C# Language Server fails to initialize: .NET 10.0 requirement blocks Unity projects on macOS ARM64

[IJsLauw]

Labels

bug, C#, language-server, Unity

Description

Summary

Serena's C# language server (via SolidLSP) attempts to install .NET 10.0 runtime, which doesn't exist yet (still in Preview), causing language server initialization to fail for Unity projects that require .NET 6.0/Standard 2.1 compatibility.

Environment

• OS: macOS 14.x (Apple Silicon ARM64)
• Serena: via Claude Code plugin (latest)
• Project: Unity 6000.3.5f2 (C# 9, .NET Standard 2.1)
• Installed .NET: 6.0.102 SDK + 6.0.2 Runtime (ARM64)
• OmniSharp: 1.39.15 (manually installed, ARM64)

Steps to Reproduce

1. Install Serena plugin in Claude Code
2. Create/open a Unity project (C# with .NET Standard 2.1 or .NET 6.0 target)
3. Create .serena/config.toml in project root:

   [project]
   name = "MyUnityProject"
   
   [language_servers.csharp]
   dotnet_version = "6.0"
   path = "/path/to/manually/installed/omnisharp"
   
   [language_servers.csharp.settings]
   unity = true
   solution = "MyUnityProject.sln"

4. Activate project in Serena
5. Attempt to use C# symbolic tools (e.g., get_symbols_overview)

Expected Behavior

• Serena should use the configured OmniSharp and .NET 6.0
• OR respect the project's target framework (.NET Standard 2.1 / .NET 6.0)
• C# language server should initialize successfully

Actual Behavior

Error log excerpt:

INFO  solidlsp.language_servers.csharp_language_server:_install_dotnet_with_script:494
      - Installing .NET 10.0 runtime using official Microsoft install script...

ERROR Failed to install .NET 10.0 runtime using install script:
      Command '['bash', '...dotnet-install.sh', '--version', '10.0', ...]'
      died with <Signals.SIGKILL: 9>.

Result:

• Language server manager fails to initialize
• All symbolic C# tools (find_symbol, get_symbols_overview, etc.) fail with:

  Error: The language server manager is not initialized
  

• Multiple bash processes spawn trying to install .NET 10.0 (4-7+ processes)
• Project becomes unusable for C# code intelligence

Root Cause Analysis

1. SolidLSP hardcoded .NET 10.0 requirement

   • The solidlsp package has hardcoded logic to install .NET 10.0
   • Code path: solidlsp/language_servers/csharp_language_server.py:_install_dotnet_with_script:494
   • This runs before reading project-specific configuration

2. .NET 10.0 doesn't exist yet

   • .NET 10 is currently in Preview, no stable release exists
   • Official Microsoft installer fails when attempting --version 10.0

3. Configuration is ignored

   • Both global ~/.serena/serena_config.yml and project .serena/config.toml are read AFTER installation attempt
   • Settings like dotnet_version = "6.0", path = "...", and external_dotnet_runtime = true have no effect

4. Unity projects specifically affected

   • Unity 2022+ uses .NET Standard 2.1 (equivalent to .NET 6.0)
   • Unity projects cannot use .NET 10.0 even when it's released
   • This affects a significant portion of C# developers using Serena

What I Tried (All Failed)

✅ Installed prerequisites manually:

• OmniSharp 1.39.15 for ARM64
• .NET 6.0 SDK (6.0.102) for ARM64
• Verified both work independently with DOTNET_ROOT=/usr/local/share/dotnet

✅ Configured project-specific settings (.serena/config.toml):

[language_servers.csharp]
dotnet_version = "6.0"
path = "/Users/username/.local/bin/omnisharp/OmniSharp"
env = { DOTNET_ROOT = "/usr/local/share/dotnet" }

✅ Configured global settings (~/.serena/serena_config.yml):

ls_specific_settings:
  csharp:
    enabled: false  # Tried to disable auto-installation
    path: "/Users/username/.local/bin/omnisharp/OmniSharp"
    dotnet_path: "/usr/local/share/dotnet/dotnet"
    external_dotnet_runtime: true
    dotnet_version: "6.0"

✅ Cleaned up multiple times:

• Removed ~/.serena/language_servers/static/CSharpLanguageServer/
• Killed all background installation processes
• Restarted Claude Code cleanly

❌ Result: SolidLSP still attempts .NET 10.0 installation on every startup

Related Issues

This appears to be the same class of problem as:

• Tries and fails to install dotnet 9 in a dotnet 8 project #663 - "Tries and fails to install dotnet 9 in a dotnet 8 project"
• [B] LSP Timeout when loading .NET 9 projects in SolidLSP (System.TimeoutException in NamedPipeClientStream) #513 - "LSP Timeout when loading .NET 9 projects"
• C# Language Server Download/Initialization Fails: Missing File #384 - "C# Language Server Download/Initialization Fails"

The pattern: Serena attempts to install newer .NET versions regardless of project requirements.

