Ship the Monaco editor as a builtin plugin

apps/server/scripts/copy-builtin-plugins.ts

@@ -96,6 +96,18 @@ async function writeRuntimePackageJson(args: {
   );
 }
 
+/**
+ * Runs `<pluginRoot>/scripts/stage-assets.mjs` when a plugin has one. The
+ * script's side effects are its contract: it populates `dist/` with runtime
+ * files the bundlers cannot produce (the Monaco plugin copies Monaco's AMD
+ * build in this way).
+ */
+async function runStageAssets(sourceRoot: string): Promise<void> {
+  const scriptPath = path.join(sourceRoot, "scripts", "stage-assets.mjs");
+  if (!(await exists(scriptPath))) return;
+  await import(pathToFileURL(scriptPath).href);
+}
+
 async function copyBuiltinPlugin(args: {
   bbVersion: string;
   build: boolean;
@@ -120,6 +132,10 @@ async function copyBuiltinPlugin(args: {
     if (packageJson.bb.host !== undefined) {
       await buildPluginHost(args.sourceRoot, args.bbVersion, toolchain);
     }
+    // A plugin that needs files on disk at runtime (rather than bundled into
+    // its server/app) stages them into `dist/` here, because `RUNTIME_DIRS`
+    // below is all that ships. Optional: most plugins have no such script.
+    await runStageAssets(args.sourceRoot);
   }
 
   const targetDir = path.join(args.targetRoot, args.name);

apps/server/src/services/plugins/builtin-registry.ts

@@ -78,6 +78,12 @@ export const BUILTIN_PLUGINS = [
     defaultEnabled: true,
     category: "Interface",
   },
+  {
+    name: "monaco-editor",
+    pluginId: "monaco-editor",
+    defaultEnabled: true,
+    category: "Interface",
+  },
   {
     name: "pdf-preview",
     pluginId: "pdf-preview",

apps/server/test/services/plugins/builtin-plugins.test.ts

@@ -229,6 +229,7 @@ describe("builtin plugin reconciliation", () => {
       ["plugin-api-tester", "Beaker"],
       ["inline-vis", "AppWindow"],
       ["keep-awake", "Coffee"],
+      ["monaco-editor", "Code"],
       ["pdf-preview", "FileText"],
       ["provider-acp", "./icons/acp.svg"],
       ["provider-claude-code", "./icons/claude-code.svg"],

apps/server/test/services/plugins/official-plugins.test.ts

@@ -100,6 +100,7 @@ describe("official plugin registry invariants", () => {
       "inline-vis": "Interface",
       "keep-awake": "Host access",
       memory: "Context & knowledge",
+      "monaco-editor": "Interface",
       "pdf-preview": "Interface",
       "provider-acp": "Agent interaction",
       "provider-claude-code": "Agent interaction",

packages/bb-app/scripts/smoke-tarball.mjs

@@ -33,6 +33,7 @@ const EXPECTED_RUNNING_BUILTIN_PLUGINS = [
   "custom-instructions",
   "inline-vis",
   "keep-awake",
+  "monaco-editor",
   "pdf-preview",
   "provider-retry",
   "secrets",

