feat(hook): improvement hook context injection for planning (#459)

* feat(shared): add improvement hook reader utility

Adds a runtime-agnostic module for reading improvement hook files from
~/.plannotator/hooks/. Uses a hardcoded base path with an allowlist of
known hook files and a 50KB size cap to prevent runaway context injection.

Designed for cross-harness reuse (Claude Code, OpenCode, Pi, etc.).

For provenance purposes, this commit was AI assisted.

* feat(hook): wire up improve-context PreToolUse hook for EnterPlanMode

Adds a PreToolUse hook on EnterPlanMode that injects corrective planning
instructions into Claude's context at plan mode entry. The new
`plannotator improve-context` subcommand reads the improvement hook file
and returns additionalContext, or silently passes through if no file exists.

For provenance purposes, this commit was AI assisted.

* fix(skill): update compound skill improvement hook path

Moves the improvement hook file path from ~/.plannotator/compound/ to
~/.plannotator/hooks/compound/ to namespace hook-injectable files
separately from other plannotator data.

For provenance purposes, this commit was AI assisted.

* fix(shared): add legacy path fallback for improvement hook reader

Users who ran the compound skill before the path migration have their
improvement hook file at ~/.plannotator/compound/ instead of the new
~/.plannotator/hooks/compound/ path. Add a fallback that checks the
legacy path only when the new path is absent — if the new path exists
but is invalid (empty, oversized), no fallback occurs to prevent
resurrecting stale instructions.

Includes tests for new-path-wins, legacy-fallback, and
invalid-new-blocks-legacy scenarios.

For provenance purposes, this commit was AI assisted.

apps/hook/hooks/hooks.json

@@ -1,5 +1,17 @@
 {
   "hooks": {
+    "PreToolUse": [
+      {
+        "matcher": "EnterPlanMode",
+        "hooks": [
+          {
+            "type": "command",
+            "command": "plannotator improve-context",
+            "timeout": 5
+          }
+        ]
+      }
+    ],
     "PermissionRequest": [
       {
         "matcher": "ExitPlanMode",

apps/hook/server/cli.ts

@@ -19,6 +19,7 @@ export function formatTopLevelHelp(): string {
     "  plannotator last",
     "  plannotator archive",
     "  plannotator sessions",
+    "  plannotator improve-context",
     "",
     "Note:",
     "  running 'plannotator' without arguments is for hook integration and expects JSON on stdin",

apps/hook/server/index.ts

@@ -1,7 +1,7 @@
 /**
  * Plannotator CLI for Claude Code & Copilot CLI
  *
- * Supports seven modes:
+ * Supports eight modes:
  *
  * 1. Plan Review (default, no args):
  *    - Spawned by ExitPlanMode hook (Claude Code)
@@ -37,6 +37,11 @@
  *    - Annotate the last assistant message from a Copilot CLI session
  *    - Parses events.jsonl from session state
  *
+ * 8. Improve Context (`plannotator improve-context`):
+ *    - Spawned by PreToolUse hook on EnterPlanMode
+ *    - Reads improvement hook file from ~/.plannotator/hooks/
+ *    - Returns additionalContext or silently passes through
+ *
  * Global flags:
  *   --help             - Show top-level usage information
  *   --browser <name>   - Override which browser to open (e.g. "Google Chrome")
@@ -68,6 +73,7 @@ import { registerSession, unregisterSession, listSessions } from "@plannotator/s
 import { openBrowser } from "@plannotator/server/browser";
 import { detectProjectName } from "@plannotator/server/project";
 import { planDenyFeedback } from "@plannotator/shared/feedback-templates";
+import { readImprovementHook } from "@plannotator/shared/improvement-hooks";
 import type { Origin } from "@plannotator/shared/agents";
 import { findSessionLogsForCwd, resolveSessionLogByPpid, findSessionLogsByAncestorWalk, getLastRenderedMessage, type RenderedMessage } from "./session-log";
 import { findCodexRolloutByThreadId, getLastCodexMessage } from "./codex-session";
@@ -700,6 +706,37 @@ if (args[0] === "sessions") {
   console.log(result.feedback || "No feedback provided.");
   process.exit(0);
 
+} else if (args[0] === "improve-context") {
+  // ============================================
+  // IMPROVEMENT HOOK CONTEXT INJECTION MODE
+  // ============================================
+  //
+  // Called by PreToolUse hook on EnterPlanMode.
+  // Reads the improvement hook file and returns additionalContext.
+  // No file = exit 0 silently (passthrough).
+
+  // Must consume stdin (Claude Code hooks deliver event JSON on stdin)
+  await Bun.stdin.text();
+
+  const hook = readImprovementHook("enterplanmode-improve");
+  if (!hook) process.exit(0);
+
+  const context = [
+    "[Plannotator Improvement Hook]",
+    "The following corrective instructions were generated from analysis of previous plan denial patterns.",
+    "Apply these guidelines when writing your plan:\n",
+    hook.content,
+  ].join("\n");
+
+  console.log(JSON.stringify({
+    hookSpecificOutput: {
+      hookEventName: "PreToolUse",
+      additionalContext: context,
+    },
+  }));
+
+  process.exit(0);
+
 } else {
   // ============================================
   // PLAN REVIEW MODE (default)

apps/skills/plannotator-compound/SKILL.md

@@ -473,10 +473,10 @@ every future planning session automatically.
 The hook file lives at:
 
 ```
-~/.plannotator/compound/enterplanmode-improve-hook.txt
+~/.plannotator/hooks/compound/enterplanmode-improve-hook.txt
 ```
 
-Create the `~/.plannotator/compound/` directory if it doesn't exist.
+Create the `~/.plannotator/hooks/compound/` directory if it doesn't exist.
 
