feat(hooks): add agent-usage-reminder hook for background agent recommendations

Implements hook that tracks whether explore/librarian agents have been used in a session.
When target tools (Grep, Glob, WebFetch, context7, websearch_exa, grep_app) are called
without prior agent usage, appends reminder message recommending parallel background_task calls.

State persists across tool calls and resets on session compaction, allowing fresh reminders
after context compaction - similar to directory-readme-injector pattern.

Files:
- src/hooks/agent-usage-reminder/: New hook implementation
  - types.ts: AgentUsageState interface
  - constants.ts: TARGET_TOOLS, AGENT_TOOLS, REMINDER_MESSAGE
  - storage.ts: File-based state persistence with compaction handling
  - index.ts: Hook implementation with tool.execute.after and event handlers
- src/config/schema.ts: Add 'agent-usage-reminder' to HookNameSchema
- src/hooks/index.ts: Export createAgentUsageReminderHook
- src/index.ts: Instantiate and register hook handlers

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/config/schema.ts

@@ -41,6 +41,7 @@ export const HookNameSchema = z.enum([
   "background-notification",
   "auto-update-checker",
   "ultrawork-mode",
+  "agent-usage-reminder",
 ])
 
 export const AgentOverrideConfigSchema = z.object({

src/hooks/agent-usage-reminder/constants.ts

@@ -0,0 +1,52 @@
+import { join } from "node:path";
+import { xdgData } from "xdg-basedir";
+
+export const OPENCODE_STORAGE = join(xdgData ?? "", "opencode", "storage");
+export const AGENT_USAGE_REMINDER_STORAGE = join(
+  OPENCODE_STORAGE,
+  "agent-usage-reminder",
+);
+
+export const TARGET_TOOLS = [
+  "Grep",
+  "safe_grep",
+  "Glob",
+  "safe_glob",
+  "WebFetch",
+  "context7_resolve-library-id",
+  "context7_get-library-docs",
+  "websearch_exa_web_search_exa",
+  "grep_app_searchGitHub",
+] as const;
+
+export const AGENT_TOOLS = [
+  "Task",
+  "call_omo_agent",
+  "background_task",
+] as const;
+
+export const REMINDER_MESSAGE = `
+[Agent Usage Reminder]
+
+You called a search/fetch tool directly without leveraging specialized agents.
+
+RECOMMENDED: Use background_task with explore/librarian agents for better results:
+
+\`\`\`
+// Parallel exploration - fire multiple agents simultaneously
+background_task(agent="explore", prompt="Find all files matching pattern X")
+background_task(agent="explore", prompt="Search for implementation of Y") 
+background_task(agent="librarian", prompt="Lookup documentation for Z")
+
+// Then continue your work while they run in background
+// System will notify you when each completes
+\`\`\`
+
+WHY:
+- Agents can perform deeper, more thorough searches
+- Background tasks run in parallel, saving time
+- Specialized agents have domain expertise
+- Reduces context window usage in main session
+
+ALWAYS prefer: Multiple parallel background_task calls > Direct tool calls
+`;

src/hooks/agent-usage-reminder/index.ts

@@ -0,0 +1,114 @@
+import type { PluginInput } from "@opencode-ai/plugin";
+import {
+  loadAgentUsageState,
+  saveAgentUsageState,
+  clearAgentUsageState,
+} from "./storage";
+import { TARGET_TOOLS, AGENT_TOOLS, REMINDER_MESSAGE } from "./constants";
+import type { AgentUsageState } from "./types";
+
+interface ToolExecuteInput {
+  tool: string;
+  sessionID: string;
+  callID: string;
+  parentSessionID?: string;
+}
+
+interface ToolExecuteOutput {
+  title: string;
+  output: string;
+  metadata: unknown;
+}
+
+interface EventInput {
+  event: {
+    type: string;
+    properties?: unknown;
+  };
+}
+
+export function createAgentUsageReminderHook(_ctx: PluginInput) {
+  const sessionStates = new Map<string, AgentUsageState>();
+
+  function getOrCreateState(sessionID: string): AgentUsageState {
+    if (!sessionStates.has(sessionID)) {
+      const persisted = loadAgentUsageState(sessionID);
+      const state: AgentUsageState = persisted ?? {
+        sessionID,
+        agentUsed: false,
+        reminderCount: 0,
+        updatedAt: Date.now(),
+      };
+      sessionStates.set(sessionID, state);
+    }
+    return sessionStates.get(sessionID)!;
+  }
+
+  function markAgentUsed(sessionID: string): void {
+    const state = getOrCreateState(sessionID);
+    state.agentUsed = true;
+    state.updatedAt = Date.now();
+    saveAgentUsageState(state);
+  }
+
+  function resetState(sessionID: string): void {
+    sessionStates.delete(sessionID);
+    clearAgentUsageState(sessionID);
+  }
+
+  const toolExecuteAfter = async (
+    input: ToolExecuteInput,
+    output: ToolExecuteOutput,
+  ) => {
+    const { tool, sessionID, parentSessionID } = input;
+
+    // Only run in root sessions (no parent = main session)
+    if (parentSessionID) {
+      return;
+    }
+
+    if ((AGENT_TOOLS as readonly string[]).includes(tool)) {
+      markAgentUsed(sessionID);
+      return;
+    }
+
+    if (!(TARGET_TOOLS as readonly string[]).includes(tool)) {
+      return;
+    }
+
+    const state = getOrCreateState(sessionID);
+
+    if (state.agentUsed) {
+      return;
+    }
+
+    output.output += REMINDER_MESSAGE;
+    state.reminderCount++;
+    state.updatedAt = Date.now();
+    saveAgentUsageState(state);
+  };
+
+  const eventHandler = async ({ event }: EventInput) => {
+    const props = event.properties as Record<string, unknown> | undefined;
+
+    if (event.type === "session.deleted") {
+      const sessionInfo = props?.info as { id?: string } | undefined;
+      if (sessionInfo?.id) {
+        resetState(sessionInfo.id);
+      }
+    }
+
+    if (event.type === "session.compacted") {
+      const sessionID = (props?.sessionID ??
+        (props?.info as { id?: string } | undefined)?.id) as string | undefined;
+      if (sessionID) {
+        resetState(sessionID);
+      }
+    }
+  };
+
+  return {
+    "tool.execute.after": toolExecuteAfter,
+    event: eventHandler,
+  };
+}