Impact

• All Unity developers cannot use Serena's C# language server
• All .NET 6.0/8.0 projects may fail if SolidLSP requires .NET 10.0
• Workaround required: Users must use VS Code + OmniSharp extension separately

Proposed Solutions

Option 1: Support Older .NET Versions (Preferred)

• Allow SolidLSP to work with .NET 6.0, 8.0, and 9.0
• Detect project target framework before choosing .NET version
• Unity projects → use .NET 6.0
• Modern projects → use .NET 8.0/9.0

Option 2: Respect Manual Configuration

• When ls_specific_settings.csharp.path is provided, skip auto-installation entirely
• When external_dotnet_runtime = true, skip .NET installation
• Read configuration BEFORE attempting any downloads

Option 3: Make .NET Version Configurable

• Allow dotnet_version in config to override hardcoded version
• Default to .NET 8.0 (stable) instead of .NET 10.0 (preview)
• Validate version exists before attempting installation

Option 4: Fallback Strategy

• Try .NET 10.0 first
• On failure, fall back to .NET 8.0, then 6.0
• Use whatever .NET is already installed on the system

Additional Context

Why .NET 6.0 for Unity:

• Unity 2022+ uses .NET Standard 2.1 API compatibility
• Unity's C# compiler maps to roughly .NET 6.0 feature set
• OmniSharp 1.39.x works perfectly with .NET 6.0 for Unity projects

System Details:

$ dotnet --version
6.0.102

$ dotnet --list-runtimes
Microsoft.AspNetCore.App 6.0.2 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.NETCore.App 6.0.2 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]

$ arch
arm64

$ file /usr/local/share/dotnet/dotnet
/usr/local/share/dotnet/dotnet: Mach-O 64-bit executable arm64

Manual OmniSharp Test (Works):

$ DOTNET_ROOT=/usr/local/share/dotnet ~/.local/bin/omnisharp/OmniSharp --version
# Successfully starts, detects .NET 6.0, finds Unity solution

Logs

Serena Log Excerpt

INFO  2026-02-10 11:46:30,842 [Task-1:init_language_server_manager]
      serena.project:create_language_server_manager:414
      - Creating language server manager for /path/to/Unity/Project

INFO  2026-02-10 11:46:31,400 [StartLS:csharp]
      serena.ls_manager:create_language_server:41
      - Creating language server instance for /path/to/Unity/Project, language=csharp.

INFO  2026-02-10 11:46:31,446 [StartLS:csharp]
      solidlsp.language_servers.csharp_language_server:_install_dotnet_with_script:494
      - Installing .NET 10.0 runtime using official Microsoft install script...

ERROR 2026-02-10 11:46:42,177 [StartLS:csharp]
      sensai.util.logging:__exit__:373
      - Language server startup (language=csharp) failed after 10.777 seconds

ERROR 2026-02-10 11:46:42,178 [StartLS:csharp]
      serena.ls_manager:start_language_server:101
      - Error starting language server for language csharp:
        Failed to install .NET 10.0 runtime using install script:
        Command '['bash', '/Users/username/.serena/language_servers/static/CSharpLanguageServer/dotnet-runtime-10.0/dotnet-install.sh',
        '--version', '10.0', '--install-dir', '/Users/username/.serena/language_servers/static/CSharpLanguageServer/dotnet-runtime-10.0',
        '--runtime', 'dotnet', '--no-path']' died with <Signals.SIGKILL: 9>.

Request

Please update SolidLSP or Serena's C# language server integration to:

1. Support .NET 6.0 and 8.0 (stable versions)
2. Respect manual OmniSharp configuration when provided
3. Detect project target framework before choosing .NET version
4. OR allow disabling automatic .NET installation entirely

This would unblock all Unity developers and users with .NET 6.0/8.0 projects.