 The file contents should be the corrective prompt instructions from Phase 3 —
 the same numbered list that appears in section 7 of the HTML report. Write them

packages/shared/improvement-hooks.test.ts

@@ -0,0 +1,135 @@
+/**
+ * Tests for improvement hook reader.
+ *
+ * Run: bun test packages/shared/improvement-hooks.test.ts
+ */
+
+import { describe, expect, test, beforeEach, afterEach } from "bun:test";
+import { mkdirSync, writeFileSync, rmSync, existsSync } from "fs";
+import { join } from "path";
+import { tmpdir } from "os";
+
+// We need to override the base dirs used by readImprovementHook.
+// Since the module uses homedir() at import time, we mock it via
+// a test harness that sets HOME to a temp directory.
+
+const TEST_HOME = join(tmpdir(), `improvement-hooks-test-${Date.now()}`);
+const NEW_BASE = join(TEST_HOME, ".plannotator", "hooks");
+const LEGACY_BASE = join(TEST_HOME, ".plannotator");
+const HOOK_RELATIVE = "compound/enterplanmode-improve-hook.txt";
+
+function setupTestHome() {
+  mkdirSync(join(NEW_BASE, "compound"), { recursive: true });
+  mkdirSync(join(LEGACY_BASE, "compound"), { recursive: true });
+}
+
+function cleanTestHome() {
+  if (existsSync(TEST_HOME)) {
+    rmSync(TEST_HOME, { recursive: true, force: true });
+  }
+}
+
+// Since the module reads homedir() at import time, we need to
+// re-import with HOME overridden. Use a helper that spawns a
+// small inline script to test each scenario in isolation.
+async function runScenario(setup: {
+  newPathContent?: string | null;
+  legacyPathContent?: string | null;
+}): Promise<{ content: string; filePath: string } | null> {
+  setupTestHome();
+
+  const newPath = join(NEW_BASE, HOOK_RELATIVE);
+  const legacyPath = join(LEGACY_BASE, HOOK_RELATIVE);
+
+  if (setup.newPathContent !== undefined && setup.newPathContent !== null) {
+    writeFileSync(newPath, setup.newPathContent);
+  }
+  if (setup.legacyPathContent !== undefined && setup.legacyPathContent !== null) {
+    writeFileSync(legacyPath, setup.legacyPathContent);
+  }
+
+  // Run in a subprocess with HOME overridden so homedir() returns TEST_HOME
+  const proc = Bun.spawn(
+    [
+      "bun",
+      "-e",
+      `
+      import { readImprovementHook } from "./packages/shared/improvement-hooks";
+      const result = readImprovementHook("enterplanmode-improve");
+      console.log(JSON.stringify(result));
+    `,
+    ],
+    {
+      env: { ...process.env, HOME: TEST_HOME },
+      cwd: join(import.meta.dir, "../.."),
+      stdout: "pipe",
+      stderr: "pipe",
+    },
+  );
+
+  const stdout = await new Response(proc.stdout).text();
+  const exitCode = await proc.exited;
+
+  if (exitCode !== 0) {
+    const stderr = await new Response(proc.stderr).text();
+    throw new Error(`Subprocess failed (exit ${exitCode}): ${stderr}`);
+  }
+
+  const parsed = JSON.parse(stdout.trim());
+  return parsed;
+}
+
+describe("readImprovementHook", () => {
+  beforeEach(setupTestHome);
+  afterEach(cleanTestHome);
+
+  test("returns content from new path when file exists", async () => {
+    const result = await runScenario({
+      newPathContent: "Focus on error handling",
+    });
+    expect(result).not.toBeNull();
+    expect(result!.content).toBe("Focus on error handling");
+    expect(result!.filePath).toContain(".plannotator/hooks/compound/");
+  });
+
+  test("new path wins over legacy path", async () => {
+    const result = await runScenario({
+      newPathContent: "New instructions",
+      legacyPathContent: "Old instructions",
+    });
+    expect(result).not.toBeNull();
+    expect(result!.content).toBe("New instructions");
+    expect(result!.filePath).toContain(".plannotator/hooks/compound/");
+  });
+
+  test("falls back to legacy path when new path is absent", async () => {
+    const result = await runScenario({
+      legacyPathContent: "Legacy instructions",
+    });
+    expect(result).not.toBeNull();
+    expect(result!.content).toBe("Legacy instructions");
+    expect(result!.filePath).toContain(".plannotator/compound/");
+    expect(result!.filePath).not.toContain(".plannotator/hooks/");
+  });
+
+  test("returns null when new path exists but is empty (no legacy fallback)", async () => {
+    const result = await runScenario({
+      newPathContent: "",
+      legacyPathContent: "Legacy instructions",
+    });
+    expect(result).toBeNull();
+  });
+
+  test("returns null when no files exist", async () => {
+    const result = await runScenario({});
+    expect(result).toBeNull();
+  });
+
+  test("returns null when new path is whitespace-only (no legacy fallback)", async () => {
+    const result = await runScenario({
+      newPathContent: "   \n  \n  ",
+      legacyPathContent: "Legacy instructions",
+    });
+    expect(result).toBeNull();
+  });
+});

