> pi can create extensions. Ask it to build one for your use case.

# Extensions

Extensions are TypeScript modules that extend pi's behavior. They can subscribe to lifecycle events, register custom tools callable by the LLM, add commands, and more.

> **Placement for /reload:** Put extensions in `~/.pi/agent/extensions/` (global) or `.pi/extensions/` (project-local) for auto-discovery. Use `pi -e ./path.ts` only for quick tests. Extensions in auto-discovered locations can be hot-reloaded with `/reload`.

**Key capabilities:**
- **Custom tools** - Register tools the LLM can call via `pi.registerTool()`
- **Event interception** - Block or modify tool calls, inject context, customize compaction
- **User interaction** - Prompt users via `ctx.ui` (select, confirm, input, notify)
- **Custom UI components** - Full TUI components with keyboard input via `ctx.ui.custom()` for complex interactions
- **Custom commands** - Register commands like `/mycommand` via `pi.registerCommand()`
- **Session persistence** - Store state that survives restarts via `pi.appendEntry()`
- **Custom rendering** - Control how tool calls/results and messages appear in TUI

**Example use cases:**
- Permission gates (confirm before `rm -rf`, `sudo`, etc.)
- Git checkpointing (stash at each turn, restore on branch)
- Path protection (block writes to `.env`, `node_modules/`)
- Custom compaction (summarize conversation your way)
- Conversation summaries (see `summarize.ts` example)
- Interactive tools (questions, wizards, custom dialogs)
- Stateful tools (todo lists, connection pools)
- External integrations (file watchers, webhooks, CI triggers)
- Games while you wait (see `snake.ts` example)

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

## Table of Contents

- [Quick Start](#quick-start)
- [Extension Locations](#extension-locations)
- [Available Imports](#available-imports)
- [Writing an Extension](#writing-an-extension)
  - [Extension Styles](#extension-styles)
- [Events](#events)
  - [Lifecycle Overview](#lifecycle-overview)
  - [Session Events](#session-events)
  - [Agent Events](#agent-events)
  - [Tool Events](#tool-events)
- [ExtensionContext](#extensioncontext)
- [ExtensionCommandContext](#extensioncommandcontext)
- [ExtensionAPI Methods](#extensionapi-methods)
- [State Management](#state-management)
- [Custom Tools](#custom-tools)
- [Custom UI](#custom-ui)
- [Error Handling](#error-handling)
- [Mode Behavior](#mode-behavior)
- [Examples Reference](#examples-reference)

## Quick Start

Create `~/.pi/agent/extensions/my-extension.ts`:

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

export default function (pi: ExtensionAPI) {
  // React to events
  pi.on("session_start", async (_event, ctx) => {
    ctx.ui.notify("Extension 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" };
    }
  });

  // Register a custom tool
  pi.registerTool({
    name: "greet",
    label: "Greet",
    description: "Greet someone by name",
    parameters: Type.Object({
      name: Type.String({ description: "Name to greet" }),
    }),
    async execute(toolCallId, params, signal, onUpdate, ctx) {
      return {
        content: [{ type: "text", text: `Hello, ${params.name}!` }],
        details: {},
      };
    },
  });

  // Register a command
  pi.registerCommand("hello", {
    description: "Say hello",
    handler: async (args, ctx) => {
      ctx.ui.notify(`Hello ${args || "world"}!`, "info");
    },
  });
}
```

Test with `--extension` (or `-e`) flag:

```bash
pi -e ./my-extension.ts
```

## Extension Locations

> **Security:** Extensions run with your full system permissions and can execute arbitrary code. Only install from sources you trust.

Extensions are auto-discovered from:

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

Additional paths via `settings.json`:

```json
{
  "packages": [
    "npm:@foo/bar@1.0.0",
    "git:github.com/user/repo@v1"
  ],
  "extensions": [
    "/path/to/local/extension.ts",
    "/path/to/local/extension/dir"
  ]
}
```

To share extensions via npm or git as pi packages, see [packages.md](packages.md).

## Available Imports

| Package | Purpose |
|---------|---------|
| `@stonega/pi-coding-agent` | Extension types (`ExtensionAPI`, `ExtensionContext`, events) |
| `@sinclair/typebox` | Schema definitions for tool parameters |
| `@stonega/pi-ai` | AI utilities (`StringEnum` for Google-compatible enums) |
| `@stonega/pi-tui` | TUI components for custom rendering |

npm dependencies work too. Add a `package.json` next to your extension (or in a parent directory), run `npm install`, and imports from `node_modules/` are resolved automatically.

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

## Writing an Extension

An extension exports a default function that receives `ExtensionAPI`:

```typescript
import type { ExtensionAPI } from "@stonega/pi-coding-agent";

export default function (pi: ExtensionAPI) {
  // Subscribe to events
  pi.on("event_name", async (event, ctx) => {
    // ctx.ui for user interaction
    const ok = await ctx.ui.confirm("Title", "Are you sure?");
    ctx.ui.notify("Done!", "success");
    ctx.ui.setStatus("my-ext", "Processing...");  // Footer status
    ctx.ui.setWidget("my-ext", ["Line 1", "Line 2"]);  // Widget above editor (default)
  });

  // Register tools, commands, shortcuts, flags
  pi.registerTool({ ... });
  pi.registerCommand("name", { ... });
  pi.registerShortcut("ctrl+x", { ... });
  pi.registerFlag("my-flag", { ... });
}
```

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

### Extension Styles

**Single file** - simplest, for small extensions:

```
~/.pi/agent/extensions/
└── my-extension.ts
```

**Directory with index.ts** - for multi-file extensions:

```
~/.pi/agent/extensions/
└── my-extension/
    ├── index.ts        # Entry point (exports default function)
    ├── tools.ts        # Helper module
    └── utils.ts        # Helper module
```

**Package with dependencies** - for extensions that need npm packages:

```
~/.pi/agent/extensions/
└── my-extension/
    ├── package.json    # Declares dependencies and entry points
    ├── package-lock.json
    ├── node_modules/   # After npm install
    └── src/
        └── index.ts
```

```json
// package.json
{
  "name": "my-extension",
  "dependencies": {
    "zod": "^3.0.0",
    "chalk": "^5.0.0"
  },
  "pi": {
    "extensions": ["./src/index.ts"]
  }
}
```

Run `npm install` in the extension directory, then imports from `node_modules/` work automatically.

## Events

### Lifecycle Overview

```
pi starts
  │
  └─► session_start
      │
      ▼
user sends prompt ─────────────────────────────────────────┐
  │                                                        │
  ├─► (extension commands checked first, bypass if found)  │
  ├─► input (can intercept, transform, or handle)          │
  ├─► (skill/template expansion if not handled)            │
  ├─► before_agent_start (can inject message, modify system prompt)
  ├─► agent_start                                          │
  ├─► message_start / message_update / message_end         │
  │                                                        │
  │   ┌─── turn (repeats while LLM calls tools) ───┐       │
  │   │                                            │       │
  │   ├─► turn_start                               │       │
  │   ├─► context (can modify messages)            │       │
  │   │                                            │       │
  │   │   LLM responds, may call tools:            │       │
  │   │     ├─► tool_call (can block)              │       │
  │   │     ├─► tool_execution_start               │       │
  │   │     ├─► tool_execution_update              │       │
  │   │     ├─► tool_execution_end                 │       │
  │   │     └─► tool_result (can modify)           │       │
  │   │                                            │       │
  │   └─► turn_end                                 │       │
  │                                                        │
  └─► agent_end                                            │
                                                           │
user sends another prompt ◄────────────────────────────────┘

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

/fork
  ├─► session_before_fork (can cancel)
  └─► session_fork

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

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

/model or Ctrl+P (model selection/cycling)
  └─► model_select

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

### Session Events

See [session.md](session.md) for session storage internals and the SessionManager API.

