oraios/serena · error · RuntimeError

Failed to verify nixd installation: {exc}

Error message

Failed to verify nixd installation: {exc}

What it means

Any exception raised while verifying nixd (timeout of the 5-second version check, OS error spawning the process, or the RuntimeError from a non-zero exit) is caught and re-raised as RuntimeError("Failed to verify nixd installation: ...") with the original exception chained.

Source

Thrown at src/solidlsp/language_servers/nixd_ls.py:138

                log.info("nixd not found. Attempting to install...")
                nixd_path = self._install_nixd_with_nix()

            if not nixd_path:
                raise RuntimeError(
                    "nixd (Nix Language Server) is not installed.\n"
                    "Please install nixd using one of the following methods:\n"
                    "  - Using Nix flakes: nix profile install github:nix-community/nixd\n"
                    "  - From nixpkgs: nix-env -iA nixpkgs.nixd\n"
                    "  - On macOS with Homebrew: brew install nixd\n\n"
                    "After installation, make sure 'nixd' is in your PATH."
                )

            try:
                result = subprocess_run([nixd_path, "--version"], capture_output=True, text=True, check=False, timeout=5)
                if result.returncode != 0:
                    raise RuntimeError(f"nixd failed to run: {result.stderr}")
            except Exception as exc:
                raise RuntimeError(f"Failed to verify nixd installation: {exc}") from exc

            return nixd_path

        def _create_launch_command(self, core_path: str) -> list[str]:
            """Return the nixd stdio launch command."""
            return [core_path]

    def _extend_nix_symbol_range_to_include_semicolon(
        self, symbol: ls_types.UnifiedSymbolInformation, file_content: str
    ) -> ls_types.UnifiedSymbolInformation:
        """
        Extend symbol range to include trailing semicolon for Nix attribute symbols.

        nixd provides ranges that exclude semicolons (expression-level), but serena needs
        statement-level ranges that include semicolons for proper replacement.
        """
        range_info = symbol["range"]
        end_line = range_info["end"]["line"]

View on GitHub (pinned to 7fcbca7e62)

Solutions

  1. Pre-build nixd before starting the server (run `nixd --version` once manually) so no network/build happens under the 5s timeout.
  2. chmod +x the nixd binary if it lacks the execute bit.
  3. Reinstall nixd via nix profile install or brew install if the binary is broken.

Example fix

// before
# first run triggers nix build, exceeding 5s timeout
// after
nix profile install github:nix-community/nixd && nixd --version  # warm up before launching
Defensive patterns

Strategy: retry

Validate before calling

import subprocess
try:
    subprocess.run(["nixd", "--version"], timeout=10, capture_output=True, check=True)
except subprocess.TimeoutExpired:
    subprocess.run(["nix", "build", "github:nix-community/nixd"], check=True)  # warm store first

Try / catch

try:
    server = NixdLanguageServer(...)
except RuntimeError as e:
    if str(e).startswith("Failed to verify nixd installation"):
        subprocess.run(["nix", "profile", "install", "github:nix-community/nixd"], check=True)  # retry once
        server = NixdLanguageServer(...)
    else:
        raise

Prevention

When it happens

Trigger: `nixd --version` hangs longer than 5 seconds; the nixd path is not executable (PermissionError); the binary is missing after a race; subprocess spawn fails.

Common situations: Slow first execution while Nix builds/realizes the store path (timeout); nixd installed without execute permission; network-sandboxed CI where nix tries to fetch during the version check.

Related errors

AI-assisted analysis of oraios/serena@7fcbca7e62 (2026-08-29). Data as JSON: /api/errors/6ac640d919e77624. Report an issue: GitHub.