Configure Neovim's Built‑in LSP Client in init.lua Without Plugins

Quick answer

Neovim ships a full LSP client in the vim.lsp namespace. You can start any language server by calling vim.lsp.start_client() with a command table, then attach buffer‑local mappings and settings inside an LspAttach autocmd. No external plugin manager is required.

Minimal worked example: TypeScript / JavaScript

The following init.lua snippet starts typescript-language-server (installed globally via npm i -g typescript-language-server) and provides the most common navigation keys.

-- init.lua
local lsp = vim.lsp
local api = vim.api

-- 1. Define the server command. Adjust the path if the binary lives elsewhere.
local ts_server = {
  name = 'tsserver',
  cmd = { 'typescript-language-server', '--stdio' },
  root_dir = function(fname)
    return lsp.util.root_pattern('package.json', 'tsconfig.json', '.git')(fname) or vim.loop.cwd()
  end,
  init_options = { preferences = { includeCompletionsWithSnippetText = true } },
}

-- 2. Start the client when a matching buffer is opened.
api.nvim_create_autocmd('FileType', {
  pattern = { 'typescript', 'typescriptreact', 'javascript', 'javascriptreact' },
  callback = function()
    lsp.start_client(ts_server)
  end,
})

-- 3. Buffer‑local keymaps and options once a client attaches.
api.nvim_create_autocmd('LspAttach', {
  callback = function(args)
    local bufnr = args.buf
    local client = lsp.get_client_by_id(args.data.client_id)
    if not client then return end

    -- Enable omnifunc for built‑in completion
    vim.bo[bufnr].omnifunc = 'v:lua.vim.lsp.omnifunc'

    local opts = { buffer = bufnr, silent = true }
    vim.keymap.set('n', 'gd', lsp.buf.definition, opts)
    vim.keymap.set('n', 'gr', lsp.buf.references, opts)
    vim.keymap.set('n', 'K', lsp.buf.hover, opts)
    vim.keymap.set('n', 'rn', lsp.buf.rename, opts)
    vim.keymap.set('n', 'ca', lsp.buf.code_action, opts)
    vim.keymap.set('n', '[d', function() vim.diagnostic.goto_prev({ buffer = bufnr }) end, opts)
    vim.keymap.set('n', ']d', function() vim.diagnostic.goto_next({ buffer = bufnr }) end, opts)
  end,
})

How it works

1. Server command table – cmd is the exact argv passed to execvp. The server must speak the Language Server Protocol over stdin/stdout.
2. Root directory detection – root_dir receives the current file name and returns a project root. The example uses lsp.util.root_pattern to look for package.json, tsconfig.json or .git.
3. FileType autocmd – Starts the client only for relevant buffers, avoiding unnecessary processes.
4. LspAttach autocmd – Runs after a client successfully attaches to a buffer. The callback receives args.data.client_id so you can fetch the client object and set buffer‑local options (omnifunc, keymaps, etc.).

Verification steps

Run these commands in Neovim after opening a TypeScript file:

• :echo vim.lsp.get_clients() – should list a client with name = "tsserver" and attached_buffers containing the current buffer number.
• :lua vim.lsp.buf.definition() – cursor jumps to the definition or prints "No definition provider" if the symbol has none.
• :LspLog – opens the LSP log; look for Starting client "tsserver" and any stderr lines that indicate a missing binary or permission error.

Limits of the built‑in client

• No bundled servers – You must install each language server yourself (via npm, cargo, pip, Mason, etc.).
• No automatic configuration – Settings such as init_options, settings, or capabilities are entirely manual.
• API stability – vim.lsp.start_client signature changed between Neovim 0.8 and 0.10. Always check :help vim.lsp for the version you run.
• Single‑threaded Lua – Long‑running callbacks (e.g., a custom on_init that does I/O) block the UI. Keep callbacks short or defer work with vim.schedule.

Common mistakes and how to avoid them

Mistake	Symptom	Fix
Hard‑coding cmd = { "tsserver" } when the binary is named typescript-language-server	Silent attach failure; :LspLog shows spawn: ENOENT	Use the exact executable name or full path; verify with which typescript-language-server in the shell.
Placing LspAttach keymaps outside the autocmd	Keymaps exist on every buffer, even those without a client	Define mappings only inside the LspAttach callback (as shown).
Assuming root_dir returns a non‑nil value	Client starts with cwd as root, causing wrong workspace symbols	Provide a fallback: or vim.loop.cwd().
Using vim.lsp.buf.definition() in a mapping without silent = true	Command echo clutters the command line	Add silent = true to the keymap options table.

Extending the pattern

Repeat the FileType + LspAttach pair for each language. For example, a Rust setup would use cmd = { "rust-analyzer" } and root_pattern("Cargo.toml"). Because the logic lives in plain Lua, you can factor the common parts into a small helper function if you manage many servers.

TL;DR checklist

• Install the language server binary in $PATH.
• Add a FileType autocmd that calls vim.lsp.start_client with a command table.
• Inside LspAttach, set omnifunc and buffer‑local keymaps.
• Verify with :echo vim.lsp.get_clients(), :lua vim.lsp.buf.definition(), and :LspLog.