diff --git a/package.json b/package.json
index 7085f87..1a1b2f5 100644
--- a/package.json
+++ b/package.json
@@ -40,6 +40,7 @@
   },
   "dependencies": {
     "@types/deep-equal": "^1.0.1",
+    "async-mutex": "^0.3.0",
     "deep-equal": "^2.0.3",
     "deepmerge": "^4.2.2",
     "path": "^0.12.7",
diff --git a/src/extension.ts b/src/extension.ts
index 7eeb272..0e78922 100644
--- a/src/extension.ts
+++ b/src/extension.ts
@@ -5,6 +5,7 @@ import { spawnSync } from 'child_process';
 import deepEqual from 'deep-equal';
 import WebRequest from 'web-request';
 import deepmerge from 'deepmerge';
+import { Mutex } from 'async-mutex';
 import vscode, { ExtensionContext } from 'vscode';
 import { LanguageClient, CloseAction, ErrorAction, InitializeError, Message, RevealOutputChannelOn } from 'vscode-languageclient';
 
@@ -54,10 +55,21 @@ let languageClient: LanguageClient | undefined;
 let languageServerDisposable: vscode.Disposable | undefined;
 let latestConfig: LanguageServerConfig | undefined;
 let crashCount = 0;
+const languageServerStartMutex = new Mutex();
+export let languageServerIsRunning = false; // TODO: use later for `start`, `stop`, and `restart` language server.
 
 export function activate(context: ExtensionContext) {
     context.subscriptions.push(
-        vscode.commands.registerCommand('arduino.languageserver.start', (config: LanguageServerConfig) => startLanguageServer(context, config)),
+        vscode.commands.registerCommand('arduino.languageserver.start', async (config: LanguageServerConfig) => {
+            const unlock = await languageServerStartMutex.acquire();
+            try {
+                const started = await startLanguageServer(context, config);
+                languageServerIsRunning = started;
+                return languageServerIsRunning ? config.board.fqbn : undefined;
+            } finally {
+                unlock();
+            }
+        }),
         vscode.commands.registerCommand('arduino.debug.start', (config: DebugConfig) => startDebug(context, config))
     );
 }
@@ -129,7 +141,7 @@ async function startDebug(_: ExtensionContext, config: DebugConfig): Promise<boo
     return vscode.debug.startDebugging(undefined, mergedDebugConfig);
 }
 
-async function startLanguageServer(context: ExtensionContext, config: LanguageServerConfig): Promise<void> {
+async function startLanguageServer(context: ExtensionContext, config: LanguageServerConfig): Promise<boolean> {
     if (languageClient) {
         if (languageClient.diagnostics) {
             languageClient.diagnostics.clear();
@@ -147,6 +159,8 @@ async function startLanguageServer(context: ExtensionContext, config: LanguageSe
 
     languageServerDisposable = languageClient.start();
     context.subscriptions.push(languageServerDisposable);
+    await languageClient.onReady();
+    return true;
 }
 
 async function buildLanguageClient(config: LanguageServerConfig): Promise<LanguageClient> {
diff --git a/yarn.lock b/yarn.lock
index 6dd931e..79f87b4 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -382,6 +382,13 @@ async-each@^1.0.1:
   resolved "https://registry.yarnpkg.com/async-each/-/async-each-1.0.3.tgz#b727dbf87d7651602f06f4d4ac387f47d91b0cbf"
   integrity sha512-z/WhQ5FPySLdvREByI2vZiTWwCnF0moMJ1hK9YQwDTHKh6I7/uSckMetoRGb5UBZPC1z0jlw+n/XCgjeH7y1AQ==
 
+async-mutex@^0.3.0:
+  version "0.3.0"
+  resolved "https://registry.yarnpkg.com/async-mutex/-/async-mutex-0.3.0.tgz#5bc765c271dfc05c48040c12032a92f1dbef2dc3"
+  integrity sha512-6VIpUM7s37EMXvnO3TvujgaS6gx4yJby13BhxovMYSap7nrbS0gJ1UzGcjD+HElNSdTz/+IlAIqj7H48N0ZlyQ==
+  dependencies:
+    tslib "^2.1.0"
+
 asynckit@^0.4.0:
   version "0.4.0"
   resolved "https://registry.yarnpkg.com/asynckit/-/asynckit-0.4.0.tgz#c79ed97f7f34cb8f2ba1bc9790bcc366474b4b79"
@@ -3763,6 +3770,11 @@ tslib@^1.9.0:
   resolved "https://registry.yarnpkg.com/tslib/-/tslib-1.13.0.tgz#c881e13cc7015894ed914862d276436fa9a47043"
   integrity sha512-i/6DQjL8Xf3be4K/E6Wgpekn5Qasl1usyw++dAA35Ue5orEn65VIxOA+YvNNl9HV3qv70T7CNwjODHZrLwvd1Q==
 
+tslib@^2.1.0:
+  version "2.1.0"
+  resolved "https://registry.yarnpkg.com/tslib/-/tslib-2.1.0.tgz#da60860f1c2ecaa5703ab7d39bc05b6bf988b97a"
+  integrity sha512-hcVC3wYEziELGGmEEXue7D75zbwIIVUMWAVbHItGPx0ziyXxrOMQx4rQEVEV45Ut/1IotuEvwqPopzIOkDMf0A==
+
 tty-browserify@0.0.0:
   version "0.0.0"
   resolved "https://registry.yarnpkg.com/tty-browserify/-/tty-browserify-0.0.0.tgz#a157ba402da24e9bf957f9aa69d524eed42901a6"