feat(FN-2121): introduce structured logging in plugin loader

- Add a reusable createLogger utility in @fusion/core for prefixed log/warn/error output
- Replace plugin-loader console logging with structured logger calls across load, reload, stop, and hook paths
- Route plugin-scoped logger methods through createLogger, including debug gating on DEBUG=plugins
- Add regression tests that mock logger.js and verify key structured log emissions and error logging flows

packages/core/src/logger.ts

@@ -0,0 +1,47 @@
+/**
+ * Lightweight structured logger for the `@fusion/core` package.
+ *
+ * Usage:
+ * ```ts
+ * import { createLogger } from "./logger.js";
+ * const log = createLogger("my-module");
+ * log.log("hello");   // → console.error("[my-module] hello")
+ * log.warn("oops");   // → console.warn("[my-module] oops")
+ * log.error("fail");  // → console.error("[my-module] fail")
+ * ```
+ *
+ * Core subsystems should use this utility rather than calling `console.*`
+ * directly so diagnostics stay consistent and easy to suppress/match in tests.
+ */
+
+export interface Logger {
+  log(message: string, ...args: unknown[]): void;
+  warn(message: string, ...args: unknown[]): void;
+  error(message: string, ...args: unknown[]): void;
+}
+
+/**
+ * Create a structured logger that prefixes every message with `[prefix]`.
+ *
+ * @param prefix - Short subsystem name, e.g. "plugin-loader".
+ * @returns A `Logger` whose output is prefixed and sent to stderr for normal
+ *          logs and errors. Keeping logs off stdout prevents command/test
+ *          output consumers from receiving Fusion execution chatter.
+ */
+export function createLogger(prefix: string): Logger {
+  const tag = `[${prefix}]`;
+  return {
+    log(message: string, ...args: unknown[]) {
+      console.error(`${tag} ${message}`, ...args);
+    },
+    warn(message: string, ...args: unknown[]) {
+      console.warn(`${tag} ${message}`, ...args);
+    },
+    error(message: string, ...args: unknown[]) {
+      console.error(`${tag} ${message}`, ...args);
+    },
+  };
+}
+
+/** Logger for the plugin loader subsystem. */
+export const pluginLoaderLog = createLogger("plugin-loader");

packages/core/src/plugin-loader.test.ts

@@ -109,6 +109,36 @@ const mockTaskStore = {
   logActivity: vi.fn(),
 } as any;
 
+type MockStructuredLogger = {
+  log: ReturnType<typeof vi.fn>;
+  warn: ReturnType<typeof vi.fn>;
+  error: ReturnType<typeof vi.fn>;
+};
+
+async function loadPluginLoaderWithMockedLogger() {
+  vi.resetModules();
+  const loggerMap = new Map<string, MockStructuredLogger>();
+  const createLoggerMock = vi.fn((prefix: string): MockStructuredLogger => {
+    const existing = loggerMap.get(prefix);
+    if (existing) return existing;
+
+    const logger: MockStructuredLogger = {
+      log: vi.fn(),
+      warn: vi.fn(),
+      error: vi.fn(),
+    };
+    loggerMap.set(prefix, logger);
+    return logger;
+  });
+
+  vi.doMock("./logger.js", () => ({
+    createLogger: createLoggerMock,
+  }));
+
+  const { PluginLoader: MockedPluginLoader } = await import("./plugin-loader.js");
+  return { MockedPluginLoader, createLoggerMock, loggerMap };
+}
+
 describe("PluginLoader", () => {
   let rootDir: string;
   let pluginStore: PluginStore;
@@ -691,6 +721,275 @@ describe("PluginLoader", () => {
     });
   });
 