#### 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 starting a new session (`/new`) or switching sessions (`/resume`).

```typescript
pi.on("session_before_switch", async (event, ctx) => {
  // event.reason - "new" or "resume"
  // event.targetSessionFile - session we're switching to (only for "resume")

  if (event.reason === "new") {
    const ok = await ctx.ui.confirm("Clear?", "Delete all messages?");
    if (!ok) return { cancel: true };
  }
});

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

#### session_before_fork / session_fork

Fired when forking via `/fork`.

```typescript
pi.on("session_before_fork", async (event, ctx) => {
  // event.entryId - ID of the entry being forked from
  return { cancel: true }; // Cancel fork
  // OR
  return { skipConversationRestore: true }; // Fork but don't rewind messages
});

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

#### 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.fromExtension - whether extension provided it
});
```

#### session_before_tree / session_tree

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

```typescript
pi.on("session_before_tree", async (event, ctx) => {
  const { preparation, signal } = event;
  return { cancel: true };
  // OR provide custom summary:
  return { summary: { summary: "...", details: {} } };
});

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

#### 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 message and/or modify the system prompt.

```typescript
pi.on("before_agent_start", async (event, ctx) => {
  // event.prompt - user's prompt text
  // event.images - attached images (if any)
  // event.systemPrompt - current system prompt

  return {
    // Inject a persistent message (stored in session, sent to LLM)
    message: {
      customType: "my-extension",
      content: "Additional context for the LLM",
      display: true,
    },
    // Replace the system prompt for this turn (chained across extensions)
    systemPrompt: event.systemPrompt + "\n\nExtra instructions for this turn...",
  };
});
```

#### 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, event.toolResults
});
```

#### message_start / message_update / message_end

Fired for message lifecycle updates.

- `message_start` and `message_end` fire for user, assistant, and toolResult messages.
- `message_update` fires for assistant streaming updates.

```typescript
pi.on("message_start", async (event, ctx) => {
  // event.message
});

pi.on("message_update", async (event, ctx) => {
  // event.message
  // event.assistantMessageEvent (token-by-token stream event)
});

pi.on("message_end", async (event, ctx) => {
  // event.message
});
```

#### tool_execution_start / tool_execution_update / tool_execution_end

Fired for tool execution lifecycle updates.

```typescript
pi.on("tool_execution_start", async (event, ctx) => {
  // event.toolCallId, event.toolName, event.args
});

pi.on("tool_execution_update", async (event, ctx) => {
  // event.toolCallId, event.toolName, event.args, event.partialResult
});

pi.on("tool_execution_end", async (event, ctx) => {
  // event.toolCallId, event.toolName, event.result, event.isError
});
```

#### context

Fired before each LLM call. Modify messages non-destructively. See [session.md](session.md) for message types.

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

### Model Events

#### model_select

Fired when the model changes via `/model` command, model cycling (`Ctrl+P`), or session restore.

```typescript
pi.on("model_select", async (event, ctx) => {
  // event.model - newly selected model
  // event.previousModel - previous model (undefined if first selection)
  // event.source - "set" | "cycle" | "restore"

  const prev = event.previousModel
    ? `${event.previousModel.provider}/${event.previousModel.id}`
    : "none";
  const next = `${event.model.provider}/${event.model.id}`;

  ctx.ui.notify(`Model changed (${event.source}): ${prev} -> ${next}`, "info");
});
```

Use this to update UI elements (status bars, footers) or perform model-specific initialization when the active model changes.

### Tool Events

#### tool_call

Fired before tool executes. **Can block.** Use `isToolCallEventType` to narrow and get typed inputs.