src/hooks/agent-usage-reminder/storage.ts

@@ -0,0 +1,42 @@
+import {
+  existsSync,
+  mkdirSync,
+  readFileSync,
+  writeFileSync,
+  unlinkSync,
+} from "node:fs";
+import { join } from "node:path";
+import { AGENT_USAGE_REMINDER_STORAGE } from "./constants";
+import type { AgentUsageState } from "./types";
+
+function getStoragePath(sessionID: string): string {
+  return join(AGENT_USAGE_REMINDER_STORAGE, `${sessionID}.json`);
+}
+
+export function loadAgentUsageState(sessionID: string): AgentUsageState | null {
+  const filePath = getStoragePath(sessionID);
+  if (!existsSync(filePath)) return null;
+
+  try {
+    const content = readFileSync(filePath, "utf-8");
+    return JSON.parse(content) as AgentUsageState;
+  } catch {
+    return null;
+  }
+}
+
+export function saveAgentUsageState(state: AgentUsageState): void {
+  if (!existsSync(AGENT_USAGE_REMINDER_STORAGE)) {
+    mkdirSync(AGENT_USAGE_REMINDER_STORAGE, { recursive: true });
+  }
+
+  const filePath = getStoragePath(state.sessionID);
+  writeFileSync(filePath, JSON.stringify(state, null, 2));
+}
+
+export function clearAgentUsageState(sessionID: string): void {
+  const filePath = getStoragePath(sessionID);
+  if (existsSync(filePath)) {
+    unlinkSync(filePath);
+  }
+}

src/hooks/agent-usage-reminder/types.ts

@@ -0,0 +1,6 @@
+export interface AgentUsageState {
+  sessionID: string;
+  agentUsed: boolean;
+  reminderCount: number;
+  updatedAt: number;
+}

src/hooks/index.ts

@@ -14,3 +14,4 @@ export { createRulesInjectorHook } from "./rules-injector";
 export { createBackgroundNotificationHook } from "./background-notification"
 export { createAutoUpdateCheckerHook } from "./auto-update-checker";
 export { createUltraworkModeHook } from "./ultrawork-mode";
+export { createAgentUsageReminderHook } from "./agent-usage-reminder";

src/index.ts

@@ -17,6 +17,7 @@ import {
   createBackgroundNotificationHook,
   createAutoUpdateCheckerHook,
   createUltraworkModeHook,
+  createAgentUsageReminderHook,
 } from "./hooks";
 import { createGoogleAntigravityAuthPlugin } from "./auth/antigravity";
 import {
@@ -207,6 +208,9 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
   const ultraworkMode = isHookEnabled("ultrawork-mode")
     ? createUltraworkModeHook()
     : null;
+  const agentUsageReminder = isHookEnabled("agent-usage-reminder")
+    ? createAgentUsageReminderHook(ctx)
+    : null;
 
   updateTerminalTitle({ sessionId: "main" });
 
@@ -320,6 +324,7 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
       await thinkMode?.event(input);
       await anthropicAutoCompact?.event(input);
       await ultraworkMode?.event(input);
+      await agentUsageReminder?.event(input);
 
       const { event } = input;
       const props = event.properties as Record<string, unknown> | undefined;
@@ -439,6 +444,7 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
       await directoryReadmeInjector?.["tool.execute.after"](input, output);
       await rulesInjector?.["tool.execute.after"](input, output);
       await emptyTaskResponseDetector?.["tool.execute.after"](input, output);
+      await agentUsageReminder?.["tool.execute.after"](input, output);
 
       if (input.sessionID === getMainSessionID()) {
         updateTerminalTitle({