+  // ── structured logging ──────────────────────────────────────────────
+
+  describe("structured logging", () => {
+    afterEach(() => {
+      vi.doUnmock("./logger.js");
+    });
+
+    it("logs when skipping a disabled plugin", async () => {
+      await pluginStore.init();
+
+      const plugin = makePlugin(makeManifest({ id: "disabled-log-test" }));
+      const pluginDir = join(rootDir, "plugins");
+      const pluginPath = await writePluginModule(pluginDir, "disabled-log.js", plugin);
+
+      await pluginStore.registerPlugin({
+        manifest: plugin.manifest,
+        path: pluginPath,
+      });
+      await pluginStore.disablePlugin("disabled-log-test");
+
+      const { MockedPluginLoader, loggerMap } = await loadPluginLoaderWithMockedLogger();
+      const loader = new MockedPluginLoader({ pluginStore, taskStore: mockTaskStore });
+
+      await expect(loader.loadPlugin("disabled-log-test")).rejects.toThrow("disabled");
+      expect(loggerMap.get("plugin-loader")?.log).toHaveBeenCalledWith(
+        "Skipping disabled plugin: disabled-log-test",
+      );
+    });
+
+    it("logs when plugin is already loaded", async () => {
+      await pluginStore.init();
+
+      const plugin = makePlugin(makeManifest({ id: "already-loaded-log" }));
+      const pluginDir = join(rootDir, "plugins");
+      const pluginPath = await writePluginModule(pluginDir, "already-loaded.js", plugin);
+
+      await pluginStore.registerPlugin({
+        manifest: plugin.manifest,
+        path: pluginPath,
+      });
+
+      const { MockedPluginLoader, loggerMap } = await loadPluginLoaderWithMockedLogger();
+      const loader = new MockedPluginLoader({ pluginStore, taskStore: mockTaskStore });
+
+      await loader.loadPlugin("already-loaded-log");
+      await loader.loadPlugin("already-loaded-log");
+
+      expect(loggerMap.get("plugin-loader")?.log).toHaveBeenCalledWith(
+        "Plugin already loaded: already-loaded-log",
+      );
+    });
+
+    it("logs when reloading a plugin", async () => {
+      await pluginStore.init();
+
+      const plugin = makePlugin(makeManifest({ id: "reload-log-test" }));
+      const pluginDir = join(rootDir, "plugins");
+      const pluginPath = await writePluginModule(pluginDir, "reload-log.js", plugin);
+
+      await pluginStore.registerPlugin({
+        manifest: plugin.manifest,
+        path: pluginPath,
+      });
+
+      const { MockedPluginLoader, loggerMap } = await loadPluginLoaderWithMockedLogger();
+      const loader = new MockedPluginLoader({ pluginStore, taskStore: mockTaskStore });
+
+      await loader.loadPlugin("reload-log-test");
+      await loader.reloadPlugin("reload-log-test");
+
+      expect(loggerMap.get("plugin-loader")?.log).toHaveBeenCalledWith(
+        "Reloading plugin: reload-log-test",
+      );
+    });
+
+    it("logs reload failures", async () => {
+      await pluginStore.init();
+
+      const pluginId = "reload-failure-log";
+      const pluginDir = join(rootDir, "plugins");
+      const pluginPath = join(pluginDir, "reload-failure.js");
+
+      await writePluginModule(pluginDir, "reload-failure.js", makePlugin(makeManifest({ id: pluginId })));
+      await pluginStore.registerPlugin({
+        manifest: makeManifest({ id: pluginId }),
+        path: pluginPath,
+      });
+
+      const { MockedPluginLoader, loggerMap } = await loadPluginLoaderWithMockedLogger();
+      const loader = new MockedPluginLoader({ pluginStore, taskStore: mockTaskStore });
+
+      await loader.loadPlugin(pluginId);
+      await writePluginWithHooks(
+        pluginDir,
+        "reload-failure.js",
+        {
+          onLoad: "(async () => { throw new Error('reload failed'); })",
+        },
+        makeManifest({ id: pluginId }),
+      );
+
+      await expect(loader.reloadPlugin(pluginId)).rejects.toThrow("reload failed");
+      expect(loggerMap.get("plugin-loader")?.error).toHaveBeenCalledWith(
+        `Reload failed for ${pluginId}, rolling back:`,
+        expect.any(Error),
+      );
+    });
+
+    it("logs rollback failures", async () => {
+      await pluginStore.init();
+
+      const pluginId = "rollback-failure-log";
+      const pluginDir = join(rootDir, "plugins");
+      const pluginPath = join(pluginDir, "rollback-failure.js");
+
+      await writePluginWithHooks(
+        pluginDir,
+        "rollback-failure.js",
+        {
+          onLoad: "((() => { let count = 0; return async () => { count += 1; if (count > 1) throw new Error('old onLoad failed on retry'); }; })())",
+        },
+        makeManifest({ id: pluginId }),
+      );
+
+      await pluginStore.registerPlugin({
+        manifest: makeManifest({ id: pluginId }),
+        path: pluginPath,
+      });
+
+      const { MockedPluginLoader, loggerMap } = await loadPluginLoaderWithMockedLogger();
