diff --git a/examples/hello/hello_test.go b/examples/hello/hello_test.go
index a019943..1302ea9 100644
--- a/examples/hello/hello_test.go
+++ b/examples/hello/hello_test.go
@@ -2,6 +2,7 @@ package main
 
 import (
 	"bytes"
+	"context"
 	"crypto/sha256"
 	"net/http"
 	"net/http/httptest"
@@ -18,6 +19,7 @@ import (
 	"git.golem15.com/golem15/summercms/phrasebook"
 	"git.golem15.com/golem15/summercms/postcard"
 	"git.golem15.com/golem15/summercms/surf"
+	"git.golem15.com/golem15/summercms/towel"
 )
 
 func TestGreeterHelloPrintsLayeredConfig(t *testing.T) {
@@ -122,6 +124,13 @@ func TestHelloPluginResolvesLang(t *testing.T) {
 	if !ok {
 		t.Fatal("translator not published")
 	}
+	ctx := towel.WithLocale(context.Background(), "pl")
+	if got := tr.Get(ctx, "golem15.hello::lang.greeting", nil); got != "Cześć" {
+		t.Fatalf("towel locale = %q", got)
+	}
+	if got := tr.Get(context.Background(), "golem15.hello::lang.greeting", nil); got != "Hello" {
+		t.Fatalf("app locale default = %q", got)
+	}
 	if got := tr.GetIn("en", "golem15.hello::lang.labels.pluginName", nil); got != "Hello plugin" {
 		t.Fatalf("en = %q", got)
 	}
diff --git a/party/registry_test.go b/party/registry_test.go
index 5273435..2b959fd 100644
--- a/party/registry_test.go
+++ b/party/registry_test.go
@@ -457,6 +457,26 @@ func TestActivateMalformedLangFailsWithPluginContext(t *testing.T) {
 	}
 }
 