```typescript
import { isToolCallEventType } from "@stonega/pi-coding-agent";

pi.on("tool_call", async (event, ctx) => {
  // event.toolName - "bash", "read", "write", "edit", etc.
  // event.toolCallId
  // event.input - tool parameters

  // Built-in tools: no type params needed
  if (isToolCallEventType("bash", event)) {
    // event.input is { command: string; timeout?: number }
    if (event.input.command.includes("rm -rf")) {
      return { block: true, reason: "Dangerous command" };
    }
  }

  if (isToolCallEventType("read", event)) {
    // event.input is { path: string; offset?: number; limit?: number }
    console.log(`Reading: ${event.input.path}`);
  }
});
```

#### Typing custom tool input

Custom tools should export their input type:

```typescript
// my-extension.ts
export type MyToolInput = Static<typeof myToolSchema>;
```

Use `isToolCallEventType` with explicit type parameters:

```typescript
import { isToolCallEventType } from "@stonega/pi-coding-agent";
import type { MyToolInput } from "my-extension";

pi.on("tool_call", (event) => {
  if (isToolCallEventType<"my_tool", MyToolInput>("my_tool", event)) {
    event.input.action;  // typed
  }
});
```

#### tool_result

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

`tool_result` handlers chain like middleware:
- Handlers run in extension load order
- Each handler sees the latest result after previous handler changes
- Handlers can return partial patches (`content`, `details`, or `isError`); omitted fields keep their current values

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

pi.on("tool_result", async (event, ctx) => {
  // event.toolName, event.toolCallId, event.input
  // event.content, event.details, event.isError

  if (isBashToolResult(event)) {
    // event.details is typed as BashToolDetails
  }

  // Modify result:
  return { content: [...], details: {...}, isError: false };
});
```

### User Bash Events

#### user_bash

Fired when user executes `!` or `!!` commands. **Can intercept.**

```typescript
pi.on("user_bash", (event, ctx) => {
  // event.command - the bash command
  // event.excludeFromContext - true if !! prefix
  // event.cwd - working directory

  // Option 1: Provide custom operations (e.g., SSH)
  return { operations: remoteBashOps };

  // Option 2: Full replacement - return result directly
  return { result: { output: "...", exitCode: 0, cancelled: false, truncated: false } };
});
```

### Input Events

#### input

Fired when user input is received, after extension commands are checked but before skill and template expansion. The event sees the raw input text, so `/skill:foo` and `/template` are not yet expanded.

**Processing order:**
1. Extension commands (`/cmd`) checked first - if found, handler runs and input event is skipped
2. `input` event fires - can intercept, transform, or handle
3. If not handled: skill commands (`/skill:name`) expanded to skill content
4. If not handled: prompt templates (`/template`) expanded to template content
5. Agent processing begins (`before_agent_start`, etc.)

```typescript
pi.on("input", async (event, ctx) => {
  // event.text - raw input (before skill/template expansion)
  // event.images - attached images, if any
  // event.source - "interactive" (typed), "rpc" (API), or "extension" (via sendUserMessage)

  // Transform: rewrite input before expansion
  if (event.text.startsWith("?quick "))
    return { action: "transform", text: `Respond briefly: ${event.text.slice(7)}` };

  // Handle: respond without LLM (extension shows its own feedback)
  if (event.text === "ping") {
    ctx.ui.notify("pong", "info");
    return { action: "handled" };
  }

  // Route by source: skip processing for extension-injected messages
  if (event.source === "extension") return { action: "continue" };

  // Intercept skill commands before expansion
  if (event.text.startsWith("/skill:")) {
    // Could transform, block, or let pass through
  }

  return { action: "continue" };  // Default: pass through to expansion
});
```

**Results:**
- `continue` - pass through unchanged (default if handler returns nothing)
- `transform` - modify text/images, then continue to expansion
- `handled` - skip agent entirely (first handler to return this wins)

Transforms chain across handlers. See [input-transform.ts](../examples/extensions/input-transform.ts).

## ExtensionContext

Every handler receives `ctx: ExtensionContext`:

### ctx.ui

UI methods for user interaction. See [Custom UI](#custom-ui) for full details.

