[Bug]: [Windows] Headless server launch always fails with error: The handle is invalid. (os error 6) (regression from #1201/#1188)

[AnaelMaayan]

What happened?

On Windows, "Start Server" (and the auto-start path) in the MCP for Unity window fails every time with:

error: The handle is invalid. (os error 6)

This is the entire contents of Library/MCPForUnity/Logs/server-launch-{port}.log — no FastMCP banner, no traceback, nothing else. The (os error N) formatting is uv's/uvx's own Rust error-printing style (Python would say [WinError 6]), so the failure happens inside uvx itself, before it gets anywhere near launching the Python server.

I believe this is a regression from #1201 (fixes #1188 — hides the terminal window on Windows by launching headless via CreateNoWindow = true with no explicit stdio redirection).

Root cause

TerminalLauncher.CreateHeadlessProcessStartInfo (Windows branch) builds:

FileName = "cmd.exe",
Arguments = $"/c \"{command} >> \"{logFilePath}\" 2>&1\"",
UseShellExecute = false,
CreateNoWindow = true,
WindowStyle = ProcessWindowStyle.Hidden

with none of RedirectStandardInput/Output/Error set — the >>/2>&1 redirection is left entirely to cmd.exe's own command-line parsing rather than the ProcessStartInfo.

The Unity Editor process itself is a GUI-subsystem process with no console at all. When it starts a child with CreateNoWindow = true and no stdio redirection, per the Win32 docs for that flag ("the console handle for the application is not set"), the resulting cmd.exe → uvx.exe chain gets no real console. cmd.exe's own >> file 2>&1 redirection gives it a valid stdout/stderr (a real file handle), but stdin is never touched, so uvx.exe inherits a console-less/invalid stdin. uvx then appears to touch that handle early during startup (plausibly for Ctrl-C/TTY setup) and dies immediately with ERROR_INVALID_HANDLE before it ever gets to spawn the Python venv / print anything.

Reproduction steps

1. On Windows, open a Unity project with MCP for Unity (HTTP transport, default settings).
2. Window > MCP for Unity > Start Server (or let auto-start trigger it).
3. Observe: Local HTTP server did not become reachable. with the log containing only error: The handle is invalid. (os error 6).
4. This is deterministic — every attempt from inside the Unity Editor process fails identically, including with a warm uv cache (so it isn't a first-run download/extraction race).
5. Confirming it's specific to the console-less parent: copy the exact command Unity prints in the error dialog ("To run it yourself, copy this command into a terminal") and run it via an equivalent ProcessStartInfo (cmd.exe /c "<command> >> log 2>&1", UseShellExecute=false, CreateNoWindow=true, no stdio redirection) but spawned from a process that already owns a console (e.g. a PowerShell/cmd session) instead of from Unity — it starts cleanly every time (FastMCP banner, Uvicorn listening). Running the identical command directly in a terminal ("To run it yourself...") also always works, which is why that fallback advice in the error dialog is misleading here — it works precisely because a terminal has a console, which is the variable that matters.

Suggested fix

Give the child process a real (even if empty) stdin instead of none, e.g. in the Windows branch of CreateHeadlessProcessStartInfo, redirect stdin from NUL:

string winRedirect = $"{command} < NUL >> \"{logFilePath}\" 2>&1";

or set RedirectStandardInput = true on the ProcessStartInfo and immediately close the stream after start. Either should give uvx.exe a valid handle to query instead of none.

Unity version

6000.0.78f1

MCP for Unity package version

10.1.0

Python server version

mcpforunityserver==10.1.0

MCP client

Claude Code

Transport

HTTP (default)

OS

Windows (11)

Relevant logs / console output

Library/MCPForUnity/Logs/server-launch-8080.log:

error: The handle is invalid. (os error 6)

Unity console:

MCP-FOR-UNITY: Local HTTP server did not become reachable. Launch log: C:\...\Library\MCPForUnity\Logs\server-launch-8080.log
error: The handle is invalid. (os error 6)
To run it yourself, copy this command into a terminal:
C:\Users\User\.local\bin\uvx.exe --from "mcpforunityserver==10.1.0" mcp-for-unity --transport http --http-url http://127.0.0.1:8080 --project-scoped-tools
UnityEngine.Debug:LogError (object)
MCPForUnity.Editor.Helpers.McpLog:Error (string) (at ./Library/PackageCache/com.coplaydev.unity-mcp@.../Editor/Helpers/McpLog.cs:50)
MCPForUnity.Editor.Services.ServerManagementService:LogLocalHttpServerLaunchFailure () (at ./Library/PackageCache/com.coplaydev.unity-mcp@.../Editor/Services/ServerManagementService.cs:1053)
MCPForUnity.Editor.Windows.Components.Connection.McpConnectionSection/<TryAutoStartSessionAsync>d__64:MoveNext () (at ./Library/PackageCache/com.coplaydev.unity-mcp@.../Editor/Windows/Components/Connection/McpConnectionSection.cs:798)
...

Checks

• I searched existing issues and did not find a duplicate
• I included logs / steps to reproduce