oraios/serena · error · SolidLSPException

Failed to start C# language server: {e}

Error message

Failed to start C# language server: {e}

What it means

Raised by _start_server when self.server.start() throws any exception while launching the Roslyn language server process; it wraps the underlying error in SolidLSPException. Commonly the DLL can't be launched (missing runtime, wrong architecture) or the process dies immediately.

Source

Thrown at src/solidlsp/language_servers/csharp_language_server.py:663

            indexing_complete.set()

        # Set up notification handlers
        self.server.on_notification("window/logMessage", window_log_message)
        self.server.on_notification("$/progress", handle_progress)
        self.server.on_notification("textDocument/publishDiagnostics", do_nothing)
        self.server.on_notification("workspace/projectInitializationComplete", handle_workspace_indexing_complete)
        self.server.on_request("workspace/configuration", handle_workspace_configuration)
        self.server.on_request("window/workDoneProgress/create", handle_work_done_progress_create)
        self.server.on_request("client/registerCapability", handle_register_capability)
        self.server.on_request("workspace/_roslyn_projectNeedsRestore", handle_project_needs_restore)

        log.info("Starting Microsoft.CodeAnalysis.LanguageServer process")

        try:
            self.server.start()
        except Exception as e:
            log.info(f"Failed to start language server process: {e}", logging.ERROR)
            raise SolidLSPException(f"Failed to start C# language server: {e}")

        # Send initialization
        initialize_params = self._create_initialize_params()

        log.info("Sending initialize request to language server")
        try:
            init_response = self.server.send.initialize(initialize_params)
            log.info(f"Received initialize response: {init_response}")
        except Exception as e:
            raise SolidLSPException(f"Failed to initialize C# language server for {self.repository_root_path}: {e}") from e

        # Apply diagnostic capabilities
        self._force_pull_diagnostics(init_response)

        # Verify required capabilities
        capabilities = init_response.get("capabilities", {})
        required_capabilities = [
            "textDocumentSync",

View on GitHub (pinned to 7fcbca7e62)

Solutions

1. Check the logged inner message ('Failed to start language server process: {e}') for the root cause
2. Install/upgrade the .NET SDK/runtime to a version compatible with the Roslyn language server
3. Re-download the language server for the correct platform (clear cache dir and rerun)
4. Verify the DLL/executable is runnable manually: `dotnet <server_dll> --help` or executing the native binary directly

Example fix

// before
$ dotnet --list-runtimes # (nothing installed)
// after
sudo apt-get install -y dotnet-sdk-9.0
$ dotnet --list-runtimes # Microsoft.NETCore.App 9.x

import subprocess
from pathlib import Path

def can_launch_roslyn(server_dll: str) -> tuple[bool, str]:
    dll = Path(server_dll)
    if not dll.is_file():
        return False, "DLL missing"
    try:
        p = subprocess.run(["dotnet", "--list-runtimes"], capture_output=True, text=True, timeout=30)
    except (OSError, subprocess.TimeoutExpired):
        return False, ".NET runtime missing or not runnable"
    if p.returncode != 0 or "Microsoft.NETCore.App" not in p.stdout:
        return False, ".NET runtime not installed"
    return True, p.stdout.splitlines()[0]

def dotnet_runtime_available() -> bool:
    import shutil
    return shutil.which("dotnet") is not None

try:
    ls._start_server()
except SolidLSPException as e:
    log.error(f"Roslyn start failed: {e}")
    ok, reason = can_launch_roslyn(server_dll)
    if not ok:
        install_dotnet_runtime()  # e.g. apt install dotnet-sdk-9.0
        ls._start_server()
    else:
        raise

When it happens

Trigger: CSharpLanguageServer._start_server when LanguageServer.start() raises — e.g. the launch command can't execute (missing .NET runtime, bad DLL path), missing dependencies for the Roslyn server, or spawn permission errors.

Common situations: .NET runtime not installed or too old for the Roslyn server version, executable bit not set (permission), Apple Silicon running x64-only build without Rosetta, or the server binary downloaded for the wrong OS.

Related errors

• Roslyn Language Server DLL not found after extraction
• Failed to initialize C# language server for {self.repository
• Language server is missing required capabilities: {', '.join
• Failed to start F# language server: {e}
• Required .NET runtime version {self._required_version_str} n

AI-assisted analysis of oraios/serena@7fcbca7e62 (2026-08-29). Data as JSON: /api/errors/536e5f56bf79e1c1. Report an issue: GitHub.