oraios/serena · error · RuntimeError

Failed to read nixd configuration file

Error message

Failed to read nixd configuration file '{config_path}': {exc}

What it means

This RuntimeError is raised when the nixd language-server configuration file referenced by the 'config_path' setting cannot be opened or read by the OS. The library reads the JSON config at startup (_load_nixd_settings) and wraps any OSError (missing file, permission denied, is-a-directory) into this error with the original OS message included. It is thrown from the nixd language server constructor, so server startup fails.

Solutions

Verify the path in the nixd 'config_path' setting exists and is a regular readable file (ls -l <path>).
Fix the path in settings to the correct absolute path of the nixd JSON config.
Fix file permissions (chmod/chown) so the process user can read the file.
If no custom config is needed, remove 'config_path' so built-in defaults are used.

Example fix

// before
config_path: "/home/user/.config/nixd/nixd.jscon"  // typo, file missing
// after
config_path: "/home/user/.config/nixd/nixd.json"

Defensive patterns

Strategy: validation

Validate before calling

import json, os

def validate_nixd_config_path(config_path: str) -> None:
    if not os.path.isabs(config_path):
        raise ValueError(f"config_path must be absolute: {config_path}")
    if not os.path.isfile(config_path):
        raise FileNotFoundError(f"nixd config not a readable file: {config_path}")
    if not os.access(config_path, os.R_OK):
        raise PermissionError(f"no read permission: {config_path}")
    with open(config_path, "r", encoding="utf-8") as f:
        json.load(f)  # also pre-checks JSON validity

Try / catch

try:
    server = NixdLanguageServer(config_path=cfg["nixd"]["config_path"], **common)
except RuntimeError as e:
    if "Failed to read nixd configuration file" in str(e):
        logger.error("Falling back to default nixd settings: %s", e)
        server = NixdLanguageServer(**common)  # omit config_path
    else:
        raise

Prevention

Keep the nixd config path in one constant/setting and verify it once at app startup.
Always store absolute paths; never rely on the process cwd.
Add the config file to deployment/packaging so it ships with the environment.
Check os.access(path, os.R_OK) after permission changes or user switches.

When it happens

Trigger: Calling the nixd language server __init__ with a 'config_path' setting pointing to a file that does not exist, has bad permissions, or is a directory; json.load then raises OSError (e.g. FileNotFoundError, PermissionError, IsADirectoryError) which is re-raised as RuntimeError.

Common situations: Typo in the config path in user settings; config file deleted or moved after settings were written; running the server as a different user without read permission; passing a directory path instead of a file path; relative path used while cwd differs from expectation (though validation usually requires absolute).

Related errors

AI-assisted analysis of oraios/serena@7fcbca7e62 (2026-08-29). Data as JSON: /api/errors/9e4f933944a701a2. Report an issue: GitHub.

Appendix: source

Thrown at src/solidlsp/language_servers/nixd_ls.py:263

        config_path_value = custom_settings.get("config_path")
        if config_path_value is None:
            return cls._create_default_nixd_settings()
        if not isinstance(config_path_value, str) or not config_path_value.strip():
            raise ValueError("ls_specific_settings.nix.config_path must be a non-empty absolute path")

        config_path = Path(config_path_value).expanduser()
        if not config_path.is_absolute():
            raise ValueError(f"ls_specific_settings.nix.config_path must be absolute: {config_path_value!r}")

        try:
            with config_path.open(encoding="utf-8") as config_file:
                settings = json.load(config_file)
        except json.JSONDecodeError as exc:
            raise ValueError(
                f"Invalid JSON in nixd configuration file '{config_path}': {exc.msg} (line {exc.lineno}, column {exc.colno})"
            ) from exc
        except OSError as exc:
            raise RuntimeError(f"Failed to read nixd configuration file '{config_path}': {exc}") from exc

        if not isinstance(settings, dict):
            raise ValueError(
                f"Invalid nixd configuration file '{config_path}': expected a JSON object containing the value of the 'nixd' section"
            )
        return settings

    @staticmethod
    def _resolve_nixd_configuration_section(settings: dict[str, Any], section: object) -> Any:
        """Resolve a ``nixd`` configuration section from the effective settings."""
        if section == "nixd":
            return deepcopy(settings)
        if not isinstance(section, str) or not section.startswith("nixd."):
            return {}

        value: Any = settings
        for key in section.removeprefix("nixd.").split("."):
            if not key or not isinstance(value, dict) or key not in value:

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