Honor NO_COLOR env var in TerminalOutputDevice (#8825)

src/Platform/Microsoft.Testing.Platform/OutputDevice/TerminalOutputDevice.cs

@@ -165,13 +165,18 @@ await _policiesService.RegisterOnAbortCallbackAsync(
         // When --ansi auto is explicitly specified, it overrides --no-ansi too.
         bool effectiveNoAnsi = noAnsi && ansiOverride == AnsiOverride.None;
 
+        // Honor the de-facto cross-tool NO_COLOR convention (https://no-color.org): when present with any
+        // non-empty value, suppress ANSI color/cursor codes unless the user explicitly opted in via
+        // `--ansi on` (which still wins, matching the precedence documented in --help).
+        bool noColorEnv = !RoslynString.IsNullOrEmpty(_environment.GetEnvironmentVariable("NO_COLOR"));
+
         bool inCI = new CIEnvironmentDetector(_environment).IsCIEnvironment();
         bool isLLMEnvironment = new LLMEnvironmentDetector(_environment).IsLLMEnvironment();
 
         AnsiMode ansiMode = ansiOverride switch
         {
-            // User explicitly forced ANSI on (e.g. `--ansi on`). Bypass CI / LLM / redirection detection
-            // so colors and cursor movement are emitted even when stdout is redirected.
+            // User explicitly forced ANSI on (e.g. `--ansi on`). Bypass CI / LLM / NO_COLOR / redirection
+            // detection so colors and cursor movement are emitted even when stdout is redirected.
             AnsiOverride.ForceOn => AnsiMode.ForceAnsi,
 
             // User explicitly disabled ANSI (`--ansi off`).
@@ -181,7 +186,8 @@ await _policiesService.RegisterOnAbortCallbackAsync(
             // No --ansi argument was provided, or `--ansi auto` was provided.
             // Fall back to environment-based detection.
             // In LLM environments, prefer simple text output so that the LLM can parse it easily.
-            _ when effectiveNoAnsi || isLLMEnvironment => AnsiMode.NoAnsi,
+            // NO_COLOR (https://no-color.org) is honored as well: any non-empty value suppresses ANSI.
+            _ when effectiveNoAnsi || noColorEnv || isLLMEnvironment => AnsiMode.NoAnsi,
             _ when inCI => AnsiMode.SimpleAnsi,
             _ => AnsiMode.AnsiIfPossible,
         };

test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/AnsiOptionTests.cs

@@ -111,6 +111,77 @@ public async Task AnsiOption_MissingArgument_FailsCommandLineValidation()
         result.AssertOutputContains("Option '--ansi' from provider 'Terminal test reporter' (UID: TerminalTestReporterCommandLineOptionsProvider) expects at least 1 arguments");
     }
 
