co-mono/packages/coding-agent/docs/custom-tools.md

# Custom Tools

Custom tools are additional tools that the LLM can call directly, just like the built-in `read`, `write`, `edit`, and `bash` tools. They are TypeScript modules that define callable functions with parameters, return values, and optional TUI rendering.

**Example use cases:**
- Ask the user questions with selectable options
- Maintain state across calls (todo lists, connection pools)
- Custom TUI rendering (progress indicators, structured output)
- Integrate external services with proper error handling
- Tools that need user confirmation before proceeding

**When to use custom tools vs. alternatives:**

| Need | Solution |
|------|----------|
| Always-needed context (conventions, commands) | AGENTS.md |
| User triggers a specific prompt template | Slash command |
| On-demand capability package (workflows, scripts, setup) | Skill |
| Additional tool directly callable by the LLM | **Custom tool** |

See [examples/custom-tools/](../examples/custom-tools/) for working examples.

## Quick Start

Create a file `~/.pi/agent/tools/hello.ts`:

```typescript
import { Type } from "@sinclair/typebox";
import type { CustomToolFactory } from "@mariozechner/pi-coding-agent";

const factory: CustomToolFactory = (pi) => ({
  name: "hello",
  label: "Hello",
  description: "A simple greeting tool",
  parameters: Type.Object({
    name: Type.String({ description: "Name to greet" }),
  }),

  async execute(toolCallId, params) {
    return {
      content: [{ type: "text", text: `Hello, ${params.name}!` }],
      details: { greeted: params.name },
    };
  },
});

export default factory;
```

The tool is automatically discovered and available in your next pi session.

## Tool Locations

| Location | Scope | Auto-discovered |
|----------|-------|-----------------|
| `~/.pi/agent/tools/*.ts` | Global (all projects) | Yes |
| `.pi/tools/*.ts` | Project-local | Yes |
| `settings.json` `customTools` array | Configured paths | Yes |
| `--tool <path>` CLI flag | One-off/debugging | No |

**Priority:** Later sources win on name conflicts. CLI `--tool` takes highest priority.

**Reserved names:** Custom tools cannot use built-in tool names (`read`, `write`, `edit`, `bash`, `grep`, `find`, `ls`).

## Available Imports

Custom tools can import from these packages (automatically resolved by pi):

| Package | Purpose |
|---------|---------|
| `@sinclair/typebox` | Schema definitions (`Type.Object`, `Type.String`, etc.) |
| `@mariozechner/pi-coding-agent` | Types (`CustomToolFactory`, `ToolSessionEvent`, etc.) |
| `@mariozechner/pi-ai` | AI utilities (`StringEnum` for Google-compatible enums) |
| `@mariozechner/pi-tui` | TUI components (`Text`, `Box`, etc. for custom rendering) |

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

## Tool Definition

```typescript
import { Type } from "@sinclair/typebox";
import { StringEnum } from "@mariozechner/pi-ai";
import { Text } from "@mariozechner/pi-tui";
import type { CustomToolFactory, ToolSessionEvent } from "@mariozechner/pi-coding-agent";

const factory: CustomToolFactory = (pi) => ({
  name: "my_tool",
  label: "My Tool",
  description: "What this tool does (be specific for LLM)",
  parameters: Type.Object({
    // Use StringEnum for string enums (Google API compatible)
    action: StringEnum(["list", "add", "remove"] as const),
    text: Type.Optional(Type.String()),
  }),

  async execute(toolCallId, params, signal, onUpdate) {
    // signal - AbortSignal for cancellation
    // onUpdate - Callback for streaming partial results
    return {
      content: [{ type: "text", text: "Result for LLM" }],
      details: { /* structured data for rendering */ },
    };
  },

  // Optional: Session lifecycle callback
  onSession(event) { /* reconstruct state from entries */ },

  // Optional: Custom rendering
  renderCall(args, theme) { /* return Component */ },
  renderResult(result, options, theme) { /* return Component */ },

  // Optional: Cleanup on session end
  dispose() { /* save state, close connections */ },
});

export default factory;
```

**Important:** Use `StringEnum` from `@mariozechner/pi-ai` instead of `Type.Union`/`Type.Literal` for string enums. The latter doesn't work with Google's API.

## ToolAPI Object

The factory receives a `ToolAPI` object (named `pi` by convention):

```typescript
interface ToolAPI {
  cwd: string;  // Current working directory
  exec(command: string, args: string[]): Promise<ExecResult>;
  ui: {
    select(title: string, options: string[]): Promise<string | null>;
    confirm(title: string, message: string): Promise<boolean>;
    input(title: string, placeholder?: string): Promise<string | null>;
    notify(message: string, type?: "info" | "warning" | "error"): void;
  };
  hasUI: boolean;  // false in --print or --mode rpc
}
```

Always check `pi.hasUI` before using UI methods.

