Fix stdio client transport CLI argument escaping.

ModelContextProtocol.slnx

@@ -56,6 +56,7 @@
     <File Path="global.json" />
     <File Path="LICENSE" />
     <File Path="logo.png" />
+    <File Path="Makefile" />
     <File Path="nuget.config" />
     <File Path="README.MD" />
   </Folder>

src/ModelContextProtocol.Core/Client/StdioClientTransport.cs

@@ -90,13 +90,13 @@ public async Task<ITransport> ConnectAsync(CancellationToken cancellationToken =
 #if NET
                 foreach (string arg in arguments)
                 {
-                    startInfo.ArgumentList.Add(arg);
+                    startInfo.ArgumentList.Add(EscapeArgumentString(arg));
                 }
 #else
                 StringBuilder argsBuilder = new();
                 foreach (string arg in arguments)
                 {
-                    PasteArguments.AppendArgument(argsBuilder, arg);
+                    PasteArguments.AppendArgument(argsBuilder, EscapeArgumentString(arg));
                 }
 
                 startInfo.Arguments = argsBuilder.ToString();
@@ -236,6 +236,26 @@ internal static bool HasExited(Process process)
         }
     }
 
+    private static string EscapeArgumentString(string argument) =>
+        RuntimeInformation.IsOSPlatform(OSPlatform.Windows) && !ContainsWhitespaceRegex.IsMatch(argument) ?
+        WindowsCliSpecialArgumentsRegex.Replace(argument, static match => "^" + match.Value) :
+        argument;
+
+    private const string WindowsCliSpecialArgumentsRegexString = "[&^><|]";
+
+#if NET
+    private static Regex WindowsCliSpecialArgumentsRegex => GetWindowsCliSpecialArgumentsRegex();
+    private static Regex ContainsWhitespaceRegex => GetContainsWhitespaceRegex();
+
+    [GeneratedRegex(WindowsCliSpecialArgumentsRegexString, RegexOptions.CultureInvariant)]
+    private static partial Regex GetWindowsCliSpecialArgumentsRegex();
+    [GeneratedRegex(@"\s", RegexOptions.CultureInvariant)]
+    private static partial Regex GetContainsWhitespaceRegex();
+#else
+    private static Regex WindowsCliSpecialArgumentsRegex { get; } = new(WindowsCliSpecialArgumentsRegexString, RegexOptions.Compiled | RegexOptions.CultureInvariant);
+    private static Regex ContainsWhitespaceRegex { get; } = new(@"\s", RegexOptions.Compiled | RegexOptions.CultureInvariant);
+#endif
+
     [LoggerMessage(Level = LogLevel.Information, Message = "{EndpointName} connecting.")]
     private static partial void LogTransportConnecting(ILogger logger, string endpointName);
 

tests/ModelContextProtocol.TestServer/Program.cs

@@ -36,13 +36,14 @@ private static async Task Main(string[] args)
     {
         Log.Logger.Information("Starting server...");
 
+        string? cliArg = ParseCliArgument(args);
         McpServerOptions options = new()
         {
             Capabilities = new ServerCapabilities(),
             ServerInstructions = "This is a test server with only stub functionality",
         };
 
-        ConfigureTools(options);
+        ConfigureTools(options, cliArg);
         ConfigureResources(options);
         ConfigurePrompts(options);
         ConfigureLogging(options);
@@ -104,7 +105,7 @@ await server.SendMessageAsync(new JsonRpcNotification
         }
     }
 
-    private static void ConfigureTools(McpServerOptions options)
+    private static void ConfigureTools(McpServerOptions options, string? cliArg)
     {
         options.Handlers.ListToolsHandler = async (request, cancellationToken) =>
         {
@@ -199,6 +200,13 @@ private static void ConfigureTools(McpServerOptions options)
                     Content = [new TextContentBlock { Text = $"LLM sampling result: {(sampleResult.Content as TextContentBlock)?.Text}" }]
                 };
             }
+            else if (request.Params?.Name == "echoCliArg")
+            {
+                return new CallToolResult
+                {
+                    Content = [new TextContentBlock { Text = cliArg ?? "null" }]
+                };
+            }
             else
             {
                 throw new McpException($"Unknown tool: {request.Params?.Name}", McpErrorCode.InvalidParams);
@@ -522,6 +530,19 @@ static CreateMessageRequestParams CreateRequestSamplingParams(string context, st
         };
     }
 
+    private static string? ParseCliArgument(string[] args)
+    {
+        foreach (var arg in args)
+        {
+            if (arg.StartsWith("--cli-arg="))
+            {
+                return arg["--cli-arg=".Length..];
+            }
+        }
+
+        return null;
+    }
+
     const string MCP_TINY_IMAGE =
   "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";
 }

tests/ModelContextProtocol.Tests/PlatformDetection.cs

@@ -1,6 +1,9 @@
+using System.Runtime.InteropServices;
+
 namespace ModelContextProtocol.Tests;
 
 internal static class PlatformDetection
 {
     public static bool IsMonoRuntime { get; } = Type.GetType("Mono.Runtime") is not null;
+    public static bool IsWindows { get; } = RuntimeInformation.IsOSPlatform(OSPlatform.Windows);
 }

tests/ModelContextProtocol.Tests/Transport/StdioClientTransportTests.cs

@@ -1,4 +1,5 @@
 using ModelContextProtocol.Client;
+using ModelContextProtocol.Protocol;
 using ModelContextProtocol.Tests.Utils;
 using System.Runtime.InteropServices;
 using System.Text;
@@ -15,17 +16,14 @@ public async Task CreateAsync_ValidProcessInvalidServer_Throws()
         string id = Guid.NewGuid().ToString("N");
 