+    [TestMethod]
+    public async Task NoColorEnvVar_NonEmpty_DisablesAnsiOutput()
+    {
+        // The de-facto NO_COLOR convention (https://no-color.org): when present with any non-empty
+        // value, color (and in our implementation, all ANSI escape codes) must be suppressed.
+        // TestHost.ExecuteAsync auto-injects `--no-ansi --no-progress` which would already suppress ESC,
+        // so we use `--ansi auto` together with the CI env (which would otherwise produce SimpleAnsi
+        // ESC) to deterministically prove NO_COLOR alone forces NoAnsi. With NO_COLOR set the output
+        // must contain no ESC even though `--ansi auto` is opted in.
+        Dictionary<string, string?> environmentVariables = new(InCIEnvironmentVariablesWithLLMCleared)
+        {
+            ["NO_COLOR"] = "1",
+        };
+
+        var testHost = TestInfrastructure.TestHost.LocateFrom(AssetFixture.TargetAssetPath, AssetName, TargetFrameworks.NetCurrent);
+        TestHostResult result = await testHost.ExecuteAsync(
+            "--ansi auto",
+            environmentVariables: environmentVariables,
+            cancellationToken: TestContext.CancellationToken);
+
+        result.AssertExitCodeIs(ExitCode.Success);
+        Assert.IsFalse(
+            result.StandardOutput.Contains(EscapeCharacter, StringComparison.Ordinal),
+            $"Expected output to NOT contain ANSI escape characters when NO_COLOR is set, but got:\n{result.StandardOutput}");
+    }
+
+    [TestMethod]
+    public async Task NoColorEnvVar_Empty_IsIgnored()
+    {
+        // Per the NO_COLOR spec, ONLY a non-empty value disables color. An empty NO_COLOR must be a no-op.
+        // With `--ansi auto` in a CI environment (and LLM detection cleared), MTP picks SimpleAnsi which
+        // does emit color ESC sequences. Asserting ESC presence proves empty NO_COLOR did not force NoAnsi.
+        Dictionary<string, string?> environmentVariables = new(InCIEnvironmentVariablesWithLLMCleared)
+        {
+            ["NO_COLOR"] = string.Empty,
+        };
+
+        var testHost = TestInfrastructure.TestHost.LocateFrom(AssetFixture.TargetAssetPath, AssetName, TargetFrameworks.NetCurrent);
+        TestHostResult result = await testHost.ExecuteAsync(
+            "--ansi auto",
+            environmentVariables: environmentVariables,
+            cancellationToken: TestContext.CancellationToken);
+
+        result.AssertExitCodeIs(ExitCode.Success);
+        Assert.IsTrue(
+            result.StandardOutput.Contains(EscapeCharacter, StringComparison.Ordinal),
+            $"Expected output to contain ANSI escape characters when NO_COLOR is empty (spec: only non-empty disables color), but got:\n{result.StandardOutput}");
+    }
+
+    [TestMethod]
+    public async Task NoColorEnvVar_OverriddenByExplicitAnsiOn()
+    {
+        // Precedence rule documented in TerminalOutputDevice: `--ansi on` short-circuits CI / LLM /
+        // NO_COLOR detection. NO_COLOR is honored "unless the user explicitly opted in via --ansi on".
+        Dictionary<string, string?> environmentVariables = new(InCIEnvironmentVariablesWithLLMCleared)
+        {
+            ["NO_COLOR"] = "1",
+        };
+
+        var testHost = TestInfrastructure.TestHost.LocateFrom(AssetFixture.TargetAssetPath, AssetName, TargetFrameworks.NetCurrent);
+        TestHostResult result = await testHost.ExecuteAsync(
+            "--ansi on",
+            environmentVariables: environmentVariables,
+            cancellationToken: TestContext.CancellationToken);
+
+        result.AssertExitCodeIs(ExitCode.Success);
+        Assert.IsTrue(
+            result.StandardOutput.Contains(EscapeCharacter, StringComparison.Ordinal),
+            $"Expected output to contain ANSI escape characters because `--ansi on` overrides NO_COLOR, but got:\n{result.StandardOutput}");
+    }
+
     public TestContext TestContext { get; set; } = null!;
 
     // Env vars that force the platform to consider it to be in CI while clearing the known LLM env
@@ -119,6 +190,7 @@ public async Task AnsiOption_MissingArgument_FailsCommandLineValidation()
     private static Dictionary<string, string?> InCIEnvironmentVariablesWithLLMCleared => new()
     {
         ["GITHUB_ACTIONS"] = "true",
+        ["NO_COLOR"] = string.Empty,
         ["CLAUDECODE"] = string.Empty,
         ["CLAUDE_CODE_ENTRYPOINT"] = string.Empty,
         ["CURSOR_EDITOR"] = string.Empty,

test/Utilities/Microsoft.Testing.TestInfrastructure/WellKnownEnvironmentVariables.cs

@@ -86,6 +86,12 @@ public static class WellKnownEnvironmentVariables
         // Isolate from the skip banner in case of parent, children tests
         "TESTINGPLATFORM_CONSOLEOUTPUTDEVICE_SKIP_BANNER",
 
+        // The de-facto cross-tool NO_COLOR convention (https://no-color.org). MTP honors any non-empty
+        // value as "disable ANSI". We filter it out so acceptance tests that rely on ANSI output are not
+        // affected when contributors / CI happen to have NO_COLOR set in their shell. Tests that need
+        // to exercise NO_COLOR behavior must set the variable explicitly.
+        "NO_COLOR",
+
         // LLM / AI agent CLI environment variables - keep in sync with
         // src/Platform/Microsoft.Testing.Platform/Helpers/LLMEnvironmentDetector.cs.
         // We filter these out so acceptance tests are not affected by the ambient