## Session Lifecycle

Tools can implement `onSession` to react to session changes:

```typescript
interface ToolSessionEvent {
  entries: SessionEntry[];      // All session entries
  sessionFile: string | null;   // Current session file
  previousSessionFile: string | null;  // Previous session file
  reason: "start" | "switch" | "branch" | "clear";
}
```

**Reasons:**
- `start`: Initial session load on startup
- `switch`: User switched to a different session (`/session`)
- `branch`: User branched from a previous message (`/branch`)
- `clear`: User cleared the session (`/clear`)

### State Management Pattern

Tools that maintain state should store it in `details` of their results, not external files. This allows branching to work correctly, as the state is reconstructed from the session history.

```typescript
interface MyToolDetails {
  items: string[];
}

const factory: CustomToolFactory = (pi) => {
  // In-memory state
  let items: string[] = [];

  // Reconstruct state from session entries
  const reconstructState = (event: ToolSessionEvent) => {
    items = [];
    for (const entry of event.entries) {
      if (entry.type !== "message") continue;
      const msg = entry.message;
      if (msg.role !== "toolResult") continue;
      if (msg.toolName !== "my_tool") continue;
      
      const details = msg.details as MyToolDetails | undefined;
      if (details) {
        items = details.items;
      }
    }
  };

  return {
    name: "my_tool",
    label: "My Tool",
    description: "...",
    parameters: Type.Object({ ... }),
    
    onSession: reconstructState,
    
    async execute(toolCallId, params) {
      // Modify items...
      items.push("new item");
      
      return {
        content: [{ type: "text", text: "Added item" }],
        // Store current state in details for reconstruction
        details: { items: [...items] },
      };
    },
  };
};
```

This pattern ensures:
- When user branches, state is correct for that point in history
- When user switches sessions, state matches that session
- When user clears, state resets

## Custom Rendering

Custom tools can provide `renderCall` and `renderResult` methods to control how they appear in the TUI. Both are optional.

### How It Works

Tool output is wrapped in a `Box` component that handles:
- Padding (1 character horizontal, 1 line vertical)
- Background color based on state (pending/success/error)

Your render methods return `Component` instances (typically `Text`) that go inside this box. Use `Text(content, 0, 0)` since the Box handles padding.

### renderCall

Renders the tool call (before/during execution):

```typescript
renderCall(args, theme) {
  let text = theme.fg("toolTitle", theme.bold("my_tool "));
  text += theme.fg("muted", args.action);
  if (args.text) {
    text += " " + theme.fg("dim", `"${args.text}"`);
  }
  return new Text(text, 0, 0);
}
```

Called when:
- Tool call starts (may have partial args during streaming)
- Args are updated during streaming

### renderResult

Renders the tool result:

```typescript
renderResult(result, { expanded, isPartial }, theme) {
  const { details } = result;

  // Handle streaming/partial results
  if (isPartial) {
    return new Text(theme.fg("warning", "Processing..."), 0, 0);
  }

  // Handle errors
  if (details?.error) {
    return new Text(theme.fg("error", `Error: ${details.error}`), 0, 0);
  }

  // Normal result
  let text = theme.fg("success", "✓ ") + theme.fg("muted", "Done");
  
  // Support expanded view (Ctrl+O)
  if (expanded && details?.items) {
    for (const item of details.items) {
      text += "\n" + theme.fg("dim", `  ${item}`);
    }
  }

  return new Text(text, 0, 0);
}
```

**Options:**
- `expanded`: User pressed Ctrl+O to expand
- `isPartial`: Result is from `onUpdate` (streaming), not final

### Best Practices

1. **Use `Text` with padding `(0, 0)`** - The Box handles padding
2. **Use `\n` for multi-line content** - Not multiple Text components
3. **Handle `isPartial`** - Show progress during streaming
4. **Support `expanded`** - Show more detail when user requests
5. **Use theme colors** - For consistent appearance
6. **Keep it compact** - Show summary by default, details when expanded

### Theme Colors

```typescript
// Foreground
theme.fg("toolTitle", text)   // Tool names
theme.fg("accent", text)      // Highlights
theme.fg("success", text)     // Success
theme.fg("error", text)       // Errors
theme.fg("warning", text)     // Warnings
theme.fg("muted", text)       // Secondary text
theme.fg("dim", text)         // Tertiary text
theme.fg("toolOutput", text)  // Output content

// Styles
theme.bold(text)
theme.italic(text)
```

### Fallback Behavior

If `renderCall` or `renderResult` is not defined or throws an error:
- `renderCall`: Shows tool name
- `renderResult`: Shows raw text output from `content`

## Execute Function

```typescript
async execute(toolCallId, args, signal, onUpdate) {
  // Type assertion for params (TypeBox schema doesn't flow through)
  const params = args as { action: "list" | "add"; text?: string };

  // Check for abort
  if (signal?.aborted) {
    return { content: [...], details: { status: "aborted" } };
  }

  // Stream progress
  onUpdate?.({
    content: [{ type: "text", text: "Working..." }],
    details: { progress: 50 },
  });

  // Return final result
  return {
    content: [{ type: "text", text: "Done" }],  // Sent to LLM
    details: { data: result },  // For rendering only
  };
}
```

## Multiple Tools from One File

Return an array to share state between related tools:

```typescript
const factory: CustomToolFactory = (pi) => {
  // Shared state
  let connection = null;

  return [
    { name: "db_connect", ... },
    { name: "db_query", ... },
    {
      name: "db_close",
      dispose() { connection?.close(); }
    },
  ];
};
```

## Examples

See [`examples/custom-tools/todo.ts`](../examples/custom-tools/todo.ts) for a complete example with:
- `onSession` for state reconstruction
- Custom `renderCall` and `renderResult`
- Proper branching support via details storage

Test with:
```bash
pi --tool packages/coding-agent/examples/custom-tools/todo.ts
```