diff --git a/.gitignore b/.gitignore
index 4e05b95..84d2869 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,6 +1,7 @@
 *.log
 *.pem
 .env
+.lsp-repl-history
 .terraform
 build
 node_modules
diff --git a/Makefile b/Makefile
index 14ef5f7..c40abd3 100644
--- a/Makefile
+++ b/Makefile
@@ -165,6 +165,10 @@ sandbox:
 	@: $${L}
 	L=$(L) node backend/sandbox.js
 
+lsp:
+	@: $${C}
+	node backend/lsp-repl.js $(C)
+
 ### Fetch artifacts from registries
 
 pull-base:
diff --git a/backend/lsp-repl.js b/backend/lsp-repl.js
index c3e5aed..b136110 100644
--- a/backend/lsp-repl.js
+++ b/backend/lsp-repl.js
@@ -5,7 +5,7 @@ import readline from "historic-readline";
 import { quote } from "shell-quote";
 import rpc from "vscode-jsonrpc";
 
-import { langs } from "./langs";
+import { langsPromise } from "./langs.js";
 
 const args = process.argv.slice(2);
 
@@ -23,6 +23,8 @@ if (["-h", "-help", "--help", "help"].includes(args[0])) {
   process.exit(0);
 }
 
+const langs = await langsPromise;
+
 let cmdline;
 if (args.length === 1 && langs[args[0]] && langs[args[0]].lsp) {
   cmdline = ["bash", "-c", langs[args[0]].lsp.start];
diff --git a/package.json b/package.json
index df5f724..0bbd4c6 100644
--- a/package.json
+++ b/package.json
@@ -19,6 +19,7 @@
     "express": "^4.17.1",
     "express-ws": "^4.0.0",
     "file-loader": "^6.2.0",
+    "historic-readline": "^1.0.8",
     "lodash": "^4.17.20",
     "monaco-editor": "0.20.0",
     "monaco-editor-webpack-plugin": "1.9.0",
diff --git a/yarn.lock b/yarn.lock
index defb8f4..4efa403 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -2203,6 +2203,16 @@ from2@^2.1.0:
     inherits "^2.0.1"
     readable-stream "^2.0.0"
 
+fs-extra@^0.24.0:
+  version "0.24.0"
+  resolved "https://registry.yarnpkg.com/fs-extra/-/fs-extra-0.24.0.tgz#d4e4342a96675cb7846633a6099249332b539952"
+  integrity sha1-1OQ0KpZnXLeEZjOmCZJJMytTmVI=
+  dependencies:
+    graceful-fs "^4.1.2"
+    jsonfile "^2.1.0"
+    path-is-absolute "^1.0.0"
+    rimraf "^2.2.8"
+
 fs-write-stream-atomic@^1.0.8:
   version "1.0.10"
   resolved "https://registry.yarnpkg.com/fs-write-stream-atomic/-/fs-write-stream-atomic-1.0.10.tgz#b47df53493ef911df75731e70a9ded0189db40c9"
@@ -2299,7 +2309,7 @@ globals@^11.1.0:
   resolved "https://registry.yarnpkg.com/globals/-/globals-11.12.0.tgz#ab8795338868a0babd8525758018c2a7eb95c42e"
   integrity sha512-WOBp/EEGUiIsJSp7wcv/y6MO+lV9UoncWqxuFfm8eBwzWNgyfBd6Gz+IeKQ9jCmyhoH99g15M3T+QaVHFjizVA==
 
-graceful-fs@^4.1.11, graceful-fs@^4.1.15, graceful-fs@^4.1.2:
+graceful-fs@^4.1.11, graceful-fs@^4.1.15, graceful-fs@^4.1.2, graceful-fs@^4.1.6:
   version "4.2.4"
   resolved "https://registry.yarnpkg.com/graceful-fs/-/graceful-fs-4.2.4.tgz#2256bde14d3632958c465ebc96dc467ca07a29fb"
   integrity sha512-WjKPNJF79dtJAVniUlGGWHYGz2jWxT6VhN/4m1NdkbZ2nOsEF+cI1Edgql5zCRhs/VsQYRvrXctxktVXZUkixw==
@@ -2369,6 +2379,13 @@ hash.js@^1.0.0, hash.js@^1.0.3:
     inherits "^2.0.3"
     minimalistic-assert "^1.0.1"
 
+historic-readline@^1.0.8:
+  version "1.0.8"
+  resolved "https://registry.yarnpkg.com/historic-readline/-/historic-readline-1.0.8.tgz#6f36e770769275113a57ae1a6007001af771cff6"
+  integrity sha1-bzbncHaSdRE6V64aYAcAGvdxz/Y=
+  dependencies:
+    fs-extra "^0.24.0"
+
 hmac-drbg@^1.0.0:
   version "1.0.1"
   resolved "https://registry.yarnpkg.com/hmac-drbg/-/hmac-drbg-1.0.1.tgz#d2745701025a6c775a6c545793ed502fc0c649a1"
@@ -2696,6 +2713,13 @@ json5@^2.1.2:
   dependencies:
     minimist "^1.2.5"
 
+jsonfile@^2.1.0:
+  version "2.4.0"
+  resolved "https://registry.yarnpkg.com/jsonfile/-/jsonfile-2.4.0.tgz#3736a2b428b87bbda0cc83b53fa3d633a35c2ae8"
+  integrity sha1-NzaitCi4e72gzIO1P6PWM6NcKug=
+  optionalDependencies:
+    graceful-fs "^4.1.6"
+
 kind-of@^3.0.2, kind-of@^3.0.3, kind-of@^3.2.0:
   version "3.2.2"
   resolved "https://registry.yarnpkg.com/kind-of/-/kind-of-3.2.2.tgz#31ea21a734bab9bbb0f32466d893aea51e4a3c64"
@@ -3655,7 +3679,7 @@ ret@~0.1.10:
   resolved "https://registry.yarnpkg.com/ret/-/ret-0.1.15.tgz#b8a4825d5bdb1fc3f6f53c2bc33f81388681c7bc"
   integrity sha512-TTlYpa+OL+vMMNG24xSlQGEJ3B/RzEfUlLct7b5G/ytav+wPrplCpVMFuwzXbkecJrb6IYo1iFb0S9v37754mg==
 
-rimraf@^2.5.4, rimraf@^2.6.3:
+rimraf@^2.2.8, rimraf@^2.5.4, rimraf@^2.6.3:
   version "2.7.1"
   resolved "https://registry.yarnpkg.com/rimraf/-/rimraf-2.7.1.tgz#35797f13a7fdadc566142c29d4f07ccad483e3ec"
   integrity sha512-uWjbaKIK3T1OSVptzX7Nl6PvQ3qAGtKEtVRjRuazjfL3Bx5eI409VZSqgND+4UNnmzLVdPj9FqFJNPqBZFve4w==