oraios/serena · error · SolidLSPException

Failed to start F# language server

Error message

Failed to start F# language server: {e}

What it means

When the underlying LSP process wrapper (self.server.start()) fails to launch the FsAutoComplete process, _start_server logs the cause and re-raises it as SolidLSPException. This typically means the command line, working directory, or environment made the server process die immediately at startup.

Solutions

1. Run fsautocomplete manually with the same flags to see the crash: fsautocomplete --adaptive-lsp-server-enabled --project-graph-enabled --use-fcs-transparent-compiler
2. Verify the .NET runtime works: dotnet --info, and reinstall FsAutoComplete if broken
3. Check that the repository_root_path (cwd) exists and is readable by the process
4. Read the preceding log.error line ('Failed to start F# language server process: ...') for the root cause and fix that underlying exception

Example fix

// before
$ fsautocomplete --adaptive-lsp-server-enabled   # fails: dotnet runtime missing
// after
$ dotnet --info        # verify runtime
$ dotnet tool update --global fsautocomplete
$ fsautocomplete --adaptive-lsp-server-enabled   # starts OK; then restart Serena

import shutil, subprocess
def fsautocomplete_launchable() -> bool:
    exe = shutil.which("fsautocomplete")
    if not exe:
        return False
    r = subprocess.run([exe, "--help"], capture_output=True, timeout=10)
    return r.returncode == 0

def server_process_can_start(cmd: list[str], cwd: str) -> bool:
    import os, shutil
    return bool(cwd and os.path.isdir(cwd)) and shutil.which(cmd[0]) is not None

try:
    server = LanguageServer.create(LanguageServerId.FSHARP, ...)
except SolidLSPException as e:
    logger.error("F# LS failed to start: %s", e)
    # check underlying cause: dotnet runtime, cwd, binary permissions

When it happens

Trigger: Calling _start_server where self.server.start() throws — fsautocomplete binary not executable/found at launch time, missing dotnet runtime when executing the wrapper script, bad cwd, or an exception raised while spawning the process.

Common situations: FsAutoComplete crashed on startup due to a missing/broken .NET runtime; binary installed but PATH/cwd invalid at runtime; port/resource conflicts or permissions preventing process spawn.

Related errors

• Failed to initialize F# language server for
• Failed to install FsAutoComplete
• Failed to start C# language server
• FsAutoComplete executable not found at
• Language server is missing required capabilities

AI-assisted analysis of oraios/serena@7fcbca7e62 (2026-08-29). Data as JSON: /api/errors/f7d3adbcf615e787. Report an issue: GitHub.

Appendix: source

Thrown at src/solidlsp/language_servers/fsharp_language_server.py:412

            """Handle window/workDoneProgress/create requests from the LSP server."""
            # Just acknowledge the request - we don't need to track progress for now
            return

        # Register custom handlers
        self.server.on_notification("window/logMessage", handle_window_log_message)
        self.server.on_notification("window/showMessage", handle_window_show_message)
        self.server.on_request("workspace/configuration", handle_workspace_configuration)
        self.server.on_request("client/registerCapability", handle_client_register_capability)
        self.server.on_request("client/unregisterCapability", handle_client_unregister_capability)
        self.server.on_request("window/workDoneProgress/create", handle_work_done_progress_create)

        log.info("Starting FsAutoComplete F# language server process")

        try:
            self.server.start()
        except Exception as e:
            log.error(f"Failed to start F# language server process: {e}")
            raise SolidLSPException(f"Failed to start F# language server: {e}")

        # Send initialization
        initialize_params = self._create_initialize_params()

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

        # Complete initialization
        self.server.notify.initialized({})

        log.info("F# language server initialized successfully")

    @override
    def _get_wait_time_for_cross_file_referencing(self) -> float:

View on GitHub (pinned to 7fcbca7e62)