diff --git a/main_test.go b/main_test.go
new file mode 100644
index 0000000..aec86c6
--- /dev/null
+++ b/main_test.go
@@ -0,0 +1,46 @@
+package main
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"sync/atomic"
+	"testing"
+)
+
+func TestLookupMissingDomain(t *testing.T) {
+	s := &server{
+		nrenASNs: make(map[uint]struct{}),
+	}
+	s.ready.Store(true)
+
+	req := httptest.NewRequest(http.MethodGet, "/lookup", nil)
+	rr := httptest.NewRecorder()
+
+	s.lookupHandler(rr, req)
+
+	if rr.Code != http.StatusBadRequest {
+		t.Fatalf("expected 400, got %d", rr.Code)
+	}
+
+	if !strings.Contains(rr.Body.String(), "missing domain") {
+		t.Fatalf("expected error message in response")
+	}
+}
+
+func TestLookupServiceNotReady(t *testing.T) {
+	s := &server{
+		nrenASNs: make(map[uint]struct{}),
+	}
+	s.ready = atomic.Bool{}
+	s.ready.Store(false)
+
+	req := httptest.NewRequest(http.MethodGet, "/lookup?domain=example.com", nil)
+	rr := httptest.NewRecorder()
+
+	s.lookupHandler(rr, req)
+
+	if rr.Code != http.StatusServiceUnavailable {
+		t.Fatalf("expected 503, got %d", rr.Code)
+	}
+}