🤖 refactor: single global extension host (VS Code architecture)

Refactors extension system from per-workspace hosts to a single global host that
manages workspace runtimes via registration. Matches VS Code's proven architecture
for multi-folder workspaces.

**Problem:** One extension host per workspace doesn't scale
- 10 workspaces = 10 Node.js processes + 10× loaded extensions
- Wasteful for cmux's parallel agentic development use case
- Inconsistent with VS Code (single host for 50+ folders)

**Solution:** Single shared extension host with workspace runtime map
- ExtensionHost maintains Map<workspaceId, Runtime>
- Extensions loaded once at app startup (initializeGlobal)
- Workspaces register/unregister dynamically
- 50 workspaces = 1 process (not 50)

**API Changes:**
- ExtensionManager: initializeGlobal(), registerWorkspace(), unregisterWorkspace(), shutdown()
- Discovery: discoverExtensions(extDir) - simplified to single directory
- Message types: 'register-workspace', 'unregister-workspace' added
- AIService: Registers workspace on first message, unregisters on removal

**Testing:**
- All unit tests pass (14 tests: 9 discovery + 5 manager)
- Integration tests need updating for new architecture (TODO)
- Static checks pass (typecheck, lint, format)

_Generated with `cmux`_

Author: ammar-agent

src/services/aiService.ts

@@ -18,6 +18,7 @@ import { getToolsForModel } from "@/utils/tools/tools";
 import { createRuntime } from "@/runtime/runtimeFactory";
 import { secretsToRecord } from "@/types/secrets";
 import type { CmuxProviderOptions } from "@/types/providerOptions";
+import { ExtensionManager } from "./extensions/extensionManager";
 import { log } from "./log";
 import {
   transformModelMessages,
@@ -112,6 +113,7 @@ export class AIService extends EventEmitter {
   private readonly initStateManager: InitStateManager;
   private readonly mockModeEnabled: boolean;
   private readonly mockScenarioPlayer?: MockScenarioPlayer;
+  private readonly extensionManager: ExtensionManager;
 
   constructor(
     config: Config,
@@ -127,7 +129,18 @@ export class AIService extends EventEmitter {
     this.historyService = historyService;
     this.partialService = partialService;
     this.initStateManager = initStateManager;
-    this.streamManager = new StreamManager(historyService, partialService);
+
+    // Initialize extension manager
+    this.extensionManager = new ExtensionManager();
+
+    // Initialize the global extension host
+    void this.extensionManager.initializeGlobal().catch((error) => {
+      log.error("Failed to initialize extension host:", error);
+    });
+
+    // Initialize stream manager with extension manager
+    this.streamManager = new StreamManager(historyService, partialService, this.extensionManager);
+
     void this.ensureSessionsDir();
     this.setupStreamEventForwarding();
     this.mockModeEnabled = process.env.CMUX_MOCK_AI === "1";
@@ -556,6 +569,20 @@ export class AIService extends EventEmitter {
       const streamToken = this.streamManager.generateStreamToken();
       const runtimeTempDir = await this.streamManager.createTempDirForStream(streamToken, runtime);
 
+      // Register workspace with extension host (non-blocking)
+      // Extensions need full workspace context including runtime and tempdir
+      void this.extensionManager
+        .registerWorkspace(
+          workspaceId,
+          metadata,
+          metadata.runtimeConfig ?? { type: "local", srcBaseDir: this.config.srcDir },
+          runtimeTempDir
+        )
+        .catch((error) => {
+          log.error(`Failed to register workspace ${workspaceId} with extension host:`, error);
+          // Don't fail the stream on extension registration errors
+        });
+
       // Get model-specific tools with workspace path (correct for local or remote)
       const allTools = await getToolsForModel(
         modelString,
@@ -822,4 +849,11 @@ export class AIService extends EventEmitter {
       return Err(`Failed to delete workspace: ${message}`);
     }
   }
+
+  /**
+   * Unregister a workspace from the extension host
+   */
+  async unregisterWorkspace(workspaceId: string): Promise<void> {
+    await this.extensionManager.unregisterWorkspace(workspaceId);
+  }
 }

