diff --git a/mod/onlyobj_test.go b/mod/onlyobj_test.go
new file mode 100644
index 0000000..ac4a5b3
--- /dev/null
+++ b/mod/onlyobj_test.go
@@ -0,0 +1,118 @@
+package mod
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+
+	"ModCreator/tests"
+	"ModCreator/types"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+// TestObjectOnlyRoundTrip covers the --objin/--objout "downloadable content"
+// feature (OnlyObjState / OnlyObjStates): a single object is decomposed into
+// files by reverse and reassembled by generate. Reverse->generate must
+// reproduce the original object, including its contained objects and states.
+func TestObjectOnlyRoundTrip(t *testing.T) {
+	orig := types.J{
+		"GUID":           "aaa111",
+		"Nickname":       "Rich Bag",
+		"LuaScript":      "function onLoad()\n  print(\"hi\")\nend",
+		"LuaScriptState": "{\"x\":1}",
+		"ContainedObjects": []interface{}{
+			types.J{"GUID": "child1", "Nickname": "Card A"},
+			types.J{"GUID": "child2", "Nickname": "Card B"},
+		},
+		"States": types.J{
+			"2": types.J{"GUID": "state2", "Nickname": "flipped"},
+		},
+	}
+
+	ff := tests.NewFF()
+
+	// Reverse (object-only): decompose the single object into files.
+	r := Reverser{
+		ModSettingsWriter: ff,
+		LuaWriter:         ff,
+		LuaSrcWriter:      ff,
+		XMLWriter:         ff,
+		XMLSrcWriter:      ff,
+		ObjWriter:         ff,
+		ObjDirCreator:     ff,
+		RootWrite:         ff,
+		OnlyObjState:      "ready.json",
+	}
+	if err := r.Write(jsonClone(t, orig)); err != nil {
+		t.Fatalf("object-only reverse: %v", err)
+	}
+
+	// The root object is written as "<sanitized nickname>.<guid>.json" at the
+	// top level; find it so we can point generate at it (mirrors how main.go
+	// derives OnlyObjStates from the --objin basename).
+	var rootFile string
+	for k := range ff.Data {
+		if strings.HasSuffix(k, ".json") && !strings.Contains(k, "/") {
+			rootFile = k
+		}
+	}
+	if rootFile == "" {
+		t.Fatalf("reverse wrote no root object file; json files: %v", ffKeys(ff))
+	}
+
+	// Generate (object-only): reassemble the single object.
+	m := &Mod{
+		Lua:           ff,
+		XML:           ff,
+		Modsettings:   ff,
+		Objs:          ff,
+		Objdirs:       ff,
+		RootRead:      ff,
+		RootWrite:     ff,
+		OnlyObjStates: rootFile,
+	}
+	if err := m.GenerateFromConfig(); err != nil {
+		t.Fatalf("object-only generate: %v", err)
+	}
+
+	// Compare through a JSON round trip so slice/map element types
+	// (e.g. []interface{} vs []types.J) don't produce spurious diffs.
+	if diff := cmp.Diff(jsonNormalize(t, orig), jsonNormalize(t, m.Data)); diff != "" {
+		t.Errorf("object-only round trip differs (-want +got):\n%s", diff)
+	}
+}
+
+func jsonClone(t *testing.T, v interface{}) map[string]interface{} {
+	t.Helper()
+	b, err := json.Marshal(v)
+	if err != nil {
+		t.Fatalf("marshal: %v", err)
+	}
+	var out map[string]interface{}
+	if err := json.Unmarshal(b, &out); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	return out
+}
+
+func jsonNormalize(t *testing.T, v interface{}) interface{} {
+	t.Helper()
+	b, err := json.Marshal(v)
+	if err != nil {
+		t.Fatalf("marshal: %v", err)
+	}
+	var out interface{}
+	if err := json.Unmarshal(b, &out); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	return out
+}
+
+func ffKeys(ff *tests.FakeFiles) []string {
+	ks := []string{}
+	for k := range ff.Data {
+		ks = append(ks, k)
+	}
+	return ks
+}