diff --git a/tests/OpenCode.Sdk.Tests/Hosting/Launcher/OpenCodeServerSpawnRequestTests.cs b/tests/OpenCode.Sdk.Tests/Hosting/Launcher/OpenCodeServerSpawnRequestTests.cs
new file mode 100644
index 00000000..15d454a2
--- /dev/null
+++ b/tests/OpenCode.Sdk.Tests/Hosting/Launcher/OpenCodeServerSpawnRequestTests.cs
@@ -0,0 +1,191 @@
+using System.ComponentModel;
+using NSubstitute;
+using OpenCode.Sdk.Internal;
+using OpenCode.Sdk.Tests.Support;
+using TUnit.Assertions.Enums;
+
+namespace OpenCode.Sdk.Tests.Hosting.Launcher;
+
+/// <summary>The standalone door's pre-spawn refusals and the request it hands to the host's strategy.</summary>
+public sealed class OpenCodeServerSpawnRequestTests
+{
+    private static string Command => OperatingSystem.IsWindows() ? @"C:\oc\opencode.exe" : "/opt/oc/opencode";
+
+    /// <summary>A rooted executable and a leading argument are both checked before spawn, without pinning #401's parameter name.</summary>
+    [Test]
+    [Arguments(true)]
+    [Arguments(false)]
+    public async Task StartAsync_Should_Refuse_A_Nul_In_A_Command_Entry_Before_Spawn(bool executable)
+    {
+        var spawn = new RecordingSpawn();
+        var options = new OpenCodeServerOptions { Command = executable ? [Command + "\0", "serve"] : [Command, "serve\0"] };
+
+        var exception = await Assert.That(async () => await OpenCodeServer.StartWithSeamsAsync(options, Seams(spawn), CancellationToken.None))
+            .Throws<ArgumentException>();
+
+        await Assert.That(exception!.Message).IsEqualTo(new ArgumentException(
+            "OpenCodeServerOptions.Command entries cannot contain NUL.", exception.ParamName).Message);
+        await Assert.That(spawn.Calls).IsEqualTo(0);
+    }
+
+    /// <summary>Neither an environment name nor its value may be silently cut at a NUL by native marshalling.</summary>
+    [Test]
+    [Arguments(true)]
+    [Arguments(false)]
+    public async Task StartAsync_Should_Refuse_A_Nul_In_An_Environment_Entry_Before_Spawn(bool name)
+    {
+        var spawn = new RecordingSpawn();
+        var options = new OpenCodeServerOptions
+        {
+            Command = [Command],
+            Environment = new Dictionary<string, string>(StringComparer.Ordinal) { [name ? "ENTRY\0" : "ENTRY"] = name ? "value" : "value\0" },
+        };
+        var seams = Seams(spawn);
+
+        var exception = await Assert.That(async () => await OpenCodeServer.StartWithSeamsAsync(options, seams, cancellationToken: CancellationToken.None))
+            .Throws<ArgumentException>();
+
+        await Assert.That(exception!.Message).IsEqualTo(new ArgumentException(
+            "OpenCodeServerOptions.Environment names and values cannot contain NUL.", exception.ParamName).Message);
+        await Assert.That(spawn.Calls).IsEqualTo(0);
+    }
+
+    /// <summary>The launcher tail follows every supplied argument; Windows starts that exact command suspended.</summary>
+    [Test]
+    public async Task StartAsync_Should_Append_The_Standalone_Arguments_After_The_Supplied_Arguments()
+    {
+        var spawn = new RecordingSpawn();
+        var options = new OpenCodeServerOptions { Command = [Command, "serve", "two words", "quoted\"argument"] };
+
+        _ = await Assert.That(async () => await OpenCodeServer.StartWithSeamsAsync(options, Seams(spawn), CancellationToken.None))
+            .Throws<OpenCodeServerException>();
+
+        string[] argv = [Command, "serve", "two words", "quoted\"argument", "--stdio", "--port", "0"];
+        if (OperatingSystem.IsWindows())
+        {
+            await Assert.That(spawn.WindowsRequest!.ApplicationPath).IsEqualTo(Command);
+            await Assert.That(spawn.WindowsRequest.CommandLine).IsEqualTo(
+                """C:\oc\opencode.exe serve "two words" "quoted\"argument" --stdio --port 0""");
+            await Assert.That(spawn.WindowsRequest.StartSuspended).IsTrue();
+        }
+        else
+        {
+            await Assert.That(new[] { spawn.PosixRequest!.ExecutablePath }.Concat(spawn.PosixRequest.Arguments))
