Language Server Protocol (LSP) Explained
The Language Server Protocol is the reason VS Code can support every language equally well. Understanding LSP helps you choose the right extensions and diagnose editor issues.
1What is LSP?
LSP is an open protocol (created by Microsoft) that decouples the editor from the language tooling. A Language Server runs as a separate process and communicates with VS Code over JSON-RPC, providing completions, diagnostics, hover info, go-to-definition, and more.
2How VS Code Uses LSP
When you open a .py file, VS Code starts the Pylance language server. Pylance analyses the file and sends diagnostics back. When you press Ctrl+Space, VS Code asks the server for completions. The server responds with a list and VS Code renders it — all in milliseconds.
3Common Language Servers
- Pylance / pyright — Python
- typescript-language-server — TypeScript / JavaScript
- rust-analyzer — Rust
- gopls — Go
- clangd — C / C++
- jdtls — Java
- solargraph — Ruby
4Diagnosing LSP Issues
If IntelliSense isn't working: open Output panel → pick the language server from the dropdown to see its log. Common issues: missing interpreter, wrong config file, or a crash in the server process. Restart the language server with Ctrl+Shift+P → Restart Language Server.
5Writing a Simple Language Server
Microsoft provides the vscode-languageserver-node npm package to write a language server in TypeScript. A minimal server can provide completions, hover, and diagnostics in under 100 lines — the starting point for any custom language or DSL extension.