+      const loader = new MockedPluginLoader({ pluginStore, taskStore: mockTaskStore });
+
+      await loader.loadPlugin(pluginId);
+      await writePluginWithHooks(
+        pluginDir,
+        "rollback-failure.js",
+        {
+          onLoad: "(async () => { throw new Error('new onLoad failed'); })",
+        },
+        makeManifest({ id: pluginId }),
+      );
+
+      await expect(loader.reloadPlugin(pluginId)).rejects.toThrow("new onLoad failed");
+      expect(loggerMap.get("plugin-loader")?.error).toHaveBeenCalledWith(
+        `Rollback failed for ${pluginId}, removing plugin:`,
+        expect.any(Error),
+      );
+    });
+
+    it("logs onUnload hook errors when stopping", async () => {
+      await pluginStore.init();
+
+      const pluginId = "stop-hook-log";
+      const pluginDir = join(rootDir, "plugins");
+      const pluginPath = await writePluginWithHooks(
+        pluginDir,
+        "stop-hook.js",
+        {
+          onUnload: "(() => { throw new Error('stop failed'); })",
+        },
+        makeManifest({ id: pluginId }),
+      );
+
+      await pluginStore.registerPlugin({
+        manifest: makeManifest({ id: pluginId }),
+        path: pluginPath,
+      });
+
+      const { MockedPluginLoader, loggerMap } = await loadPluginLoaderWithMockedLogger();
+      const loader = new MockedPluginLoader({ pluginStore, taskStore: mockTaskStore });
+
+      await loader.loadPlugin(pluginId);
+      await loader.stopPlugin(pluginId);
+
+      expect(loggerMap.get("plugin-loader")?.error).toHaveBeenCalledWith(
+        `Error in onUnload for ${pluginId}:`,
+        expect.any(Error),
+      );
+    });
+
+    it("logs loadAllPlugins failures", async () => {
+      await pluginStore.init();
+
+      const pluginDir = join(rootDir, "plugins");
+      const goodPlugin = makePlugin(makeManifest({ id: "good-load-all-log" }));
+      const goodPath = await writePluginModule(pluginDir, "good-load-all.js", goodPlugin);
+      const badPath = await writePluginWithHooks(
+        pluginDir,
+        "bad-load-all.js",
+        {
+          onLoad: "(async () => { throw new Error('load all failure'); })",
+        },
+        makeManifest({ id: "bad-load-all-log" }),
+      );
+
+      await pluginStore.registerPlugin({ manifest: goodPlugin.manifest, path: goodPath });
+      await pluginStore.registerPlugin({
+        manifest: makeManifest({ id: "bad-load-all-log" }),
+        path: badPath,
+      });
+
+      const { MockedPluginLoader, loggerMap } = await loadPluginLoaderWithMockedLogger();
+      const loader = new MockedPluginLoader({ pluginStore, taskStore: mockTaskStore });
+
+      await loader.loadAllPlugins();
+
+      expect(loggerMap.get("plugin-loader")?.error).toHaveBeenCalledWith(
+        "Failed to load plugin bad-load-all-log:",
+        expect.any(Error),
+      );
+    });
+
+    it("logs invokeHook failures", async () => {
+      await pluginStore.init();
+
+      const { MockedPluginLoader, loggerMap } = await loadPluginLoaderWithMockedLogger();
+      const loader = new MockedPluginLoader({ pluginStore, taskStore: mockTaskStore });
+
+      (loader as any).plugins.set("hook-error-log", {
+        manifest: makeManifest({ id: "hook-error-log" }),
+        state: "started",
+        hooks: {
+          onTaskCreated: () => {
+            throw new Error("hook failure");
+          },
+        },
+        tools: [],
+        routes: [],
+      } as FusionPlugin);
+
+      await loader.invokeHook("onTaskCreated", { id: "FN-123" } as any);
+
+      expect(loggerMap.get("plugin-loader")?.error).toHaveBeenCalledWith(
+        "Error in onTaskCreated hook for hook-error-log:",
+        expect.any(Error),
+      );
+    });
+
+    it("logs custom events from createContext through structured logger", async () => {
+      await pluginStore.init();
+
+      const pluginId = "custom-event-log";
+      const pluginDir = join(rootDir, "plugins");
+      const pluginPath = await writePluginWithHooks(
+        pluginDir,
+        "custom-event.js",
+        {
+          onLoad: "(async (ctx) => { ctx.emitEvent('custom-event', { payload: 'ok' }); })",
+        },
+        makeManifest({ id: pluginId }),
+      );
+
+      await pluginStore.registerPlugin({
+        manifest: makeManifest({ id: pluginId }),
+        path: pluginPath,
+      });
+
+      const { MockedPluginLoader, loggerMap } = await loadPluginLoaderWithMockedLogger();
+      const loader = new MockedPluginLoader({ pluginStore, taskStore: mockTaskStore });
+
+      await loader.loadPlugin(pluginId);
+
+      expect(loggerMap.get("plugin-loader")?.log).toHaveBeenCalledWith(
+        `[plugin:${pluginId}] Custom event: custom-event`,
+        { payload: "ok" },
+      );
+    });
+  });
+
   // ── getPluginTools ─────────────────────────────────────────────────
 
