CoplayDev/unity-mcp · error · Exception

Failed to download {packageId}: {request.error}

Error message

Failed to download {packageId}: {request.error}

What it means

Thrown by RoslynInstaller.Install when a UnityWebRequest.Get against the NuGet flat container (api.nuget.org) does not return Success. The installer downloads .nupkg zips and extracts the Roslyn DLLs that power the runtime_compilation tool; a failed download aborts the whole install.

Source

Thrown at MCPForUnity/Editor/Setup/RoslynInstaller.cs:99

                    {
                        EditorUtility.DisplayProgressBar(
                            "Installing Roslyn",
                            $"Downloading {packageId} v{pkgVersion}...",
                            (float)i / NuGetEntries.Length);
                    }

                    string url =
                        $"https://api.nuget.org/v3-flatcontainer/{packageId}/{pkgVersion}/{packageId}.{pkgVersion}.nupkg";

                    using (var request = UnityWebRequest.Get(url))
                    {
                        request.timeout = 30;
                        request.SendWebRequest();
                        while (!request.isDone)
                            System.Threading.Thread.Sleep(50);

                        if (request.result != UnityWebRequest.Result.Success)
                            throw new Exception($"Failed to download {packageId}: {request.error}");

                        byte[] nupkgBytes = request.downloadHandler.data;
                        byte[] dllBytes = ExtractFileFromZip(nupkgBytes, dllPathInZip);

                        if (dllBytes == null)
                        {
                            Debug.LogError($"[MCP] Could not find {dllPathInZip} in {packageId}.{pkgVersion}.nupkg");
                            continue;
                        }

                        string destPath = Path.Combine(destFolder, dllName);
                        File.WriteAllBytes(destPath, dllBytes);
                        Debug.Log($"[MCP] Extracted {dllName} ({dllBytes.Length / 1024}KB) → Assets/{PluginsRelPath}/{dllName}");
                    }
                }

                if (interactive)
                    EditorUtility.DisplayProgressBar("Installing Roslyn", "Refreshing assets...", 0.95f);

View on GitHub (pinned to c21bf496bc)

Solutions

1. Verify connectivity to https://api.nuget.org from the Unity machine.
2. Retry the install once network is stable.
3. Manually place the required DLLs (Microsoft.CodeAnalysis 4.12.0, CSharp, System.Collections.Immutable 8.0.0, System.Reflection.Metadata 8.0.0, System.Runtime.CompilerServices.Unsafe 6.0.0) into Assets/Plugins/Roslyn.
4. Configure a working proxy in Unity/OS settings if behind a corporate network.

try
{
    RoslynInstaller.Install(interactive: true);
}
catch (Exception ex) when (ex.Message.Contains("Failed to download"))
{
    Debug.LogError($"NuGet download failed (network): {ex.Message}. Place DLLs manually in Assets/Plugins/Roslyn.");
}

When it happens

Trigger: Network is offline or api.nuget.org is unreachable; the 30-second request timeout is exceeded on a slow link; a corporate proxy or SSL/certificate error fails the request; the specific package/version was delisted from NuGet.

Common situations: Corporate firewall blocks nuget.org; air-gapped Unity machine; transient NuGet outage; slow connection timing out at the hardcoded 30s.

Related errors

• Port must be positive.
• Port {port} is already in use.
• No available ports found in range {DefaultPort}-{DefaultPort
• fal submit returned no request_id: {truncated_response}
• fal {phase} failed (status={res?.Status}): {detail}

AI-assisted analysis of CoplayDev/unity-mcp@c21bf496bc (2026-08-13). Data as JSON: /api/errors/dfa58694a7acd6fb. Report an issue: GitHub.