+func TestActivateMalformedLangLeafNamesFileAndKey(t *testing.T) {
+	p := &langPlugin{
+		id: "golem15.hello",
+		fsys: fstest.MapFS{
+			"lang/en/lang.yaml": {Data: []byte("labels:\n  count: 1\n")},
+		},
+	}
+	app := backpack.New(&compass.Config{})
+	_, err := activate([]Plugin{p}, app, []string{"golem15.hello"})
+	if err == nil {
+		t.Fatal("expected non-string leaf error")
+	}
+	msg := err.Error()
+	for _, want := range []string{"golem15.hello", "lang/en/lang.yaml", "labels.count", "non-string leaf"} {
+		if !strings.Contains(msg, want) {
+			t.Fatalf("error %q missing %s", msg, want)
+		}
+	}
+}
+
 type mailPlugin struct {
 	id        string
 	fsys      fs.FS
diff --git a/phrasebook/translator_test.go b/phrasebook/translator_test.go
index 911750b..ea36eba 100644
--- a/phrasebook/translator_test.go
+++ b/phrasebook/translator_test.go
@@ -5,6 +5,8 @@ import (
 	"context"
 	"io/fs"
 	"log/slog"
+	"os"
+	"path/filepath"
 	"strings"
 	"testing"
 	"testing/fstest"
@@ -249,3 +251,95 @@ func TestLocaleFallbackSmoke(t *testing.T) {
 		t.Fatalf("production lookup must be silent, got %q", buf.String())
 	}
 }
+
+func TestCatalogBoundaries(t *testing.T) {
+	t.Parallel()
+
+	t.Run("invalid yaml names plugin and file", func(t *testing.T) {
+		err := NewCatalog().Load("golem15.hello", fstest.MapFS{
+			"lang/en/lang.yaml": {Data: []byte("greeting: [\n")},
+		})
+		if err == nil || !strings.Contains(err.Error(), "golem15.hello") || !strings.Contains(err.Error(), "lang/en/lang.yaml") {
+			t.Fatalf("parse error = %v", err)
+		}
+	})
+	t.Run("nested group file flattens under group name", func(t *testing.T) {
+		cat := NewCatalog()
+		err := cat.Load("golem15.hello", fstest.MapFS{
+			"lang/en/validation.yaml": {Data: []byte("fields:\n  name: Name is required\n")},
+		})
+		if err != nil {
+			t.Fatal(err)
+		}
+		tr := NewTranslator(cat, Options{})
+		if got := tr.GetIn("en", "golem15.hello::validation.fields.name", nil); got != "Name is required" {
+			t.Fatalf("nested group = %q", got)
+		}
+	})
+	t.Run("path extra segments and wrong suffix are malformed", func(t *testing.T) {
+		for _, file := range []string{"lang/en/nested/lang.yaml", "lang/en/lang.yml", "i18n/en/lang.yaml"} {
+			err := NewCatalog().Load("golem15.hello", fstest.MapFS{
+				file: {Data: []byte("greeting: Hi\n")},
+			})
+			if err == nil || !strings.Contains(err.Error(), "golem15.hello") || !strings.Contains(err.Error(), "malformed path") {
+				t.Fatalf("%s error = %v", file, err)
+			}
+		}
+	})
+	t.Run("inverted bracket range", func(t *testing.T) {
+		err := NewCatalog().Load("golem15.hello", fstest.MapFS{
+			"lang/en/lang.yaml": {Data: []byte("items: \"[5,2] nope|{1} one\"\n")},
+		})
+		if err == nil || !strings.Contains(err.Error(), "golem15.hello") || !strings.Contains(err.Error(), "lang/en/lang.yaml") || !strings.Contains(err.Error(), "malformed pipe") {
+			t.Fatalf("inverted range error = %v", err)
+		}
+	})
+	t.Run("empty pipe form", func(t *testing.T) {
+		err := NewCatalog().Load("golem15.hello", fstest.MapFS{
+			"lang/en/lang.yaml": {Data: []byte("items: \"one||other\"\n")},
+		})
+		if err == nil || !strings.Contains(err.Error(), "malformed pipe") {
+			t.Fatalf("empty form error = %v", err)
+		}
+	})
+	t.Run("get substitutes name case variants", func(t *testing.T) {
+		cat := NewCatalog()
+		if err := cat.Load("golem15.hello", fstest.MapFS{
+			"lang/en/lang.yaml": {Data: []byte("helloName: \"hi :name :Name :NAME\"\n")},
+		}); err != nil {
+			t.Fatal(err)
+		}
+		tr := NewTranslator(cat, Options{})
+		if got := tr.GetIn("en", "golem15.hello::lang.helloName", map[string]string{"name": "alice"}); got != "hi alice Alice ALICE" {
+			t.Fatalf("Get params = %q", got)
+		}
+	})
+	t.Run("configured locale and fallback", func(t *testing.T) {
+		dir := t.TempDir()
+		if err := os.WriteFile(filepath.Join(dir, "app.yaml"), []byte("locale: pl\nfallback_locale: en\n"), 0o644); err != nil {
+			t.Fatal(err)
+		}
+		cfg, err := compass.Load(dir)
+		if err != nil {
+			t.Fatal(err)
+		}
+		app := backpack.New(cfg)
+		p := &langPlugin{id: "golem15.hello", fsys: fstest.MapFS{
+			"lang/en/lang.yaml": {Data: []byte("greeting: Hello\nonlyEn: English only\n")},
+			"lang/pl/lang.yaml": {Data: []byte("greeting: Cześć\n")},
+		}}
+		if err := Activate(app, []*langPlugin{p}); err != nil {
+			t.Fatal(err)
+		}
+		tr, ok := app.Lookup[*Translator]()
+		if !ok {
+			t.Fatal("translator not published")
+		}
+		if got := tr.Get(context.Background(), "golem15.hello::lang.greeting", nil); got != "Cześć" {
+			t.Fatalf("configured app.locale = %q", got)
+		}
+		if got := tr.GetIn("pl", "golem15.hello::lang.onlyEn", nil); got != "English only" {
+			t.Fatalf("configured fallback = %q", got)
+		}
+	})
+}