oraios/serena · error · SvelteCompanionPreparationError

Failed to open {len(failed_svelte_files)} Svelte file(s) on

Error message

Failed to open {len(failed_svelte_files)} Svelte file(s) on companion TypeScript server: {listing}

What it means

During startup the Svelte language server spawns a companion TypeScript server and opens every `.svelte` file on it so typescript-svelte-plugin adds them to the TS program. If any open_file call throws, the files are collected and this SvelteCompanionPreparationError is raised, listing up to _MAX_FAILED_FILES_IN_ERROR sorted paths (plus 'and N more'). The original first open error is attached as __cause__.

Source

Thrown at src/solidlsp/language_servers/svelte_language_server.py:383

        failed_svelte_files = []
        first_open_error: Exception | None = None
        for svelte_file in svelte_files:
            try:
                with self._ts_server.open_file(svelte_file) as file_buffer:
                    file_buffer.ref_count += 1
                    self._indexed_svelte_file_uris.append(file_buffer.uri)
            except Exception as exc:
                log.debug("Failed to open %s on companion TS server: %s", svelte_file, exc)
                if first_open_error is None:
                    first_open_error = exc
                failed_svelte_files.append(svelte_file)

        if failed_svelte_files:
            shown_files = sorted(failed_svelte_files)[:_MAX_FAILED_FILES_IN_ERROR]
            remainder = len(failed_svelte_files) - len(shown_files)
            listing = ", ".join(shown_files) + (f" and {remainder} more" if remainder else "")
            raise SvelteCompanionPreparationError(
                f"Failed to open {len(failed_svelte_files)} Svelte file(s) on companion TypeScript server: {listing}"
            ) from first_open_error

        self._svelte_files_indexed = True
        log.info("Svelte file indexing complete; waiting for companion TS server to finish processing")

        timeout = self._get_companion_indexing_timeout()
        if self._ts_server._wait_for_indexing_start_or_completion(timeout=timeout):
            log.info("Companion TypeScript server finished indexing .svelte files")
        else:
            raise TimeoutError(
                f"Companion TypeScript server did not finish indexing {len(svelte_files)} .svelte files within {timeout:.0f}s "
                f"({self._ts_server.describe_indexing_state()})"
            )

    def _cleanup_indexed_svelte_files(self) -> None:
        """Decrement ref-counts for all .svelte files opened during indexing."""
        if not self._indexed_svelte_file_uris or self._ts_server is None:

View on GitHub (pinned to 7fcbca7e62)

Solutions

Read the chained `__cause__` (first_open_error) in logs — it carries the real failure from the first file that could not be opened.
Verify the companion TypeScript server is healthy: check that typescript-language-server and typescript were installed in the svelte-lsp-<version> dir and that node is on PATH.
Reduce repo scope or move huge/generated .svelte trees (e.g. inside build output) out of the project; note node_modules and dot-dirs are already skipped.
Check file permissions/readability of the listed .svelte files and that paths are valid on the current OS.
Restart the language server session after fixing the underlying cause; the failure is collected at startup only.

Example fix

// before: unreadable files cause open_file to throw
chmod 000 src/App.svelte
// after
chmod 644 src/App.svelte

Defensive patterns

Strategy: try-catch

Validate before calling

from pathlib import Path
svelte_files = [p for p in Path(repo).rglob("*.svelte")
                if "node_modules" not in p.parts and os.access(p, os.R_OK)]
if not svelte_files:
    print("nothing to index")  # or investigate unreadable files before starting

Type guard

def can_open_svelte_files(repo: str) -> bool:
    return all(os.access(p, os.R_OK)
               for p in Path(repo).rglob("*.svelte")
               if "node_modules" not in p.parts)

Try / catch

try:
    server.start()
except SvelteCompanionPreparationError as e:
    log.error("companion open failed: %s", e.__cause__)
    # check tsserver process health, fix listed files, then restart
    raise

Prevention

Read the __cause__ of SvelteCompanionPreparationError — it names the first failing file and reason
Keep .svelte files readable and avoid exotic characters in paths
Ensure node and the installed typescript-language-server binary are functional before startup
Exclude generated/build .svelte output from the repo
Monitor companion tsserver logs for crashes during startup

When it happens

Trigger: Starting the Svelte LS (which calls _start_typescript_server -> _ensure_svelte_files_indexed_on_ts_server) when `self._ts_server.open_file(svelte_file)` raises for one or more .svelte files — e.g. the companion tsserver process died, an LSP didOpen/didChange request timed out or returned an error, or the file buffer/URI handling failed.

Common situations: Companion TypeScript server crashed at startup (bad node, missing typescript-language-server binary) so every open fails; repo with unreadable or locked .svelte files; invalid characters in paths that break URI construction; tsserver request timeout under a huge repo; earlier companion preparation failures leaving the server in a bad state.

Related errors

AI-assisted analysis of oraios/serena@7fcbca7e62 (2026-08-29). Data as JSON: /api/errors/56aebcd2dce6ad6f. Report an issue: GitHub.