co-mono/packages/coding-agent/docs/hooks.md

# Hooks

Hooks are TypeScript modules that extend pi's behavior by subscribing to lifecycle events. They can intercept tool calls, prompt the user, modify results, inject messages, and more.

**Example use cases:**
- Block dangerous commands (permission gates for `rm -rf`, `sudo`)
- Checkpoint code state (git stash at each turn, restore on branch)
- Protect paths (block writes to `.env`, `node_modules/`)
- Inject messages from external sources (file watchers, webhooks)
- Custom slash commands and UI components

See [examples/hooks/](../examples/hooks/) for working implementations.

## Quick Start

Create `~/.pi/agent/hooks/my-hook.ts`:

```typescript
import type { HookAPI } from "@mariozechner/pi-coding-agent/hooks";

export default function (pi: HookAPI) {
  pi.on("session_start", async (_event, ctx) => {
    ctx.ui.notify("Hook loaded!", "info");
  });

  pi.on("tool_call", async (event, ctx) => {
    if (event.toolName === "bash" && event.input.command?.includes("rm -rf")) {
      const ok = await ctx.ui.confirm("Dangerous!", "Allow rm -rf?");
      if (!ok) return { block: true, reason: "Blocked by user" };
    }
  });
}
```

Test with `--hook` flag:

```bash
pi --hook ./my-hook.ts
```

## Hook Locations

Hooks are auto-discovered from:

| Location | Scope |
|----------|-------|
| `~/.pi/agent/hooks/*.ts` | Global (all projects) |
| `.pi/hooks/*.ts` | Project-local |

Additional paths via `settings.json`:

```json
{
  "hooks": ["/path/to/hook.ts"],
  "hookTimeout": 30000
}
```

The `hookTimeout` (default 30s) applies to most events. `tool_call` has no timeout since it may prompt the user.

## Available Imports

| Package | Purpose |
|---------|---------|
| `@mariozechner/pi-coding-agent/hooks` | Hook types (`HookAPI`, `HookContext`, events) |
| `@mariozechner/pi-coding-agent` | Additional types if needed |
| `@mariozechner/pi-ai` | AI utilities |
| `@mariozechner/pi-tui` | TUI components |

Node.js built-ins (`node:fs`, `node:path`, etc.) are also available.

## Writing a Hook

A hook exports a default function that receives `HookAPI`:

```typescript
import type { HookAPI } from "@mariozechner/pi-coding-agent/hooks";

export default function (pi: HookAPI) {
  // Subscribe to events
  pi.on("event_name", async (event, ctx) => {
    // Handle event
  });
}
```

Hooks are loaded via [jiti](https://github.com/unjs/jiti), so TypeScript works without compilation.

## Events

### Lifecycle Overview

```
pi starts
  │
  └─► session_start
      │
      ▼
user sends prompt ─────────────────────────────────────────┐
  │                                                        │
  ├─► before_agent_start (can inject message)              │
  ├─► agent_start                                          │
  │                                                        │
  │   ┌─── turn (repeats while LLM calls tools) ───┐       │
  │   │                                            │       │
  │   ├─► turn_start                               │       │
  │   ├─► context (can modify messages)            │       │
  │   │                                            │       │
  │   │   LLM responds, may call tools:            │       │
  │   │     ├─► tool_call (can block)              │       │
  │   │     │   tool executes                      │       │
  │   │     └─► tool_result (can modify)           │       │
  │   │                                            │       │
  │   └─► turn_end                                 │       │
  │                                                        │
  └─► agent_end                                            │
                                                           │
user sends another prompt ◄────────────────────────────────┘

/new (new session)
  ├─► session_before_new (can cancel)
  └─► session_new

/resume (switch session)
  ├─► session_before_switch (can cancel)
  └─► session_switch

/branch
  ├─► session_before_branch (can cancel)
  └─► session_branch

/compact or auto-compaction
  ├─► session_before_compact (can cancel or customize)
  └─► session_compact

/tree navigation
  ├─► session_before_tree (can cancel or customize)
  └─► session_tree

exit (Ctrl+C, Ctrl+D)
  └─► session_shutdown
```

### Session Events

#### session_start

Fired on initial session load.

```typescript
pi.on("session_start", async (_event, ctx) => {
  ctx.ui.notify(`Session: ${ctx.sessionManager.getSessionFile() ?? "ephemeral"}`, "info");
});
```