packages/shared/improvement-hooks.ts

@@ -0,0 +1,105 @@
+/**
+ * Improvement Hook Reader
+ *
+ * Reads improvement hook files from ~/.plannotator/hooks/.
+ * Falls back to the legacy path (~/.plannotator/) when the new-path
+ * file is absent, for compatibility with files written before the
+ * path migration. If the new-path file exists but is invalid (empty,
+ * oversized, not a regular file), the legacy path is NOT consulted —
+ * this prevents resurrecting stale instructions.
+ *
+ * Runtime-agnostic: uses only node:fs, node:path, node:os.
+ *
+ * Security model:
+ * - Hardcoded base paths (no user input determines file path)
+ * - KNOWN_HOOKS allowlist (only pre-registered relative paths)
+ * - Size cap to prevent runaway context injection
+ * - Same trust model as ~/.plannotator/config.json
+ */
+
+import { homedir } from "os";
+import { join } from "path";
+import { readFileSync, statSync } from "fs";
+
+/** Base directory for hook-injectable files (new path) */
+const HOOKS_BASE_DIR = join(homedir(), ".plannotator", "hooks");
+
+/** Legacy base directory (pre-migration path) */
+const LEGACY_BASE_DIR = join(homedir(), ".plannotator");
+
+/** Maximum file size to read (50 KB) */
+const MAX_FILE_SIZE = 50 * 1024;
+
+/**
+ * Known improvement hook file paths, keyed by hook name.
+ * `path` is relative to HOOKS_BASE_DIR (~/.plannotator/hooks/).
+ * `legacyPath` is relative to LEGACY_BASE_DIR (~/.plannotator/).
+ */
+const KNOWN_HOOKS = {
+  "enterplanmode-improve": {
+    path: "compound/enterplanmode-improve-hook.txt",
+    legacyPath: "compound/enterplanmode-improve-hook.txt",
+  },
+} as const;
+
+export type ImprovementHookName = keyof typeof KNOWN_HOOKS;
+
+export interface ImprovementHookResult {
+  content: string;
+  hookName: ImprovementHookName;
+  filePath: string;
+}
+
+/** Check whether a path exists on disk (any file type). */
+function fileExists(path: string): boolean {
+  try {
+    statSync(path);
+    return true;
+  } catch {
+    return false;
+  }
+}
+
+/** Validate and read a hook file. Returns the result, or null if invalid. */
+function tryReadHookFile(
+  filePath: string,
+  hookName: ImprovementHookName,
+): ImprovementHookResult | null {
+  try {
+    const stat = statSync(filePath);
+    if (!stat.isFile() || stat.size === 0 || stat.size > MAX_FILE_SIZE) return null;
+
+    const content = readFileSync(filePath, "utf-8").trim();
+    if (!content) return null;
+
+    return { content, hookName, filePath };
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * Read an improvement hook file by name.
+ *
+ * Lookup order:
+ * 1. New path (HOOKS_BASE_DIR + path). If it exists and validates, return it.
+ * 2. If the new path exists but is invalid (empty, oversized, etc.), return null.
+ * 3. Only if the new path does not exist, try the legacy path (LEGACY_BASE_DIR + legacyPath).
+ */
+export function readImprovementHook(
+  hookName: ImprovementHookName,
+): ImprovementHookResult | null {
+  const entry = KNOWN_HOOKS[hookName];
+  if (!entry) return null;
+
+  const newPath = join(HOOKS_BASE_DIR, entry.path);
+
+  // New path exists — use it exclusively (even if invalid)
+  if (fileExists(newPath)) {
+    return tryReadHookFile(newPath, hookName);
+  }
+
+  // New path absent — fall back to legacy path
+  const legacyFilePath = join(LEGACY_BASE_DIR, entry.legacyPath);
+  return tryReadHookFile(legacyFilePath, hookName);
+}

packages/shared/package.json

@@ -21,6 +21,7 @@
     "./resolve-file": "./resolve-file.ts",
     "./external-annotation": "./external-annotation.ts",
     "./agent-jobs": "./agent-jobs.ts",
-    "./config": "./config.ts"
+    "./config": "./config.ts",
+    "./improvement-hooks": "./improvement-hooks.ts"
   }
 }