+                .IsEquivalentTo(argv, CollectionOrdering.Matching);
+        }
+    }
+
+    /// <summary>Each start injects a fresh base64url lease credential, never the caller's replacement.</summary>
+    [Test]
+    public async Task StartAsync_Should_Inject_A_Fresh_Base64Url_Password_Over_The_Callers_Entry()
+    {
+        var spawn = new RecordingSpawn();
+        var seams = Seams(spawn);
+        const string callerPassword = "caller-password";
+        var options = new OpenCodeServerOptions
+        {
+            Command = [Command],
+            Environment = new Dictionary<string, string>(StringComparer.Ordinal) { ["OPENCODE_PASSWORD"] = callerPassword },
+        };
+
+        _ = await Assert.That(async () => await OpenCodeServer.StartWithSeamsAsync(options, seams, CancellationToken.None))
+            .Throws<OpenCodeServerException>();
+        var first = spawn.Environment["OPENCODE_PASSWORD"];
+        _ = await Assert.That(async () => await OpenCodeServer.StartWithSeamsAsync(options, seams, CancellationToken.None))
+            .Throws<OpenCodeServerException>();
+        var second = spawn.Environment["OPENCODE_PASSWORD"];
+
+        await Assert.That(first).Matches("^[A-Za-z0-9_-]{43}$");
+        await Assert.That(second).Matches("^[A-Za-z0-9_-]{43}$");
+        await Assert.That(second).IsNotEqualTo(first);
+        await Assert.That(first).IsNotEqualTo(callerPassword);
+        await Assert.That(second).IsNotEqualTo(callerPassword);
+    }
+
+    /// <summary>Only the host's literal 1 inherits stderr; the ownership lease and readiness stream remain pipes.</summary>
+    [Test]
+    [Arguments("1", true)]
+    [Arguments("true", false)]
+    public async Task StartAsync_Should_Inherit_Stderr_Only_For_The_Hosts_Print_Logs_Switch(string printLogs, bool inherit)
+    {
+        var spawn = new RecordingSpawn();
+        var host = new Dictionary<string, string>(StringComparer.Ordinal) { ["OPENCODE_PRINT_LOGS"] = printLogs };
+        var options = new OpenCodeServerOptions { Command = [Command] };
+        var expected = inherit ? ChildStreamRoute.Inherit : ChildStreamRoute.Pipe;
+
+        _ = await Assert.That(async () => await OpenCodeServer.StartWithSeamsAsync(options, Seams(spawn, host), CancellationToken.None))
+            .Throws<OpenCodeServerException>();
+
+        if (OperatingSystem.IsWindows())
+        {
+            await Assert.That(spawn.WindowsRequest!.StandardStreams!.Input).IsEqualTo(ChildStreamRoute.Pipe);
+            await Assert.That(spawn.WindowsRequest.StandardStreams.Output).IsEqualTo(ChildStreamRoute.Pipe);
+            await Assert.That(spawn.WindowsRequest.StandardStreams.Error).IsEqualTo(expected);
+        }
+        else
+        {
+            await Assert.That(spawn.PosixRequest!.StandardInput).IsEqualTo(ChildStreamRoute.Pipe);
+            await Assert.That(spawn.PosixRequest.StandardOutput).IsEqualTo(ChildStreamRoute.Pipe);
+            await Assert.That(spawn.PosixRequest.StandardError).IsEqualTo(expected);
+        }
+    }
+
+    /// <summary>A spawn failure names a meaningful directory, while a blank directory is passed as null and omitted.</summary>
+    [Test]
+    [Arguments(true)]
+    [Arguments(false)]
+    public async Task StartAsync_Should_Name_The_Working_Directory_Only_When_It_Is_Not_Blank(bool directory)
+    {
+        var spawn = new RecordingSpawn();
+        var work = OperatingSystem.IsWindows() ? @"C:\work" : "/work";
+        var options = new OpenCodeServerOptions { Command = [Command], WorkingDirectory = directory ? work : " " };
+
+        var exception = await Assert.That(async () => await OpenCodeServer.StartWithSeamsAsync(options, Seams(spawn), CancellationToken.None))
+            .Throws<OpenCodeServerException>();
+
+        await Assert.That(exception!.Message).IsEqualTo(directory
+            ? $"Failed to start the server command '{Command}' in the working directory '{work}'."