#### session_before_switch / session_switch

Fired when switching sessions via `/resume`.

```typescript
pi.on("session_before_switch", async (event, ctx) => {
  // event.targetSessionFile - session we're switching to
  return { cancel: true }; // Cancel the switch
});

pi.on("session_switch", async (event, ctx) => {
  // event.previousSessionFile - session we came from
});
```

#### session_before_new / session_new

Fired when starting a new session via `/new`.

```typescript
pi.on("session_before_new", async (_event, ctx) => {
  const ok = await ctx.ui.confirm("Clear?", "Delete all messages?");
  if (!ok) return { cancel: true };
});

pi.on("session_new", async (_event, ctx) => {
  // New session started
});
```

#### session_before_branch / session_branch

Fired when branching via `/branch`.

```typescript
pi.on("session_before_branch", async (event, ctx) => {
  // event.entryIndex - entry index being branched from
  
  return { cancel: true }; // Cancel branch
  // OR
  return { skipConversationRestore: true }; // Branch but don't rewind messages
});

pi.on("session_branch", async (event, ctx) => {
  // event.previousSessionFile - previous session file
});
```

The `skipConversationRestore` option is useful for checkpoint hooks that restore code state separately.

#### session_before_compact / session_compact

Fired on compaction. See [compaction.md](compaction.md) for details.

```typescript
pi.on("session_before_compact", async (event, ctx) => {
  const { preparation, branchEntries, customInstructions, signal } = event;
  
  // Cancel:
  return { cancel: true };
  
  // Custom summary:
  return {
    compaction: {
      summary: "...",
      firstKeptEntryId: preparation.firstKeptEntryId,
      tokensBefore: preparation.tokensBefore,
    }
  };
});

pi.on("session_compact", async (event, ctx) => {
  // event.compactionEntry - the saved compaction
  // event.fromHook - whether hook provided it
});
```

#### session_before_tree / session_tree

Fired on `/tree` navigation. See [compaction.md](compaction.md) for details.

```typescript
pi.on("session_before_tree", async (event, ctx) => {
  const { preparation, signal } = event;
  // preparation.targetId, oldLeafId, commonAncestorId, entriesToSummarize, userWantsSummary
  
  return { cancel: true };
  // OR (if userWantsSummary):
  return { summary: { summary: "...", details: {} } };
});

pi.on("session_tree", async (event, ctx) => {
  // event.newLeafId, oldLeafId, summaryEntry, fromHook
});
```

#### session_shutdown

Fired on exit (Ctrl+C, Ctrl+D, SIGTERM).

```typescript
pi.on("session_shutdown", async (_event, ctx) => {
  // Cleanup, save state, etc.
});
```

### Agent Events

#### before_agent_start

Fired after user submits prompt, before agent loop. Can inject a persistent message.

```typescript
pi.on("before_agent_start", async (event, ctx) => {
  // event.prompt - user's prompt text
  // event.images - attached images (if any)
  
  return {
    message: {
      customType: "my-hook",
      content: "Additional context for the LLM",
      display: true,  // Show in TUI
    }
  };
});
```

The injected message is persisted as `CustomMessageEntry` and sent to the LLM.

#### agent_start / agent_end

Fired once per user prompt.

```typescript
pi.on("agent_start", async (_event, ctx) => {});

pi.on("agent_end", async (event, ctx) => {
  // event.messages - messages from this prompt
});
```

#### turn_start / turn_end

Fired for each turn (one LLM response + tool calls).

```typescript
pi.on("turn_start", async (event, ctx) => {
  // event.turnIndex, event.timestamp
});

pi.on("turn_end", async (event, ctx) => {
  // event.turnIndex
  // event.message - assistant's response
  // event.toolResults - tool results from this turn
});
```

#### context

Fired before each LLM call. Modify messages non-destructively (session unchanged).

```typescript
pi.on("context", async (event, ctx) => {
  // event.messages - deep copy, safe to modify
  
  // Filter or transform messages
  const filtered = event.messages.filter(m => !shouldPrune(m));
  return { messages: filtered };
});
```

### Tool Events

#### tool_call

Fired before tool executes. **Can block.** No timeout.

```typescript
pi.on("tool_call", async (event, ctx) => {
  // event.toolName - "bash", "read", "write", "edit", etc.
  // event.toolCallId
  // event.input - tool parameters
  
  if (shouldBlock(event)) {
    return { block: true, reason: "Not allowed" };
  }
});
```

