fix(cli): route prompt selection hints to stderr

Co-Authored-By: Virgil <virgil@lethean.io>

pkg/cli/prompt.go

@@ -75,12 +75,14 @@ func Select(label string, options []string) (string, error) {
 	r := newReader()
 	input, err := r.ReadString('\n')
 	if err != nil && strings.TrimSpace(input) == "" {
+		promptHint("No input received. Selection cancelled.")
 		return "", fmt.Errorf("selection cancelled: %w", err)
 	}
 
 	trimmed := strings.TrimSpace(input)
 	n, err := strconv.Atoi(trimmed)
 	if err != nil || n < 1 || n > len(options) {
+		promptHint(fmt.Sprintf("Please enter a number between 1 and %d.", len(options)))
 		return "", fmt.Errorf("invalid selection %q: choose a number between 1 and %d", trimmed, len(options))
 	}
 	return options[n-1], nil

pkg/cli/prompt_test.go

@@ -10,6 +10,34 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
+func captureStderr(t *testing.T, fn func()) string {
+	t.Helper()
+
+	oldErr := os.Stderr
+	r, w, err := os.Pipe()
+	if !assert.NoError(t, err) {
+		return ""
+	}
+	os.Stderr = w
+
+	defer func() {
+		os.Stderr = oldErr
+	}()
+
+	fn()
+
+	if !assert.NoError(t, w.Close()) {
+		return ""
+	}
+
+	var buf bytes.Buffer
+	_, err = io.Copy(&buf, r)
+	if !assert.NoError(t, err) {
+		return ""
+	}
+	return buf.String()
+}
+
 func TestPrompt_Good(t *testing.T) {
 	SetStdin(strings.NewReader("hello\n"))
 	defer SetStdin(nil) // reset
@@ -59,9 +87,13 @@ func TestSelect_Bad_Invalid(t *testing.T) {
 	SetStdin(strings.NewReader("5\n"))
 	defer SetStdin(nil)
 
-	_, err := Select("Pick", []string{"a", "b"})
+	var err error
+	stderr := captureStderr(t, func() {
+		_, err = Select("Pick", []string{"a", "b"})
+	})
 	assert.Error(t, err)
 	assert.Contains(t, err.Error(), "choose a number between 1 and 2")
+	assert.Contains(t, stderr, "Please enter a number between 1 and 2.")
 }
 
 func TestSelect_Bad_EOF(t *testing.T) {
@@ -223,6 +255,19 @@ func TestChoose_Bad_FilteredDefaultDoesNotFallBackToFirstVisible(t *testing.T) {
 	assert.Equal(t, "apricot", val)
 }
 
+func TestChoose_Bad_InvalidNumberUsesStderrHint(t *testing.T) {
+	SetStdin(strings.NewReader("5\n2\n"))
+	defer SetStdin(nil)
+
+	var val string
+	stderr := captureStderr(t, func() {
+		val = Choose("Pick", []string{"a", "b"})
+	})
+
+	assert.Equal(t, "b", val)
+	assert.Contains(t, stderr, "Please enter a number between 1 and 2.")
+}
+
 func TestChooseMulti_Good_Filter(t *testing.T) {
 	SetStdin(strings.NewReader("ap\n1 2\n"))
 	defer SetStdin(nil)

pkg/cli/utils.go

@@ -396,7 +396,7 @@ func Choose[T any](prompt string, items []T, opts ...ChooseOption[T]) T {
 			if n >= 1 && n <= len(visible) {
 				return items[visible[n-1]]
 			}
-			fmt.Printf("Please enter a number between 1 and %d.\n", len(visible))
+			promptHint(fmt.Sprintf("Please enter a number between 1 and %d.", len(visible)))
 			continue
 		}