+            : $"Failed to start the server command '{Command}'.");
+        await Assert.That(OperatingSystem.IsWindows() ? spawn.WindowsRequest!.WorkingDirectory : spawn.PosixRequest!.WorkingDirectory)
+            .IsEqualTo(directory ? work : null);
+    }
+
+    private static LauncherSeams Seams(RecordingSpawn spawn, IReadOnlyDictionary<string, string>? host = null) => new()
+    {
+        WindowsSpawn = spawn,
+        Spawn = spawn,
+        Job = Substitute.For<IProcessJob>(),
+        WindowsExits = Substitute.For<IWindowsChildExit>(),
+        TreeKill = Substitute.For<IWindowsTreeKill>(),
+        Signals = Substitute.For<IProcessGroupSignal>(),
+        Exits = Substitute.For<IChildExitStatus>(),
+        Time = new ManualTimeProvider(),
+        HostEnvironment = host ?? new Dictionary<string, string>(StringComparer.Ordinal),
+    };
+
+    /// <summary>Records the host strategy's request and refuses to create a process.</summary>
+    private sealed class RecordingSpawn : IPosixSpawn, IWindowsSpawn
+    {
+        public int Calls { get; private set; }
+
+        public PosixSpawnRequest? PosixRequest { get; private set; }
+
+        public WindowsSpawnRequest? WindowsRequest { get; private set; }
+
+        public IReadOnlyDictionary<string, string> Environment => WindowsRequest?.Environment ?? PosixRequest!.Environment;
+
+        public PosixSpawnedChild Spawn(PosixSpawnRequest request)
+        {
+            Calls++;
+            PosixRequest = request;
+            throw new Win32Exception(2);
+        }
+
+        public WindowsSpawnedChild Spawn(WindowsSpawnRequest request)
+        {
+            Calls++;
+            WindowsRequest = request;
+            throw new Win32Exception(2);
+        }
+    }
+}
diff --git a/tests/OpenCode.Sdk.Tests/Hosting/Launcher/WindowsServerChildTests.cs b/tests/OpenCode.Sdk.Tests/Hosting/Launcher/WindowsServerChildTests.cs
new file mode 100644
index 00000000..3b82d817
--- /dev/null
+++ b/tests/OpenCode.Sdk.Tests/Hosting/Launcher/WindowsServerChildTests.cs
@@ -0,0 +1,145 @@
+using System.ComponentModel;
+using System.Diagnostics;
+using Microsoft.Win32.SafeHandles;
+using NSubstitute;
+using NSubstitute.ExceptionExtensions;
+using OpenCode.Sdk.Internal;
+using OpenCode.Sdk.Tests.Harness;
+using OpenCode.Sdk.Tests.Support;
+using Testably.Abstractions;
+
+namespace OpenCode.Sdk.Tests.Hosting.Launcher;
+
+/// <summary>The Windows strategy's spawn and adoption failures, including the running child it must end.</summary>
+[NotInParallel(ParallelConstraintKeys.ServerProcess)]
+public sealed class WindowsServerChildTests
+{
+    private static readonly RealFileSystem FileSystem = new();
+    private static readonly TimeSpan Bound = TimeSpan.FromSeconds(30);
+
+    /// <summary>A native spawn failure preserves its cause and names the resolution and directory outside the command quotes.</summary>
+    [Test]
+    [Arguments(true)]
+    [Arguments(false)]
+    public async Task SpawnAsync_Should_Name_The_Command_And_Preserve_The_Spawn_Failure(bool details)
+    {
+        var seams = Seams();
+        var failure = new Win32Exception(2);
+        seams.WindowsSpawn.Spawn(Arg.Any<WindowsSpawnRequest>()).Throws(failure);
+        var start = Start(new ResolvedExecutable("opencode", details ? @"C:\tools\opencode.exe" : "opencode", IsBatchScript: false)) with
+        { WorkingDirectory = details ? @"C:\work" : null };
+
+        var exception = await Assert.That(async () => await WindowsServerChild.SpawnAsync(start, seams))
+            .Throws<OpenCodeServerException>();
+
+        await Assert.That(exception!.Message).IsEqualTo(details
+            ? @"Failed to start the server command 'opencode' (resolved to 'C:\tools\opencode.exe') in the working directory 'C:\work'."
