diff --git a/main_width_test.go b/main_width_test.go
new file mode 100644
index 0000000..5068831
--- /dev/null
+++ b/main_width_test.go
@@ -0,0 +1,78 @@
+package main
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestResolveWidth(t *testing.T) {
+	tt := []struct {
+		name            string
+		isTerminal      bool
+		configuredWidth uint
+		flagChanged     bool
+		detectedWidth   int
+		detectErr       error
+		want            uint
+	}{
+		{
+			name:            "explicit width keeps configured value",
+			isTerminal:      true,
+			configuredWidth: 40,
+			flagChanged:     true,
+			detectedWidth:   180,
+			want:            40,
+		},
+		{
+			name:            "explicit zero width is preserved",
+			isTerminal:      true,
+			configuredWidth: 0,
+			flagChanged:     true,
+			detectedWidth:   180,
+			want:            0,
+		},
+		{
+			name:            "auto width uses detected terminal width",
+			isTerminal:      true,
+			configuredWidth: 0,
+			flagChanged:     false,
+			detectedWidth:   100,
+			want:            100,
+		},
+		{
+			name:            "auto width no longer caps terminal width",
+			isTerminal:      true,
+			configuredWidth: 0,
+			flagChanged:     false,
+			detectedWidth:   180,
+			want:            180,
+		},
+		{
+			name:            "auto width falls back when detection fails",
+			isTerminal:      true,
+			configuredWidth: 0,
+			flagChanged:     false,
+			detectErr:       errors.New("boom"),
+			want:            80,
+		},
+		{
+			name:            "non-tty fallback remains 80",
+			isTerminal:      false,
+			configuredWidth: 0,
+			flagChanged:     false,
+			want:            80,
+		},
+	}
+
+	for _, tc := range tt {
+		t.Run(tc.name, func(t *testing.T) {
+			got := resolveWidth(tc.isTerminal, tc.configuredWidth, tc.flagChanged, func() (int, error) {
+				return tc.detectedWidth, tc.detectErr
+			})
+
+			if got != tc.want {
+				t.Fatalf("resolveWidth() = %d, want %d", got, tc.want)
+			}
+		})
+	}
+}