Tool inputs:
- `bash`: `{ command, timeout? }`
- `read`: `{ path, offset?, limit? }`
- `write`: `{ path, content }`
- `edit`: `{ path, oldText, newText }`
- `ls`: `{ path?, limit? }`
- `find`: `{ pattern, path?, limit? }`
- `grep`: `{ pattern, path?, glob?, ignoreCase?, literal?, context?, limit? }`

#### tool_result

Fired after tool executes. **Can modify result.**

```typescript
pi.on("tool_result", async (event, ctx) => {
  // event.toolName, event.toolCallId, event.input
  // event.content - array of TextContent | ImageContent
  // event.details - tool-specific (see below)
  // event.isError
  
  // Modify result:
  return { content: [...], details: {...}, isError: false };
});
```

Use type guards for typed details:

```typescript
import { isBashToolResult } from "@mariozechner/pi-coding-agent/hooks";

pi.on("tool_result", async (event, ctx) => {
  if (isBashToolResult(event)) {
    // event.details is BashToolDetails | undefined
    if (event.details?.truncation?.truncated) {
      // Full output at event.details.fullOutputPath
    }
  }
});
```

Available guards: `isBashToolResult`, `isReadToolResult`, `isEditToolResult`, `isWriteToolResult`, `isGrepToolResult`, `isFindToolResult`, `isLsToolResult`.

## HookContext

Every handler receives `ctx: HookContext`:

### ctx.ui

UI methods for user interaction:

```typescript
// Select from options
const choice = await ctx.ui.select("Pick one:", ["A", "B", "C"]);
// Returns selected string or undefined if cancelled

// Confirm dialog
const ok = await ctx.ui.confirm("Delete?", "This cannot be undone");
// Returns true or false

// Text input
const name = await ctx.ui.input("Name:", "placeholder");
// Returns string or undefined if cancelled

// Notification
ctx.ui.notify("Done!", "info");  // "info" | "warning" | "error"

// Custom component with keyboard focus
const handle = ctx.ui.custom(myComponent);
// Returns { close: () => void, requestRender: () => void }
// Component can implement handleInput(data: string) for keyboard
// Call handle.close() when done
```

### ctx.hasUI

`false` in print mode (`-p`) and RPC mode. Always check before using `ctx.ui`:

```typescript
if (ctx.hasUI) {
  const choice = await ctx.ui.select(...);
} else {
  // Default behavior
}
```

### ctx.cwd

Current working directory.

### ctx.sessionManager

Read-only access to session state:

```typescript
// Get all entries (excludes header)
const entries = ctx.sessionManager.getEntries();

// Get current branch (root to leaf)
const branch = ctx.sessionManager.getBranch();

// Get specific entry by ID
const entry = ctx.sessionManager.getEntry(id);

// Get session file (undefined with --no-session)
const file = ctx.sessionManager.getSessionFile();

// Get tree structure
const tree = ctx.sessionManager.getTree();

// Get entry label
const label = ctx.sessionManager.getLabel(entryId);
```

Use `pi.sendMessage()` or `pi.appendEntry()` for writes.

### ctx.modelRegistry

Access to models and API keys:

```typescript
// Get API key for a model
const apiKey = await ctx.modelRegistry.getApiKey(model);

// Get available models
const models = ctx.modelRegistry.getAvailableModels();
```

### ctx.model

Current model, or `undefined` if none selected yet. Use for LLM calls in hooks:

```typescript
if (ctx.model) {
  const apiKey = await ctx.modelRegistry.getApiKey(ctx.model);
  // Use with @mariozechner/pi-ai complete()
}
```

## HookAPI Methods

### pi.on(event, handler)

Subscribe to events. See [Events](#events) for all event types.

### pi.sendMessage(message, triggerTurn?)

Inject a message into the session. Creates `CustomMessageEntry` (participates in LLM context).

```typescript
pi.sendMessage({
  customType: "my-hook",      // Your hook's identifier
  content: "Message text",    // string or (TextContent | ImageContent)[]
  display: true,              // Show in TUI
  details: { ... },           // Optional metadata (not sent to LLM)
}, triggerTurn);              // If true, triggers LLM response
```

### pi.appendEntry(customType, data?)

Persist hook state. Creates `CustomEntry` (does NOT participate in LLM context).

```typescript
// Save state
pi.appendEntry("my-hook-state", { count: 42 });

// Restore on reload
pi.on("session_start", async (_event, ctx) => {
  for (const entry of ctx.sessionManager.getEntries()) {
    if (entry.type === "custom" && entry.customType === "my-hook-state") {
      // Reconstruct from entry.data
    }
  }
});
```

### pi.registerCommand(name, options)

Register a custom slash command:

```typescript
pi.registerCommand("stats", {
  description: "Show session statistics",
  handler: async (args, ctx) => {
    // args = everything after /stats
    const count = ctx.sessionManager.getEntries().length;
    ctx.ui.notify(`${count} entries`, "info");
  }
});
```

To trigger LLM after command, call `pi.sendMessage(..., true)`.