src/services/extensions/extensionHost.ts

@@ -0,0 +1,238 @@
+/**
+ * Extension Host Process
+ *
+ * This script runs as a separate Node.js process (spawned via fork()).
+ * It receives IPC messages from the main cmux process, loads extensions once,
+ * maintains a map of workspace runtimes, and dispatches hooks to extensions.
+ *
+ * A single shared extension host serves all workspaces (VS Code architecture).
+ */
+
+import type { Runtime } from "../../runtime/Runtime";
+import type {
+  Extension,
+  ExtensionHostMessage,
+  ExtensionHostResponse,
+  ExtensionInfo,
+} from "../../types/extensions";
+
+const workspaceRuntimes = new Map<string, Runtime>();
+const extensions: Array<{ id: string; module: Extension }> = [];
+
+/**
+ * Send a message to the parent process
+ */
+function sendMessage(message: ExtensionHostResponse): void {
+  if (process.send) {
+    process.send(message);
+  }
+}
+
+/**
+ * Load an extension from its entrypoint path
+ */
+async function loadExtension(extInfo: ExtensionInfo): Promise<void> {
+  try {
+    // Dynamic import to load the extension module
+    // Extensions must export a default object with hook handlers
+    // eslint-disable-next-line no-restricted-syntax, @typescript-eslint/no-unsafe-assignment -- Dynamic import required for user extensions
+    const module = await import(extInfo.path);
+
+    // eslint-disable-next-line @typescript-eslint/no-unsafe-member-access -- User-provided extension module
+    if (!module.default) {
+      throw new Error(`Extension ${extInfo.id} does not export a default object`);
+    }
+
+    extensions.push({
+      id: extInfo.id,
+      // eslint-disable-next-line @typescript-eslint/no-unsafe-member-access -- User-provided extension module
+      module: module.default as Extension,
+    });
+
+    console.log(`[ExtensionHost] Loaded extension: ${extInfo.id}`);
+  } catch (error) {
+    const errorMsg = error instanceof Error ? error.message : String(error);
+    console.error(`[ExtensionHost] Failed to load extension ${extInfo.id}:`, errorMsg);
+    sendMessage({
+      type: "extension-load-error",
+      id: extInfo.id,
+      error: errorMsg,
+    });
+  }
+}
+
+/**
+ * Initialize the extension host (load extensions globally)
+ */
+async function handleInit(msg: Extract<ExtensionHostMessage, { type: "init" }>): Promise<void> {
+  try {
+    const { extensions: extensionList } = msg;
+
+    console.log(`[ExtensionHost] Initializing with ${extensionList.length} extension(s)`);
+
+    // Load all extensions once
+    for (const extInfo of extensionList) {
+      await loadExtension(extInfo);
+    }
+
+    // Send ready message
+    sendMessage({
+      type: "ready",
+      extensionCount: extensions.length,
+    });
+
+    console.log(`[ExtensionHost] Ready with ${extensions.length} loaded extension(s)`);
+  } catch (error) {
+    console.error("[ExtensionHost] Failed to initialize:", error);
+    process.exit(1);
+  }
+}
+
+/**
+ * Register a workspace with the extension host
+ */
+async function handleRegisterWorkspace(
+  msg: Extract<ExtensionHostMessage, { type: "register-workspace" }>
+): Promise<void> {
+  try {
+    const { workspaceId, runtimeConfig } = msg;
+
+    // Dynamically import createRuntime to avoid bundling issues
+    // eslint-disable-next-line no-restricted-syntax -- Required in child process to avoid circular deps
+    const { createRuntime } = await import("../../runtime/runtimeFactory");
+
+    // Create runtime for this workspace
+    const runtime = createRuntime(runtimeConfig);
+    workspaceRuntimes.set(workspaceId, runtime);
+
+    console.log(`[ExtensionHost] Registered workspace ${workspaceId}`);
+
+    // Send confirmation
+    sendMessage({
+      type: "workspace-registered",
+      workspaceId,
+    });
+  } catch (error) {
+    console.error(`[ExtensionHost] Failed to register workspace:`, error);
+  }
+}
+
+/**
+ * Unregister a workspace from the extension host
+ */
+function handleUnregisterWorkspace(
+  msg: Extract<ExtensionHostMessage, { type: "unregister-workspace" }>
+): void {
+  const { workspaceId } = msg;
+
+  workspaceRuntimes.delete(workspaceId);
+  console.log(`[ExtensionHost] Unregistered workspace ${workspaceId}`);
+
+  sendMessage({
+    type: "workspace-unregistered",
+    workspaceId,
+  });
+}
+
+/**
+ * Dispatch post-tool-use hook to all extensions
+ */
+async function handlePostToolUse(
+  msg: Extract<ExtensionHostMessage, { type: "post-tool-use" }>
+): Promise<void> {
+  const { payload } = msg;
+
+  // Get runtime for this workspace
+  const runtime = workspaceRuntimes.get(payload.workspaceId);
+  if (!runtime) {
+    console.warn(
+      `[ExtensionHost] Runtime not found for workspace ${payload.workspaceId}, skipping hook`
+    );
+    sendMessage({
+      type: "hook-complete",
+      hookType: "post-tool-use",
+    });
+    return;
+  }
+
+  // Dispatch to all extensions sequentially
+  for (const { id, module } of extensions) {
+    if (!module.onPostToolUse) {
+      continue;
+    }
+
+    try {
+      // Call the extension's hook handler with runtime access
+      await module.onPostToolUse({
+        ...payload,
+        runtime,
+      });
+    } catch (error) {
+      const errorMsg = error instanceof Error ? error.message : String(error);
+      console.error(`[ExtensionHost] Extension ${id} threw error in onPostToolUse:`, errorMsg);
+      sendMessage({
+        type: "extension-error",
+        extensionId: id,
+        error: errorMsg,
+      });
+    }
+  }
+
+  // Acknowledge completion
+  sendMessage({
+    type: "hook-complete",
+    hookType: "post-tool-use",
+  });
+}
+
+/**
+ * Handle shutdown request
+ */
+function handleShutdown(): void {
+  console.log("[ExtensionHost] Shutting down");
+  process.exit(0);
+}
+
+/**
+ * Main message handler
+ */
+process.on("message", (msg: ExtensionHostMessage) => {
+  void (async () => {
+    try {
+      switch (msg.type) {
+        case "init":
+          await handleInit(msg);
+          break;
+        case "register-workspace":
+          await handleRegisterWorkspace(msg);
+          break;
+        case "unregister-workspace":
+          handleUnregisterWorkspace(msg);
+          break;
+        case "post-tool-use":
+          await handlePostToolUse(msg);
+          break;
+        case "shutdown":
+          handleShutdown();
+          break;
+        default:
+          console.warn(`[ExtensionHost] Unknown message type:`, msg);
+      }
+    } catch (error) {
+      console.error("[ExtensionHost] Error handling message:", error);
+    }
+  })();
+});
+
+// Handle process errors
+process.on("uncaughtException", (error) => {
+  console.error("[ExtensionHost] Uncaught exception:", error);
+  process.exit(1);
+});
+
+process.on("unhandledRejection", (reason) => {
+  console.error("[ExtensionHost] Unhandled rejection:", reason);
+  process.exit(1);
+});
+
+console.log("[ExtensionHost] Process started, waiting for init message");