plugins/monaco-editor/README.md

@@ -0,0 +1,95 @@
+# bb-plugin-monaco-editor
+
+Opens files in BB using [Monaco](https://microsoft.github.io/monaco-editor/),
+the editor from VS Code, instead of BB's read-only file preview.
+
+It applies everywhere BB opens a file: links clicked in chat, the secondary
+panel's file search, and `bb thread open`.
+
+## Features
+
+- **Edit and save.** <kbd>⌘S</kbd> writes the file. If it changed on disk
+  since you opened it — often because the agent edited it — the save stops and
+  offers Reload or Overwrite rather than clobbering the change.
+- **Find in file** with <kbd>⌘F</kbd>, plus Monaco's usual editing: multiple
+  cursors, block selection, bracket matching, code folding.
+- **Syntax highlighting** for ~86 common file types.
+- **File tree.** Toggle it from the file bar to browse the project, filter by
+  path, expand and collapse directories, and jump to another file. It opens
+  with the current file revealed. Right-click any row to copy its absolute
+  path, relative path, or filename.
+- **Quick palette commands.** Open the quick palette (<kbd>⌘⇧P</kbd>) and
+  type "fold", "sort", or "copy" to reach *fold level 1–5*, *fold
+  recursively*, *unfold all*, *unfold recursively*, *unfold at cursor*,
+  *sort selected lines ascending/descending*, and *copy the path / relative
+  path of the current file*. The sort rows appear only with a multi-line
+  selection, and every row acts on the Monaco tab you last worked in.
+- **Follows your theme,** including light/dark switches and custom palettes.
+
+## Development
+
+Ships with BB as a builtin; there is nothing to install.
+
+```
+pnpm exec turbo run typecheck test --filter=bb-plugin-monaco-editor
+```
+
+`scripts/stage-assets.mjs` builds the Monaco bundle the editor loads, into
+`dist/monaco`. Packaging runs it (`apps/server/scripts/copy-builtin-plugins.ts`),
+since only a builtin's `dist/` ships. A source checkout never runs that path —
+the dev server loads builtins straight from `plugins/<name>` — so the plugin
+builds the bundle itself when it is missing or older than `monaco-bundle/`,
+which makes that one file open a few seconds slow.
+`pnpm --filter bb-plugin-monaco-editor build:monaco` does it up front.
+
+The dev loop already rebuilds `dist/app.js` and reloads `server.ts` on save,
+so editing `app.tsx`, `components/`, `lib/`, or `server.ts` needs nothing
+extra. It knows nothing about the Monaco bundle, which is why the staleness
+check exists: edit `monaco-bundle/` and the next file open rebuilds.
+
+Monaco is built rather than bundled into `app.js` because `bb plugin build`
+emits one file with no code splitting: Monaco would parse at app boot for
+everyone, including users who never open a file, and its worker could not be
+emitted at all. `lib/monaco-loader.ts` loads the built files from a
+`files.createPreview` URL the first time a file tab opens.
+
+`monaco-bundle/editor.js` is the entry: Monaco's own `editor.main`, which is
+the API plus its contribution modules (find, folding, word navigation,
+sorting, …) and every Monarch grammar. What it leaves out is the language
+*services* for CSS, HTML, JSON, and TypeScript — completion and type checking
+this plugin has no use for. esbuild proves what is reachable, so the result is
+4.6 MB rather than the 24 MB of Monaco's prebuilt tree.
+
+Do not trim that entry to `editor.api` to save the difference. The API without
+the contributions still opens files and still types, so the editor looks fine
+while find, word navigation, and folding silently do not exist. The build
+script asserts each of those is present for that reason.
+
+## Which files it opens
+
+The plugin claims the extensions listed in `lib/languages.ts` — common code,
+config, and text formats. Binaries like `png` and `pdf` are left to BB's own
+preview, which renders them properly.
+
+To change any file type back, use **Settings → File openers**, which offers
+Automatic, BB's built-in preview, or Monaco per extension. Right-clicking a
+file link also offers a one-off "Open with…".
+
+## Roadmap
+
+- **Language intelligence.** There is no language server, so no
+  go-to-definition, find-references, or type checking. Monaco ships a
+  TypeScript checker, but it can only see the one open file, so every import
+  looks unresolved — it is switched off rather than showing errors that are
+  wrong.
+- **File operations.** The tree is read-only; renaming, creating, and
+  deleting files are not implemented yet.
+- **Hidden files and `node_modules`** never appear in the tree. BB's path
+  listing excludes them and offers no way to ask for them
+  ([#2093](https://github.com/get-bb/bb/issues/2093)).
+- **Opening a file from the tree reuses the current tab,** so the tab title
+  keeps naming the file it was opened with. A plugin cannot ask BB to open a
+  file or retitle its tab ([#2102](https://github.com/get-bb/bb/issues/2102)).
+- **No "open in editor" button** like BB's preview has; that capability is not
+  available to plugins.
+- **Thread-storage files on a remote machine** fail to open.