   describe("getPluginTools", () => {

packages/core/src/plugin-loader.ts

@@ -24,9 +24,11 @@ import type {
   PluginInstallation,
 } from "./plugin-types.js";
 import { validatePluginManifest } from "./plugin-types.js";
+import { createLogger } from "./logger.js";
 
 // Minimum Fusion version for plugin compatibility checks (can be expanded later)
 const MINIMUM_FUSION_VERSION = "0.1.0";
+const log = createLogger("plugin-loader");
 
 export interface PluginLoaderOptions {
   /** Plugin store for persistence */
@@ -102,20 +104,20 @@ export class PluginLoader extends EventEmitter<{
       emitEvent: (event: string, data: unknown) => {
         this.emit("plugin:error", { pluginId: plugin.manifest.id, error: new Error(`Custom event: ${event}`) });
         // Custom events are logged but not surfaced as errors
-        console.log(`[plugin:${plugin.manifest.id}] Custom event: ${event}`, data);
+        log.log(`[plugin:${plugin.manifest.id}] Custom event: ${event}`, data);
       },
     };
   }
 
   private createLogger(pluginId: string): PluginLogger {
-    const prefix = `[plugin:${pluginId}]`;
+    const pluginLog = createLogger(`plugin:${pluginId}`);
     return {
-      info: (...args: unknown[]) => console.log(prefix, ...args),
-      warn: (...args: unknown[]) => console.warn(prefix, ...args),
-      error: (...args: unknown[]) => console.error(prefix, ...args),
-      debug: (...args: unknown[]) => {
+      info: (message: string, ...args: unknown[]) => pluginLog.log(message, ...args),
+      warn: (message: string, ...args: unknown[]) => pluginLog.warn(message, ...args),
+      error: (message: string, ...args: unknown[]) => pluginLog.error(message, ...args),
+      debug: (message: string, ...args: unknown[]) => {
         if (process.env.DEBUG?.includes("plugins")) {
-          console.log(prefix, ...args);
+          pluginLog.log(message, ...args);
         }
       },
     };
@@ -146,7 +148,7 @@ export class PluginLoader extends EventEmitter<{
 
     // Skip disabled plugins
     if (!installation.enabled) {
-      console.log(`[plugin-loader] Skipping disabled plugin: ${pluginId}`);
+      log.log(`Skipping disabled plugin: ${pluginId}`);
       throw Object.assign(new Error(`Plugin "${pluginId}" is disabled`), {
         code: "PLUGIN_DISABLED",
       });
@@ -154,7 +156,7 @@ export class PluginLoader extends EventEmitter<{
 
     // Skip already loaded plugins
     if (this.plugins.has(pluginId)) {
-      console.log(`[plugin-loader] Plugin already loaded: ${pluginId}`);
+      log.log(`Plugin already loaded: ${pluginId}`);
       return this.plugins.get(pluginId)!;
     }
 
@@ -181,8 +183,8 @@ export class PluginLoader extends EventEmitter<{
           plugin.manifest.fusionVersion,
         );
         if (!compatible) {
-          console.warn(
-            `[plugin-loader] Plugin ${pluginId} requires Fusion ${plugin.manifest.fusionVersion}, minimum is ${MINIMUM_FUSION_VERSION}`,
+          log.warn(
+            `Plugin ${pluginId} requires Fusion ${plugin.manifest.fusionVersion}, minimum is ${MINIMUM_FUSION_VERSION}`,
           );
         }
       }
@@ -284,7 +286,7 @@ export class PluginLoader extends EventEmitter<{
    */
   private invalidateModuleCache(path: string): void {
     this.loadedModules.delete(path);
-    console.log(`[plugin-loader] Module cache invalidated for: ${path}`);
+    log.log(`Module cache invalidated for: ${path}`);
   }
 