Workaround (For Others Experiencing This)

Until fixed, use VS Code with C# extension for IntelliSense, while using Claude Code for AI assistance. OmniSharp in VS Code works perfectly with the same Unity projects.

---

Thank you for this excellent tool! Looking forward to using Serena with Unity projects once this is resolved.

[opcode81]

.NET 10.0 runtime, which doesn't exist yet (still in Preview)

.NET 10 is not in preview, it was released in November 2025.
Was there a problem installing it?

Serena should use the configured OmniSharp and .NET 6.0

SolidLSP uses not OmniSharp but Roslyn by default.
We also support OmniSharp though, and you can use it by setting your project language to csharp_omnisharp instead of just csharp.

Also pinging @thomhurst because Roslyn C# language server support is mostly maintained by him.
Is the LS using .NET 10 a problem for projects that use older .NET versions or can it support them just fine?

[thomhurst]

.net 10 still in preview for that architecture!? I didn't know that.

The new Roslyn language server only supports .net 10 (and should be backwards compatible still, but yeah a blocker if it can't install it) - switching to omnisharp via the instructions above are probably the best workaround for now until it's officially released

[thomhurst]

Alternatively does installing the preview runtime work? (Not ideal if its not stable though I know)

[opcode81]

.net 10 still in preview for that architecture!? I didn't know that.

It is not in preview, it is fully released - and the Roslyn LS works fine in our macOS CI tests.
I believe the claim may stem from an AI having an early training cutoff ;)

[opcode81]

Create .serena/config.toml in project root

@IJsLauw I believe something went very wrong here. Serena does not use such a file at all.

[IJsLauw]

Please ignore, installing the claude plugin wen't wrong for my unity project, and relying too much on AI got me deeper into it, but i think i got it fixed now. Not thanks to gemini, or was it claude ...

[AncientPixel]

Please ignore, installing the claude plugin wen't wrong for my unity project, and relying too much on AI got me deeper into it, but i think i got it fixed now. Not thanks to gemini, or was it claude ...

I have the same issue with my Serena / Unity install, how did you fix it? @IJsLauw

Just want to add, I'm on Windows 11, I've installed the .net10 runtime, and it works if I switch it to csharp_omnisharp but would prefer csharp.

[opcode81]

@AncientPixel please provide some concrete information on the error you are seeing when using csharp.

[IJsLauw]

Well, I think i broke it by setting up Serena in Claude code from the desktop app and inside rider from the terminal at the same time... which mixed up paths and configs since rider starts at the unity root instead of the project root. I just removed everything and let Claude code the desktop app fix it from the top. Don't ask me how exactly, it took some time and retries since i was not familiar with Serena I let my agent handle it... Verzonden vanaf Outlook voor Android<https://aka.ms/AAb9ysg>

…

________________________________ From: Gustave ***@***.***> Sent: Friday, February 13, 2026 11:17:54 PM To: oraios/serena ***@***.***> Cc: Laurent de Goede ***@***.***>; Mention ***@***.***> Subject: Re: [oraios/serena] C# Language Server fails to initialize: .NET 10.0 requirement blocks Unity projects on macOS ARM64 (Issue #1014) [https://avatars.githubusercontent.com/u/4158280?s=20&v=4]AncientPixel left a comment (oraios/serena#1014)<#1014 (comment)> Please ignore, installing the claude plugin wen't wrong for my unity project, and relying too much on AI got me deeper into it, but i think i got it fixed now. Not thanks to gemini, or was it claude ... I have the same issue with my Serena / Unity install, how did you fix it? — Reply to this email directly, view it on GitHub<#1014 (comment)>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/ABTVDR2SP5KDQIZUVYBYN7L4LZERFAVCNFSM6AAAAACUTD6AFKVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZTQOJZHAZTQMZWG4>. You are receiving this because you were mentioned.Message ID: ***@***.***>

[Trapulo]

I have same problem. Enabling C# via language csharp (Serena on docker, windows 11 host), it reports:

INFO  2026-02-19 17:07:05,846 [StartLS:csharp] solidlsp.language_servers.csharp_language_server:_install_dotnet_with_script:494 - Installing .NET 10.0 runtime using official Microsoft install script...
ERROR 2026-02-19 17:07:15,201 [StartLS:csharp] sensai.util.logging:__exit__:342 - Language server startup (language=csharp) failed after 9.372 seconds
ERROR 2026-02-19 17:07:15,201 [StartLS:csharp] serena.ls_manager:start_language_server:101 - Error starting language server for language csharp: Failed to install .NET 10.0 runtime using install script: dotnet_install: Error: Could not find `.NET Core Runtime` with version = 10.0
dotnet_install: Error: Refer to: https://aka.ms/dotnet-os-lifecycle for information on .NET Core support
Traceback (most recent call last):
  File "/workspaces/serena/src/solidlsp/language_servers/csharp_language_server.py", line 539, in _install_dotnet_with_script
    result = subprocess.run(cmd, capture_output=True, text=True, check=True)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/subprocess.py", line 571, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['bash', '/workspaces/serena/config/language_servers/static/CSharpLanguageServer/dotnet-runtime-10.0/dotnet-install.sh', '--version', '10.0', '--install-dir', '/workspaces/serena/config/language_servers/static/CSharpLanguageServer/dotnet-runtime-10.0', '--runtime', 'dotnet', '--no-path']' returned non-zero exit status 1.

The above exception was the direct cause of the following exception:

[vbuldakov]

I'm running Serena in docker (WSL2 backend, win 11 host), I'm trying to index my .NET 10 project and getting same error:

ERROR 2026-02-23 15:23:15,394 sensai.util.logging:__exit__:373 - Language server startup (language=csharp) failed after 15.059 seconds
ERROR 2026-02-23 15:23:15,394 serena.ls_manager:start_language_server:101 - Error starting language server for language csharp: Failed to install .NET 10.0 runtime using install script: dotnet_install: Error: Could not find `.NET Core Runtime` with version = 10.0
dotnet_install: Error: Refer to: https://aka.ms/dotnet-os-lifecycle for information on .NET Core support
Traceback (most recent call last):
  File "/workspaces/serena/src/solidlsp/language_servers/csharp_language_server.py", line 540, in _install_dotnet_with_script
    result = subprocess.run(cmd, capture_output=True, text=True, check=True)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/subprocess.py", line 571, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['bash', '/workspaces/serena/config/language_servers/static/CSharpLanguageServer/dotnet-runtime-10.0/dotnet-install.sh', '--version', '10.0', '--install-dir', '/workspaces/serena/config/language_servers/static/CSharpLanguageServer/dotnet-runtime-10.0', '--runtime', 'dotnet', '--no-path']' returned non-zero exit status 1.

[AncientPixel]

@AncientPixel please provide some concrete information on the error you are seeing when using csharp.

I get the following in the logs:
ERROR 2026-02-25 18:31:02,918 [StartLS:csharp] sensai.util.logging:exit:373 - Language server startup (language=csharp) failed after 18.838 seconds
ERROR 2026-02-25 18:31:02,918 [StartLS:csharp] serena.ls_manager:start_language_server:102 - Error starting language server for language csharp: Failed to install .NET 10.0 runtime using install script: dotnet-install: Downloading from "primary" link has failed with error:
Uri: https://builds.dotnet.microsoft.com/dotnet/Runtime/10.0/dotnet-runtime-10.0-win-x64.zip
StatusCode: 404
Error: Unable to download https://builds.dotnet.microsoft.com/dotnet/Runtime/10.0/dotnet-runtime-10.0-win-x64.zip. Returned HTTP status code: 404 Downloading from "legacy" link has failed with error:
Uri: https://builds.dotnet.microsoft.com/dotnet/Runtime/10.0/dotnet-win-x64.10.0.zip

That Uri does not exist but this one does:
https://builds.dotnet.microsoft.com/dotnet/Runtime/10.0.3/dotnet-runtime-10.0.3-win-x64.zip

[Nrconlon]

I was having the same issue for my C# project, but when I told claude .Net 10 WAS actually released it walked me through installing it and setting everything up properly.
"So here's the thing — Roslyn is the better backend (csharp in Serena's config), but it's the one that hardcodes .NET 10 which doesn't exist yet. That's why we had to fall back to csharp_omnisharp."
Told it to look it up
"Well shoot - .NET 10 is released (November 2025). My training data was wrong."
Then it proceeded to set it all up correctly. Serena fully working now.
Issue is with claude codes out of date training data.

[AncientPixel]

Thanks all, so the fix is on windows is installing the .net 10.0.3 SDK, then telling Claude to fix Serena MCP by installing csharp-ls, it will install version 0.22.0, exit and restart Claude and it worked.