+            : "Failed to start the server command 'opencode'.");
+        await Assert.That(exception.InnerException).IsSameReferenceAs(failure);
+    }
+
+    /// <summary>A watch creation failure ends the already-running child with code 1 and closes the handle it offered the watch; the session leak guard proves no watch is left.</summary>
+    [Test]
+    [Timeout(120_000)]
+    public async Task SpawnAsync_Should_End_The_Child_When_Its_Exit_Cannot_Be_Watched(CancellationToken cancellationToken)
+    {
+        if (!OperatingSystem.IsWindows())
+        {
+            BranchReport.Print("POSIX — the Windows watch adoption path requires a Windows process handle");
+            return;
+        }
+
+        await using var exits = new RefusingChildExit();
+        var seams = Seams(exits, exits);
+        var fixture = new ServiceFixtureCommand(FileSystem).Resolve();
+        var resolved = new ExecutableResolver(ExecutableSearchEnvironment.ForCurrentProcess()).Resolve(fixture[0]);
+        var executable = new ResolvedExecutable("fixture", resolved.Path, IsBatchScript: false);
+        var start = Start(executable) with { SuppliedArguments = [fixture[1], "idle"] };
+
+        var exception = await Assert.That(async () => await WindowsServerChild.SpawnAsync(start, seams))
+            .Throws<OpenCodeServerException>();
+
+        await Assert.That(exception!.Message).IsEqualTo(
+            $"The server command 'fixture' (resolved to '{resolved.Path}') was started, but its exit could not be watched or its output read, so it was ended.");
+        await Assert.That(exception.InnerException).IsSameReferenceAs(exits.Failure);
+        await Assert.That(exits.Held).IsNotNull();
+        await Assert.That(await ProcessObservation.ObserveExitWithinAsync(exits.Held!, Bound, cancellationToken)).IsTrue();
+        await Assert.That(exits.Held!.ExitCode).IsEqualTo(1);
+        await Assert.That(exits.Watched!.IsClosed).IsTrue();
+    }
+
+    private static ServerChildStart Start(ResolvedExecutable executable) => new()
+    {
+        Executable = executable,
+        SuppliedArguments = [],
+        LauncherArguments = [],
+        Password = nameof(WindowsServerChildTests),
+        GracefulShutdownTimeout = TimeSpan.FromSeconds(3),
+        OnStandardOutput = static _ => { },
+        OnStandardError = static _ => { },
+    };
+
+    private static LauncherSeams Seams(IWindowsSpawn? spawn = null, IWindowsChildExit? exits = null) => new()
+    {
+        WindowsSpawn = spawn ?? Substitute.For<IWindowsSpawn>(),
+        WindowsExits = exits ?? Substitute.For<IWindowsChildExit>(),
+        Job = Substitute.For<IProcessJob>(),
+        Signals = Substitute.For<IProcessGroupSignal>(),
+        TreeKill = Substitute.For<IWindowsTreeKill>(),
+        Exits = Substitute.For<IChildExitStatus>(),
+        Time = new ManualTimeProvider(),
+        Spawn = Substitute.For<IPosixSpawn>(),
+        HostEnvironment = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase),
+    };
+
+    /// <summary>Takes an independent observation handle before refusing to create any exit watch.</summary>
+    private sealed class RefusingChildExit : IWindowsChildExit, IWindowsSpawn, IAsyncDisposable
+    {
+        private readonly WindowsSpawn _spawn = new();
+
+        public Win32Exception Failure { get; } = new(6);
+
+        public Process? Held { get; private set; }
+
+        public SafeProcessHandle? Watched { get; private set; }
+
+        public WindowsSpawnedChild Spawn(WindowsSpawnRequest request)
+        {
+            var child = _spawn.Spawn(request);
+            try
+            {
+                Held = Process.GetProcessById(child.ProcessId);
+                _ = Held.Handle;
+                return child;
+            }
+            catch
+            {
+                // The strategy never saw this child, so nothing else would end it or close its handles.
