fix(lsp): improve error messages when LSP server is not installed (#305)

Previously, when an LSP server was configured but not installed, the error
message said "No LSP server configured" which was misleading. Now the
error message distinguishes between:

1. Server not configured at all
2. Server configured but not installed (with installation hints)

The new error messages include:
- Clear indication of whether server is configured vs installed
- Installation commands for each built-in server
- Supported file extensions
- Configuration examples for custom servers

Fixes #304

Co-authored-by: sisyphus-dev-ai <sisyphus-dev-ai@users.noreply.github.com>

src/tools/lsp/client.ts

@@ -1,9 +1,8 @@
 import { spawn, type Subprocess } from "bun"
 import { readFileSync } from "fs"
 import { extname, resolve } from "path"
-import type { ResolvedServer } from "./config"
 import { getLanguageId } from "./config"
-import type { Diagnostic } from "./types"
+import type { Diagnostic, ResolvedServer } from "./types"
 
 interface ManagedClient {
   client: LSPClient

src/tools/lsp/config.ts

@@ -1,16 +1,8 @@
 import { existsSync, readFileSync } from "fs"
 import { join } from "path"
 import { homedir } from "os"
-import { BUILTIN_SERVERS, EXT_TO_LANG } from "./constants"
-
-export interface ResolvedServer {
-  id: string
-  command: string[]
-  extensions: string[]
-  priority: number
-  env?: Record<string, string>
-  initialization?: Record<string, unknown>
-}
+import { BUILTIN_SERVERS, EXT_TO_LANG, LSP_INSTALL_HINTS } from "./constants"
+import type { ResolvedServer, ServerLookupResult } from "./types"
 
 interface LspEntry {
   disabled?: boolean
@@ -120,23 +112,47 @@ function getMergedServers(): ServerWithSource[] {
   })
 }
 
-export function findServerForExtension(ext: string): ResolvedServer | null {
+export function findServerForExtension(ext: string): ServerLookupResult {
   const servers = getMergedServers()
 
   for (const server of servers) {
     if (server.extensions.includes(ext) && isServerInstalled(server.command)) {
       return {
-        id: server.id,
-        command: server.command,
-        extensions: server.extensions,
-        priority: server.priority,
-        env: server.env,
-        initialization: server.initialization,
+        status: "found",
+        server: {
+          id: server.id,
+          command: server.command,
+          extensions: server.extensions,
+          priority: server.priority,
+          env: server.env,
+          initialization: server.initialization,
+        },
       }
     }
   }
 
-  return null
+  for (const server of servers) {
+    if (server.extensions.includes(ext)) {
+      const installHint =
+        LSP_INSTALL_HINTS[server.id] || `Install '${server.command[0]}' and ensure it's in your PATH`
+      return {
+        status: "not_installed",
+        server: {
+          id: server.id,
+          command: server.command,
+          extensions: server.extensions,
+        },
+        installHint,
+      }
+    }
+  }
+
+  const availableServers = [...new Set(servers.map((s) => s.id))]
+  return {
+    status: "not_configured",
+    extension: ext,
+    availableServers,
+  }
 }
 
 export function getLanguageId(ext: string): string {

src/tools/lsp/constants.ts

@@ -40,6 +40,37 @@ export const DEFAULT_MAX_REFERENCES = 200
 export const DEFAULT_MAX_SYMBOLS = 200
 export const DEFAULT_MAX_DIAGNOSTICS = 200
 
