Questions about monaco-lsp-client: migration from monaco-languageclient and roadmap

[Weiry Gonçalves (weirygon)]

Hi,

I have a few questions about the new monaco-lsp-client that was recently added to the monaco-editor project.

Some context: I’ve built a web application that provides a code editor using monaco-editor together with TypeFox/monaco-languageclient, which communicates over WebSocket with my LSP server. This project is currently running, and I’d like to understand how monaco-lsp-client will work so I can plan a possible future migration.

1. How it works: Will monaco-lsp-client work similarly to monaco-languageclient, e.g. connecting to an external language server over WebSocket, but with native Monaco support (without the VS Code API layer)?
2. Migration: For projects already using monaco-languageclient, is there anything we should keep in mind to make a future migration easier? Any differences in supported LSP features?
3. Roadmap: The package is currently in alpha. Is there a roadmap or an estimated timeline for when it will be released as stable and ready for production use?

[Outsider (LeandroCustodio2002)]

I dug through the source a bit since I was curious too. Short answers below, but keep in mind I'm not on the team, so for the roadmap part only a maintainer can really answer.

1. How it works

Yes, it's the same basic idea: a client in the browser talking JSON-RPC to an external language server, but wired straight into Monaco's language APIs, with no VS Code API layer in between. It's not a separate npm package. It ships inside monaco-editor (added in 0.55.0, "native LSP support, see new lsp namespace"), so you get it from monaco.lsp:

import * as monaco from 'monaco-editor';

const transport = await monaco.lsp.WebSocketTransport.connectTo({
  address: 'ws://localhost:3000/lsp',
});
const client = new monaco.lsp.MonacoLspClient(transport);

Besides WebSocketTransport there's also createTransportToWorker and createTransportToIFrame, if you ever want to run the server in a worker.

On the wire, WebSocketTransport sends one JSON-RPC message per text frame, with no Content-Length headers. That's the same framing vscode-ws-jsonrpc uses with monaco-languageclient, so your existing WebSocket ↔ LSP bridge on the server side should most likely keep working as is.

2. Migration: things I noticed in the source that differ

• Feature set is fixed in LspClient.ts: completion, hover, signature help, definition/declaration/type definition/implementation, references, document highlight, document symbols, rename, code actions, code lens, document links, formatting (full/range/on-type), folding, selection range, inlay hints, semantic tokens and diagnostics (push and pull). You don't get the VS Code extension-API-level stuff that monaco-languageclient can emulate (workspace configuration, custom commands in the VS Code sense, etc.).
• It syncs every model. The TextDocumentSynchronizer subscribes to monaco.editor.onDidCreateModel and sends all models to the server, with no document selector. If you have models in several languages, or several servers, you'll need to handle that yourself (e.g. separate clients or keeping models scoped).
• No workspace root. initialize is sent with rootUri: null and processId: null. Servers that need a workspace folder to work properly may behave differently than they do with monaco-languageclient.
• URIs are lowercased when it maps server responses back to models. Worth testing if your server or file paths are case-sensitive.

To make a future migration easier: keep your LSP setup behind one small module of your own (connect + create client + dispose), and don't lean too much on monaco-languageclient's VS Code-specific services.

3. Roadmap

The package README still says "This package is in alpha stage and might contain many bugs". The 0.57.0 changelog adds "typed native LSP client and transport APIs", but I couldn't find a public roadmap or a stable ETA. The last changes in monaco-lsp-client/ are from June. So for now I'd stay on monaco-languageclient in production and experiment with monaco.lsp on the side.