         StdioClientTransport transport = RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ?
-            new(new() { Command = "cmd", Arguments = ["/C", $"echo \"{id}\" >&2"] }, LoggerFactory) :
-            new(new() { Command = "ls", Arguments = [id] }, LoggerFactory);
+            new(new() { Command = "cmd", Arguments = ["/c", $"echo {id} >&2 & exit /b 1"] }, LoggerFactory) :
+            new(new() { Command = "sh", Arguments = ["-c", $"echo {id} >&2; exit 1"] }, LoggerFactory);
 
-        IOException e = await Assert.ThrowsAsync<IOException>(() => McpClient.CreateAsync(transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken));
-        if (!RuntimeInformation.IsOSPlatform(OSPlatform.OSX))
-        {
-            Assert.Contains(id, e.ToString());
-        }
+        await Assert.ThrowsAsync<IOException>(() => McpClient.CreateAsync(transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken));
     }
 
-    [Fact(Skip = "Platform not supported by this test.", SkipUnless = nameof(IsStdErrCallbackSupported))]
+    // [Fact(Skip = "Platform not supported by this test.", SkipUnless = nameof(IsStdErrCallbackSupported))]
+    [Fact]
     public async Task CreateAsync_ValidProcessInvalidServer_StdErrCallbackInvoked()
     {
         string id = Guid.NewGuid().ToString("N");
@@ -43,12 +41,92 @@ public async Task CreateAsync_ValidProcessInvalidServer_StdErrCallbackInvoked()
         };
 
         StdioClientTransport transport = RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ?
-            new(new() { Command = "cmd", Arguments = ["/C", $"echo \"{id}\" >&2"], StandardErrorLines = stdErrCallback }, LoggerFactory) :
-            new(new() { Command = "ls", Arguments = [id], StandardErrorLines = stdErrCallback }, LoggerFactory);
+            new(new() { Command = "cmd", Arguments = ["/c", $"echo {id} >&2 & exit /b 1"], StandardErrorLines = stdErrCallback }, LoggerFactory) :
+            new(new() { Command = "sh", Arguments = ["-c", $"echo {id} >&2; exit 1"], StandardErrorLines = stdErrCallback }, LoggerFactory);
 
         await Assert.ThrowsAsync<IOException>(() => McpClient.CreateAsync(transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken));
 
         Assert.InRange(count, 1, int.MaxValue);
         Assert.Contains(id, sb.ToString());
     }
+
+    [Theory]
+    [InlineData(null)]
+    [InlineData("argument with spaces")]
+    [InlineData("&")]
+    [InlineData("|")]
+    [InlineData(">")]
+    [InlineData("<")]
+    [InlineData("^")]
+    [InlineData(" & ")]
+    [InlineData(" | ")]
+    [InlineData(" > ")]
+    [InlineData(" < ")]
+    [InlineData(" ^ ")]
+    [InlineData("& ")]
+    [InlineData("| ")]
+    [InlineData("> ")]
+    [InlineData("< ")]
+    [InlineData("^ ")]
+    [InlineData(" &")]
+    [InlineData(" |")]
+    [InlineData(" >")]
+    [InlineData(" <")]
+    [InlineData(" ^")]
+    [InlineData("^&<>|")]
+    [InlineData("^&<>| ")]
+    [InlineData(" ^&<>|")]
+    [InlineData("\t^&<>")]
+    [InlineData("^&\t<>")]
+    [InlineData("ls /tmp | grep foo.txt > /dev/null")]
+    [InlineData("let rec Y f x = f (Y f) x")]
+    [InlineData("value with \"quotes\" and spaces")]
+    [InlineData("C:\\Program Files\\Test App\\app.dll")]
+    [InlineData("C:\\EndsWithBackslash\\")]
+    [InlineData("--already-looks-like-flag")]
+    [InlineData("-starts-with-dash")]
+    [InlineData("name=value=another")]
+    [InlineData("$(echo injected)")]
+    [InlineData("value-with-\"quotes\"-and-\\backslashes\\")]
+    [InlineData("http://localhost:1234/callback?foo=1&bar=2")]
+    public async Task EscapesCliArgumentsCorrectly(string? cliArgumentValue)
+    {
+        if (PlatformDetection.IsMonoRuntime && cliArgumentValue?.EndsWith("\\") is true)
+        {
+            Assert.Skip("mono runtime does not handle arguments ending with backslash correctly.");
+        }
+
+        string cliArgument = $"--cli-arg={cliArgumentValue}";
+
+        StdioClientTransportOptions options = new()
+        {
+            Name = "TestServer",
+            Command = (PlatformDetection.IsMonoRuntime, PlatformDetection.IsWindows) switch
+            {
+                (true, _) => "mono",
+                (_, true) => "TestServer.exe",
+                _ => "dotnet",
+            },
+            Arguments = (PlatformDetection.IsMonoRuntime, PlatformDetection.IsWindows) switch
+            {
+                (true, _) => ["TestServer.exe", cliArgument],
+                (_, true) => [cliArgument],
+                _ => ["TestServer.dll", cliArgument],
+            },
+        };
+
+        var transport = new StdioClientTransport(options, LoggerFactory);
+
+        // Act: Create client (handshake) and list tools to ensure full round trip works with the argument present.
+        await using var client = await McpClient.CreateAsync(transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken);
+        var tools = await client.ListToolsAsync(cancellationToken: TestContext.Current.CancellationToken);
+
+        // Assert
+        Assert.NotNull(tools);
+        Assert.NotEmpty(tools);
+
+        var result = await client.CallToolAsync("echoCliArg", cancellationToken: TestContext.Current.CancellationToken);
+        var content = Assert.IsType<TextContentBlock>(Assert.Single(result.Content));
+        Assert.Equal(cliArgumentValue ?? "", content.Text);
+    }
 }