+                _ = WindowsInterop.TerminateProcess(child.Process, 1);
+                child.MainThread?.Dispose();
+                child.StandardInput?.Dispose();
+                child.StandardOutput?.Dispose();
+                child.StandardError?.Dispose();
+                child.Process.Dispose();
+                throw;
+            }
+        }
+
+        public IWindowsExitWatch Watch(SafeProcessHandle process)
+        {
+            Watched = process;
+            throw Failure;
+        }
+
+        public async ValueTask DisposeAsync()
+        {
+            if (Held is { HasExited: false })
+            {
+                Held.Kill();
+                await Held.WaitForExitAsync();
+            }
+
+            Held?.Dispose();
+        }
+    }
+}
diff --git a/tests/OpenCode.Sdk.Tests/Hosting/Platform/Posix/PosixSignalNamesTests.cs b/tests/OpenCode.Sdk.Tests/Hosting/Platform/Posix/PosixSignalNamesTests.cs
index ff33288e..80c1b3e0 100644
--- a/tests/OpenCode.Sdk.Tests/Hosting/Platform/Posix/PosixSignalNamesTests.cs
+++ b/tests/OpenCode.Sdk.Tests/Hosting/Platform/Posix/PosixSignalNamesTests.cs
@@ -11,6 +11,20 @@ public sealed class PosixSignalNamesTests
 {
     public static IEnumerable<int> StandardSignals() => Enumerable.Range(1, 31);
 
+    /// <summary>The host selects macOS's numbering only on macOS; Windows exposes the otherwise-unused Linux table.</summary>
+    [Test]
+    public async Task ForCurrentProcess_Should_Select_The_Hosts_Signal_Numbering()
+    {
+        if (OperatingSystem.IsMacOS())
+        {
+            await Assert.That(PosixSignalNames.ForCurrentProcess.NameOf(10)).IsEqualTo("SIGBUS");
+        }
+        else
+        {
+            await Assert.That(PosixSignalNames.ForCurrentProcess.NameOf(10)).IsEqualTo("SIGUSR1");
+        }
+    }
+
     [Test]
     [MethodDataSource(nameof(StandardSignals))]
     public async Task NameOf_Should_Name_Every_Standard_Signal_On_Both_Systems(int number)
diff --git a/tests/OpenCode.Sdk.Tests/Hosting/Platform/Windows/WindowsExitWatchTests.cs b/tests/OpenCode.Sdk.Tests/Hosting/Platform/Windows/WindowsExitWatchTests.cs
index a0489db8..e968abbf 100644
--- a/tests/OpenCode.Sdk.Tests/Hosting/Platform/Windows/WindowsExitWatchTests.cs
+++ b/tests/OpenCode.Sdk.Tests/Hosting/Platform/Windows/WindowsExitWatchTests.cs
@@ -27,6 +27,7 @@ public sealed class WindowsExitWatchTests
     [Timeout(120_000)]
     [Arguments(AccessViolation)]
     [Arguments(7)]
+    [Arguments(259)]
     public async Task Exited_Should_Report_The_Code_In_Its_Full_32_Bits(int code, CancellationToken cancellationToken)
     {
         if (!OperatingSystem.IsWindows())