   /**
@@ -312,7 +314,7 @@ export class PluginLoader extends EventEmitter<{
     const installation = await this.options.pluginStore.getPlugin(pluginId);
     const pluginPath = this.resolvePluginPath(installation.path);
 
-    console.log(`[plugin-loader] Reloading plugin: ${pluginId}`);
+    log.log(`Reloading plugin: ${pluginId}`);
 
     // Call onUnload with timeout
     try {
@@ -322,7 +324,7 @@ export class PluginLoader extends EventEmitter<{
         `onUnload timeout for ${pluginId}`,
       );
     } catch (err) {
-      console.warn(`[plugin-loader] onUnload for ${pluginId} timed out or failed:`, err);
+      log.warn(`onUnload for ${pluginId} timed out or failed:`, err);
       // Continue with reload despite onUnload issues
     }
 
@@ -362,13 +364,13 @@ export class PluginLoader extends EventEmitter<{
       // State is already "started", no need to update store
       // (avoiding started -> started transition which is disallowed)
 
-      console.log(`[plugin-loader] Plugin ${pluginId} reloaded successfully`);
+      log.log(`Plugin ${pluginId} reloaded successfully`);
 
       this.emit("plugin:reloaded", { pluginId, plugin: newPlugin });
       return newPlugin;
     } catch (err) {
       // Rollback: restore old plugin
-      console.error(`[plugin-loader] Reload failed for ${pluginId}, rolling back:`, err);
+      log.error(`Reload failed for ${pluginId}, rolling back:`, err);
 
       try {
         // Restore old plugin
@@ -385,12 +387,12 @@ export class PluginLoader extends EventEmitter<{
         // Update store state back to started
         await this.options.pluginStore.updatePluginState(pluginId, "started");
 
-        console.warn(`[plugin-loader] Rollback successful for ${pluginId}`);
+        log.warn(`Rollback successful for ${pluginId}`);
         throw err; // Still throw the original error
       } catch (rollbackErr) {
         // Rollback also failed - remove plugin and set error state
-        console.error(
-          `[plugin-loader] Rollback failed for ${pluginId}, removing plugin:`,
+        log.error(
+          `Rollback failed for ${pluginId}, removing plugin:`,
           rollbackErr,
         );
 
@@ -520,8 +522,8 @@ export class PluginLoader extends EventEmitter<{
       } catch (err) {
         if ((err as any).code !== "PLUGIN_DISABLED") {
           errors++;
-          console.error(
-            `[plugin-loader] Failed to load plugin ${installation.id}:`,
+          log.error(
+            `Failed to load plugin ${installation.id}:`,
             err,
           );
         }
@@ -576,7 +578,7 @@ export class PluginLoader extends EventEmitter<{
   async stopPlugin(pluginId: string): Promise<void> {
     const plugin = this.plugins.get(pluginId);
     if (!plugin) {
-      console.log(`[plugin-loader] Plugin not loaded: ${pluginId}`);
+      log.log(`Plugin not loaded: ${pluginId}`);
       return;
     }
 
@@ -588,7 +590,7 @@ export class PluginLoader extends EventEmitter<{
       // Call onUnload hook
       await this.safeCallHook(plugin, "onUnload", []);
     } catch (err) {
-      console.error(`[plugin-loader] Error in onUnload for ${pluginId}:`, err);
+      log.error(`Error in onUnload for ${pluginId}:`, err);
     }
 
     // Update state
@@ -633,7 +635,7 @@ export class PluginLoader extends EventEmitter<{
       try {
         await this.stopPlugin(plugin.id);
       } catch (err) {
-        console.error(`[plugin-loader] Error stopping plugin ${plugin.id}:`, err);
+        log.error(`Error stopping plugin ${plugin.id}:`, err);
       }
     }
   }
@@ -655,8 +657,8 @@ export class PluginLoader extends EventEmitter<{
       try {
         await this.safeCallHook(plugin, hookName, args);
       } catch (err) {
-        console.error(
-          `[plugin-loader] Error in ${hookName} hook for ${pluginId}:`,
+        log.error(
+          `Error in ${hookName} hook for ${pluginId}:`,
           err,
         );