAllowed)
+	}
+}
+
+func TestPutMetrics(t *testing.T) {
+	srv := httptest.NewServer(&fakeS3{data: make(map[string][]byte)})
+	defer srv.Close()
+
+	local, err := cachedir.New(t.TempDir())
+	if err != nil {
+		t.Fatalf("Create local cache: %v", err)
+	}
+	cache := &gobuild.S3Cache{
+		Local: local,
+		S3Client: &s3util.Client{
+			Client: s3.New(s3.Options{
+				BaseEndpoint: aws.String(srv.URL),
+				Region:       "us-east-1",
+				Credentials:  credentials.NewStaticCredentialsProvider("id", "secret", ""),
+				UsePathStyle: true,
+			}),
+			Bucket: "test-bucket",
+		},
+	}
+
+	ctx := context.Background()
+	var metrics expvar.Map
+	cache.SetMetrics(ctx, &metrics)
+	count := func(name string) string { return metrics.Get(name).String() }
+
+	const (
+		outputID = "aabbccddeeff00112233445566778899aabbccddeeff00112233445566778899"
+		action1  = "11bbccddeeff00112233445566778899aabbccddeeff00112233445566778899"
+		action2  = "22bbccddeeff00112233445566778899aabbccddeeff00112233445566778899"
+	)
+	put := func(actionID string) {
+		t.Helper()
+		if _, err := cache.Put(ctx, gocache.Object{
+			ActionID: actionID,
+			OutputID: outputID,
+			Size:     5,
+			Body:     strings.NewReader("hello"),
+		}); err != nil {
+			t.Fatalf("Put %s: %v", actionID, err)
+		}
+		if err := cache.Close(ctx); err != nil {
+			t.Fatalf("Close: %v", err)
+		}
+	}
+
+	// The first write stores the object in S3.
+	put(action1)
+	if got := count("put_s3_object"); got != "1" {
+		t.Errorf("After new object: put_s3_object = %s, want 1", got)
+	}
+	if got := count("put_s3_found"); got != "0" {
+		t.Errorf("After new object: put_s3_found = %s, want 0", got)
+	}
+
+	// A second action with the same output finds the object already present.
+	put(action2)
+	if got := count("put_s3_object"); got != "1" {
+		t.Errorf("After existing object: put_s3_object = %s, want 1", got)
+	}
+	if got := count("put_s3_found"); got != "1" {
+		t.Errorf("After existing object: put_s3_found = %s, want 1", got)
+	}
+	if got := count("put_s3_action"); got != "2" {
+		t.Errorf("put_s3_action = %s, want 2", got)
+	}
+}