### pi.registerMessageRenderer(customType, renderer)

Custom TUI rendering for `CustomMessageEntry`:

```typescript
import { Text } from "@mariozechner/pi-tui";

pi.registerMessageRenderer("my-hook", (message, options, theme) => {
  // message.content, message.details
  // options.expanded (user pressed Ctrl+O)
  return new Text(theme.fg("accent", `[MY-HOOK] ${message.content}`), 0, 0);
});
```

### pi.exec(command, args, options?)

Execute a shell command:

```typescript
const result = await pi.exec("git", ["status"], {
  signal,      // AbortSignal
  timeout,     // Milliseconds
});

// result.stdout, result.stderr, result.code, result.killed
```

## Examples

### Permission Gate

```typescript
import type { HookAPI } from "@mariozechner/pi-coding-agent/hooks";

export default function (pi: HookAPI) {
  const dangerous = [/\brm\s+(-rf?|--recursive)/i, /\bsudo\b/i];

  pi.on("tool_call", async (event, ctx) => {
    if (event.toolName !== "bash") return;

    const cmd = event.input.command as string;
    if (dangerous.some(p => p.test(cmd))) {
      if (!ctx.hasUI) {
        return { block: true, reason: "Dangerous (no UI)" };
      }
      const ok = await ctx.ui.confirm("Dangerous!", `Allow: ${cmd}?`);
      if (!ok) return { block: true, reason: "Blocked by user" };
    }
  });
}
```

### Protected Paths

```typescript
import type { HookAPI } from "@mariozechner/pi-coding-agent/hooks";

export default function (pi: HookAPI) {
  const protectedPaths = [".env", ".git/", "node_modules/"];

  pi.on("tool_call", async (event, ctx) => {
    if (event.toolName !== "write" && event.toolName !== "edit") return;

    const path = event.input.path as string;
    if (protectedPaths.some(p => path.includes(p))) {
      ctx.ui.notify(`Blocked: ${path}`, "warning");
      return { block: true, reason: `Protected: ${path}` };
    }
  });
}
```

### Git Checkpoint

```typescript
import type { HookAPI } from "@mariozechner/pi-coding-agent/hooks";

export default function (pi: HookAPI) {
  const checkpoints = new Map<number, string>();

  pi.on("turn_start", async (event) => {
    const { stdout } = await pi.exec("git", ["stash", "create"]);
    if (stdout.trim()) checkpoints.set(event.turnIndex, stdout.trim());
  });

  pi.on("session_before_branch", async (event, ctx) => {
    const ref = checkpoints.get(event.entryIndex);
    if (!ref || !ctx.hasUI) return;

    const ok = await ctx.ui.confirm("Restore?", "Restore code to checkpoint?");
    if (ok) {
      await pi.exec("git", ["stash", "apply", ref]);
      ctx.ui.notify("Code restored", "info");
    }
  });

  pi.on("agent_end", () => checkpoints.clear());
}
```

### Custom Command

See [examples/hooks/snake.ts](../examples/hooks/snake.ts) for a complete example with `registerCommand()`, `ui.custom()`, and session persistence.

## Mode Behavior

| Mode | UI Methods | Notes |
|------|-----------|-------|
| Interactive | Full TUI | Normal operation |
| RPC | JSON protocol | Host handles UI |
| Print (`-p`) | No-op (returns null/false) | Hooks run but can't prompt |

In print mode, `select()` returns `undefined`, `confirm()` returns `false`, `input()` returns `undefined`. Design hooks to handle this.

## Error Handling

- Hook errors are logged, agent continues
- `tool_call` errors block the tool (fail-safe)
- Timeout errors (default 30s) are logged but don't block
- Errors display in UI with hook path and message

## Debugging

1. Open VS Code in hooks directory
2. Open JavaScript Debug Terminal (Ctrl+Shift+P → "JavaScript Debug Terminal")
3. Set breakpoints
4. Run `pi --hook ./my-hook.ts`