oraios/serena · error · ValueError

No language server for language {language.value} present; ca

Error message

No language server for language {language.value} present; cannot restart

What it means

restart_language_server only restarts servers already tracked in _language_servers; it never creates a server for an unmanaged language. Requesting a restart for an absent language raises ValueError. This typically surfaces when _ensure_functional_ls detects a dead server it assumed was managed, or user code calls restart for a language never added.

Source

Thrown at src/serena/ls_manager.py:200

    def _create_and_start_language_server(self, ls_id: LanguageServerId) -> SolidLanguageServer:
        if self._language_server_factory is None:
            raise ValueError(f"No language server factory available to create language server for {ls_id}")
        language_server = self._language_server_factory.create_language_server(ls_id)
        language_server.start()
        self._language_servers[ls_id] = language_server
        return language_server

    def restart_language_server(self, language: LanguageServerId) -> SolidLanguageServer:
        """
        Forces recreation and restart of the language server for the given language.
        It is assumed that the language server for the given language is no longer running.

        :param language: the language
        :return: the newly created language server
        """
        if language not in self._language_servers:
            raise ValueError(f"No language server for language {language.value} present; cannot restart")
        return self._create_and_start_language_server(language)

    def add_language_server(self, ls_id: LanguageServerId) -> SolidLanguageServer:
        """
        Dynamically adds a new language server for the given language.

        :param ls_id: the language server to add
        :return: the newly created language server
        """
        if ls_id in self._language_servers:
            raise ValueError(f"Language server for language {ls_id.value} already present")
        return self._create_and_start_language_server(ls_id)

    def remove_language_server(self, language: LanguageServerId, save_cache: bool = False) -> None:
        """
        Removes the language server for the given language, stopping it if it is running.

        :param language: the language

View on GitHub (pinned to 7fcbca7e62)

Solutions

Check language in manager.get_active_language_server_ids() before restarting
Use add_language_server(language) instead if the server was removed and you want it created
Ensure the manager was created with all languages the application will use
Guard against races by serializing remove/restart calls

Example fix

// before
if not ls.is_running():
    ls = manager.restart_language_server(LanguageServerId.TYPESCRIPT)
// after
if LanguageServerId.TYPESCRIPT in manager.get_active_language_server_ids():
    ls = manager.restart_language_server(LanguageServerId.TYPESCRIPT)
else:
    ls = manager.add_language_server(LanguageServerId.TYPESCRIPT)

Defensive patterns

Strategy: validation

Validate before calling

active = manager.get_active_language_server_ids()
if language not in active:
    manager.add_language_server(language)
else:
    manager.restart_language_server(language)

Try / catch

try:
    ls = manager.restart_language_server(language)
except ValueError as e:
    if "cannot restart" in str(e):
        ls = manager.add_language_server(language)
    else:
        raise

Prevention

Check get_active_language_server_ids() before restarting
Use add_language_server for languages not yet managed
Serialize remove/restart calls to avoid state races
Keep the configured language list consistent with runtime usage

When it happens

Trigger: Calling restart_language_server(LanguageServerId.TYPESCRIPT) when only PYTHON is managed; _ensure_functional_ls attempting a restart after the server entry was removed; manager built with fewer languages than callers assume.

Common situations: Language server crashed and was removed via remove_language_server; tooling hardcodes a language list differing from project config; race where another thread removed the server before restart.

Understand the failure class

Background: 'Could not be found', 'does not exist', 'not found in database': the resource-not-found family when an ID, slug, key, or URI lookup comes back empty — this error's family across 20 libraries.

Related errors

AI-assisted analysis of oraios/serena@7fcbca7e62 (2026-08-29). Data as JSON: /api/errors/cf98c00bda92ec23. Report an issue: GitHub.