diff --git a/internal/notifier/alertmanager_test.go b/internal/notifier/alertmanager_test.go
index 308f533a3..834b1e7f6 100644
--- a/internal/notifier/alertmanager_test.go
+++ b/internal/notifier/alertmanager_test.go
@@ -23,10 +23,27 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"testing"
+	"time"
 
 	. "github.com/onsi/gomega"
 )
 
+func TestNewAlertmanager(t *testing.T) {
+	t.Run("valid URL", func(t *testing.T) {
+		g := NewWithT(t)
+		am, err := NewAlertmanager("https://alertmanager.example.com/api/v2/alerts", "", nil, "", "", "")
+		g.Expect(err).ToNot(HaveOccurred())
+		g.Expect(am.URL).To(Equal("https://alertmanager.example.com/api/v2/alerts"))
+	})
+
+	t.Run("invalid URL", func(t *testing.T) {
+		g := NewWithT(t)
+		_, err := NewAlertmanager("not a url", "", nil, "", "", "")
+		g.Expect(err).To(HaveOccurred())
+		g.Expect(err.Error()).To(ContainSubstring("invalid Alertmanager URL"))
+	})
+}
+
 func TestAlertmanager_Post(t *testing.T) {
 	g := NewWithT(t)
 	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
@@ -36,6 +53,17 @@ func TestAlertmanager_Post(t *testing.T) {
 		err = json.Unmarshal(b, &payload)
 		g.Expect(err).ToNot(HaveOccurred())
 
+		g.Expect(payload).To(HaveLen(1))
+		alert := payload[0]
+		g.Expect(alert.Status).To(Equal("firing"))
+		g.Expect(alert.Labels["alertname"]).To(Equal("FluxGitRepositoryReason"))
+		g.Expect(alert.Labels["severity"]).To(Equal("info"))
+		g.Expect(alert.Labels["reason"]).To(Equal("reason"))
+		g.Expect(alert.Labels["kind"]).To(Equal("GitRepository"))
+		g.Expect(alert.Labels["name"]).To(Equal("webapp"))
+		g.Expect(alert.Labels["namespace"]).To(Equal("gitops-system"))
+		g.Expect(alert.Labels["reportingcontroller"]).To(Equal("source-controller"))
+		g.Expect(alert.Annotations["message"]).To(Equal("message"))
 	}))
 	defer ts.Close()
 
@@ -45,3 +73,97 @@ func TestAlertmanager_Post(t *testing.T) {
 	err = alertmanager.Post(context.TODO(), testEvent())
 	g.Expect(err).ToNot(HaveOccurred())
 }
+
+func TestAlertmanager_PostWithBearerToken(t *testing.T) {
+	g := NewWithT(t)
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		g.Expect(r.Header.Get("Authorization")).To(Equal("Bearer test-token"))
+	}))
+	defer ts.Close()
+
+	alertmanager, err := NewAlertmanager(ts.URL, "", nil, "test-token", "", "")
+	g.Expect(err).ToNot(HaveOccurred())
+
+	err = alertmanager.Post(context.TODO(), testEvent())
+	g.Expect(err).ToNot(HaveOccurred())
+}
+
+func TestAlertmanager_PostWithBasicAuth(t *testing.T) {
+	g := NewWithT(t)
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		username, password, ok := r.BasicAuth()
+		g.Expect(ok).To(BeTrue())
+		g.Expect(username).To(Equal("admin"))
+		g.Expect(password).To(Equal("secret"))
+	}))
+	defer ts.Close()
+
+	alertmanager, err := NewAlertmanager(ts.URL, "", nil, "", "admin", "secret")
+	g.Expect(err).ToNot(HaveOccurred())
+
+	err = alertmanager.Post(context.TODO(), testEvent())
+	g.Expect(err).ToNot(HaveOccurred())
+}
+
+func TestAlertmanager_PostWithSummaryMetadata(t *testing.T) {
+	g := NewWithT(t)
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		b, err := io.ReadAll(r.Body)
+		g.Expect(err).ToNot(HaveOccurred())
+		var payload []AlertManagerAlert
+		err = json.Unmarshal(b, &payload)
+		g.Expect(err).ToNot(HaveOccurred())
+
+		alert := payload[0]
+		g.Expect(alert.Annotations["summary"]).To(Equal("deployment failed"))
+		// summary should not appear as a label
+		_, hasSummaryLabel := alert.Labels["summary"]
+		g.Expect(hasSummaryLabel).To(BeFalse())
+	}))
+	defer ts.Close()
+
+	alertmanager, err := NewAlertmanager(ts.URL, "", nil, "", "", "")
+	g.Expect(err).ToNot(HaveOccurred())
+
+	event := testEvent()
+	event.Metadata["summary"] = "deployment failed"
+	err = alertmanager.Post(context.TODO(), event)
+	g.Expect(err).ToNot(HaveOccurred())
+}
+
+func TestAlertmanager_PostServerError(t *testing.T) {
+	g := NewWithT(t)
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusInternalServerError)
+	}))
+	defer ts.Close()
+
+	alertmanager, err := NewAlertmanager(ts.URL, "", nil, "", "", "")
+	g.Expect(err).ToNot(HaveOccurred())
+
+	ctx, cancel := context.WithTimeout(context.Background(), 1*time.Second)
+	defer cancel()
+	err = alertmanager.Post(ctx, testEvent())
+	g.Expect(err).To(HaveOccurred())
+	g.Expect(err.Error()).To(ContainSubstring("postMessage failed"))
+}
+
+func TestAlertManagerTime_MarshalJSON(t *testing.T) {
+	g := NewWithT(t)
+	ts := time.Date(2026, 3, 1, 12, 0, 0, 0, time.UTC)
+	amt := AlertManagerTime(ts)
+
+	b, err := json.Marshal(amt)
+	g.Expect(err).ToNot(HaveOccurred())
+	g.Expect(string(b)).To(Equal(`"2026-03-01T12:00:00Z"`))
+}
+
+func TestAlertManagerTime_UnmarshalJSON(t *testing.T) {
+	g := NewWithT(t)
+	var amt AlertManagerTime
+	err := json.Unmarshal([]byte(`"2026-03-01T12:00:00Z"`), &amt)
+	g.Expect(err).ToNot(HaveOccurred())
+
+	expected := time.Date(2026, 3, 1, 12, 0, 0, 0, time.UTC)
+	g.Expect(time.Time(amt)).To(Equal(expected))
+}