From 056a4d6b0d67582a6a0044c4233e9a74113e773d Mon Sep 17 00:00:00 2001
From: Waldek Mastykarz <waldek@mastykarz.nl>
Date: Wed, 21 Oct 2026 15:54:18 +0200
Subject: [PATCH] Only disable color when NO_COLOR is non-empty. Closes #1967

Per https://no-color.org, NO_COLOR applies only when present and not empty.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 09b222bb-b264-4461-926d-32f935613554
---
 DevProxy.Tests/DevProxyCommandNoColorTests.cs | 37 +++++++++++++++++++
 DevProxy/Commands/DevProxyCommand.cs          | 14 +++++--
 2 files changed, 47 insertions(+), 4 deletions(-)
 create mode 100644 DevProxy.Tests/DevProxyCommandNoColorTests.cs

diff --git a/DevProxy.Tests/DevProxyCommandNoColorTests.cs b/DevProxy.Tests/DevProxyCommandNoColorTests.cs
new file mode 100644
index 00000000..3596fe72
--- /dev/null
+++ b/DevProxy.Tests/DevProxyCommandNoColorTests.cs
@@ -0,0 +1,37 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using DevProxy.Commands;
+
+using Xunit;
+
+namespace DevProxy.Tests;
+
+public class DevProxyCommandNoColorTests
+{
+    [Theory]
+    [InlineData(null, false)]
+    [InlineData("", false)]
+    [InlineData("1", true)]
+    [InlineData("true", true)]
+    public void ShouldDisableColor_RespectsNoColorEnv(string? noColor, bool expected)
+    {
+        Assert.Equal(expected, DevProxyCommand.ShouldDisableColor([], noColor, null));
+    }
+
+    [Fact]
+    public void ShouldDisableColor_NoColorOption_DisablesColor()
+    {
+        Assert.True(DevProxyCommand.ShouldDisableColor(["devproxy", "--no-color"], "", null));
+    }
+
+    [Theory]
+    [InlineData("dumb", true)]
+    [InlineData("DUMB", true)]
+    [InlineData("xterm-256color", false)]
+    public void ShouldDisableColor_RespectsTerm(string term, bool expected)
+    {
+        Assert.Equal(expected, DevProxyCommand.ShouldDisableColor([], null, term));
+    }
+}
diff --git a/DevProxy/Commands/DevProxyCommand.cs b/DevProxy/Commands/DevProxyCommand.cs
index dfc9b763..ce6834d5 100644
--- a/DevProxy/Commands/DevProxyCommand.cs
+++ b/DevProxy/Commands/DevProxyCommand.cs
@@ -231,15 +231,21 @@ public static bool NoColor
                 return field;
             }
 
-            var args = Environment.GetCommandLineArgs();
-            field = args.Contains(NoColorOptionName) ||
-                Environment.GetEnvironmentVariable("NO_COLOR") is not null ||
-                string.Equals(Environment.GetEnvironmentVariable("TERM"), "dumb", StringComparison.OrdinalIgnoreCase);
+            field = ShouldDisableColor(
+                Environment.GetCommandLineArgs(),
+                Environment.GetEnvironmentVariable("NO_COLOR"),
+                Environment.GetEnvironmentVariable("TERM"));
             _noColorResolved = true;
             return field;
         }
     }
 
+    // Per https://no-color.org, NO_COLOR disables color only when present and not empty.
+    internal static bool ShouldDisableColor(string[] args, string? noColorEnv, string? termEnv) =>
+        args.Contains(NoColorOptionName) ||
+        !string.IsNullOrEmpty(noColorEnv) ||
+        string.Equals(termEnv, "dumb", StringComparison.OrdinalIgnoreCase);
+
     public DevProxyCommand(
         IEnumerable<IPlugin> plugins,
         ISet<UrlToWatch> urlsToWatch,