+export const LSP_INSTALL_HINTS: Record<string, string> = {
+  typescript: "npm install -g typescript-language-server typescript",
+  deno: "Install Deno from https://deno.land",
+  vue: "npm install -g @vue/language-server",
+  eslint: "npm install -g vscode-langservers-extracted",
+  oxlint: "npm install -g oxlint",
+  biome: "npm install -g @biomejs/biome",
+  gopls: "go install golang.org/x/tools/gopls@latest",
+  "ruby-lsp": "gem install ruby-lsp",
+  basedpyright: "pip install basedpyright",
+  pyright: "pip install pyright",
+  ty: "pip install ty",
+  ruff: "pip install ruff",
+  "elixir-ls": "See https://github.com/elixir-lsp/elixir-ls",
+  zls: "See https://github.com/zigtools/zls",
+  csharp: "dotnet tool install -g csharp-ls",
+  fsharp: "dotnet tool install -g fsautocomplete",
+  "sourcekit-lsp": "Included with Xcode or Swift toolchain",
+  rust: "rustup component add rust-analyzer",
+  clangd: "See https://clangd.llvm.org/installation",
+  svelte: "npm install -g svelte-language-server",
+  astro: "npm install -g @astrojs/language-server",
+  "bash-ls": "npm install -g bash-language-server",
+  jdtls: "See https://github.com/eclipse-jdtls/eclipse.jdt.ls",
+  "yaml-ls": "npm install -g yaml-language-server",
+  "lua-ls": "See https://github.com/LuaLS/lua-language-server",
+  php: "npm install -g intelephense",
+  dart: "Included with Dart SDK",
+  "terraform-ls": "See https://github.com/hashicorp/terraform-ls",
+}
+
 // Synced with OpenCode's server.ts
 // https://github.com/sst/opencode/blob/main/packages/opencode/src/lsp/server.ts
 export const BUILTIN_SERVERS: Record<string, Omit<LSPServerConfig, "id">> = {

src/tools/lsp/types.ts

@@ -135,3 +135,23 @@ export interface CodeAction {
   command?: Command
   data?: unknown
 }
+
+export interface ServerLookupInfo {
+  id: string
+  command: string[]
+  extensions: string[]
+}
+
+export type ServerLookupResult =
+  | { status: "found"; server: ResolvedServer }
+  | { status: "not_configured"; extension: string; availableServers: string[] }
+  | { status: "not_installed"; server: ServerLookupInfo; installHint: string }
+
+export interface ResolvedServer {
+  id: string
+  command: string[]
+  extensions: string[]
+  priority: number
+  env?: Record<string, string>
+  initialization?: Record<string, unknown>
+}

src/tools/lsp/utils.ts

@@ -17,6 +17,7 @@ import type {
   TextEdit,
   CodeAction,
   Command,
+  ServerLookupResult,
 } from "./types"
 
 export function findWorkspaceRoot(filePath: string): string {
@@ -40,15 +41,51 @@ export function findWorkspaceRoot(filePath: string): string {
   return require("path").dirname(resolve(filePath))
 }
 
+export function formatServerLookupError(result: Exclude<ServerLookupResult, { status: "found" }>): string {
+  if (result.status === "not_installed") {
+    const { server, installHint } = result
+    return [
+      `LSP server '${server.id}' is configured but NOT INSTALLED.`,
+      ``,
+      `Command not found: ${server.command[0]}`,
+      ``,
+      `To install:`,
+      `  ${installHint}`,
+      ``,
+      `Supported extensions: ${server.extensions.join(", ")}`,
+      ``,
+      `After installation, the server will be available automatically.`,
+      `Run 'lsp_servers' tool to verify installation status.`,
+    ].join("\n")
+  }
+
+  return [
+    `No LSP server configured for extension: ${result.extension}`,
+    ``,
+    `Available servers: ${result.availableServers.slice(0, 10).join(", ")}${result.availableServers.length > 10 ? "..." : ""}`,
+    ``,
+    `To add a custom server, configure 'lsp' in oh-my-opencode.json:`,
+    `  {`,
+    `    "lsp": {`,
+    `      "my-server": {`,
+    `        "command": ["my-lsp", "--stdio"],`,
+    `        "extensions": ["${result.extension}"]`,
+    `      }`,
+    `    }`,
+    `  }`,
+  ].join("\n")
+}
+
 export async function withLspClient<T>(filePath: string, fn: (client: LSPClient) => Promise<T>): Promise<T> {
   const absPath = resolve(filePath)
   const ext = extname(absPath)
-  const server = findServerForExtension(ext)
+  const result = findServerForExtension(ext)
 
-  if (!server) {
-    throw new Error(`No LSP server configured for extension: ${ext}`)
+  if (result.status !== "found") {
+    throw new Error(formatServerLookupError(result))
   }
 
+  const server = result.server
   const root = findWorkspaceRoot(absPath)
   const client = await lspManager.getClient(root, server)