### ctx.hasUI

`false` in print mode (`-p`) and JSON mode. `true` in interactive and RPC mode. In RPC mode, dialog methods (`select`, `confirm`, `input`, `editor`) work via the extension UI sub-protocol, and fire-and-forget methods (`notify`, `setStatus`, `setWidget`, `setTitle`, `setEditorText`) emit requests to the client. Some TUI-specific methods are no-ops or return defaults (see [rpc.md](rpc.md#extension-ui-protocol)).

### ctx.cwd

Current working directory.

### ctx.sessionManager

Read-only access to session state. See [session.md](session.md) for the full SessionManager API and entry types.

```typescript
ctx.sessionManager.getEntries()       // All entries
ctx.sessionManager.getBranch()        // Current branch
ctx.sessionManager.getLeafId()        // Current leaf entry ID
```

### ctx.modelRegistry / ctx.model

Access to models and API keys.

### ctx.isIdle() / ctx.abort() / ctx.hasPendingMessages()

Control flow helpers.

### ctx.shutdown()

Request a graceful shutdown of pi.

- **Interactive mode:** Deferred until the agent becomes idle (after processing all queued steering and follow-up messages).
- **RPC mode:** Deferred until the next idle state (after completing the current command response, when waiting for the next command).
- **Print mode:** No-op. The process exits automatically when all prompts are processed.

Emits `session_shutdown` event to all extensions before exiting. Available in all contexts (event handlers, tools, commands, shortcuts).

```typescript
pi.on("tool_call", (event, ctx) => {
  if (isFatal(event.input)) {
    ctx.shutdown();
  }
});
```

### ctx.getContextUsage()

Returns current context usage for the active model. Uses last assistant usage when available, then estimates tokens for trailing messages.

```typescript
const usage = ctx.getContextUsage();
if (usage && usage.tokens > 100_000) {
  // ...
}
```

### ctx.compact()

Trigger compaction without awaiting completion. Use `onComplete` and `onError` for follow-up actions.

```typescript
ctx.compact({
  customInstructions: "Focus on recent changes",
  onComplete: (result) => {
    ctx.ui.notify("Compaction completed", "info");
  },
  onError: (error) => {
    ctx.ui.notify(`Compaction failed: ${error.message}`, "error");
  },
});
```

### ctx.getSystemPrompt()

Returns the current effective system prompt. This includes any modifications made by `before_agent_start` handlers for the current turn.

```typescript
pi.on("before_agent_start", (event, ctx) => {
  const prompt = ctx.getSystemPrompt();
  console.log(`System prompt length: ${prompt.length}`);
});
```

## ExtensionCommandContext

Command handlers receive `ExtensionCommandContext`, which extends `ExtensionContext` with session control methods. These are only available in commands because they can deadlock if called from event handlers.

### ctx.waitForIdle()

Wait for the agent to finish streaming:

```typescript
pi.registerCommand("my-cmd", {
  handler: async (args, ctx) => {
    await ctx.waitForIdle();
    // Agent is now idle, safe to modify session
  },
});
```

### ctx.newSession(options?)

Create a new session:

```typescript
const result = await ctx.newSession({
  parentSession: ctx.sessionManager.getSessionFile(),
  setup: async (sm) => {
    sm.appendMessage({
      role: "user",
      content: [{ type: "text", text: "Context from previous session..." }],
      timestamp: Date.now(),
    });
  },
});

if (result.cancelled) {
  // An extension cancelled the new session
}
```

### ctx.fork(entryId)

Fork from a specific entry, creating a new session file:

```typescript
const result = await ctx.fork("entry-id-123");
if (!result.cancelled) {
  // Now in the forked session
}
```

### ctx.navigateTree(targetId, options?)

Navigate to a different point in the session tree:

```typescript
const result = await ctx.navigateTree("entry-id-456", {
  summarize: true,
  customInstructions: "Focus on error handling changes",
  replaceInstructions: false, // true = replace default prompt entirely
  label: "review-checkpoint",
});
```

Options:
- `summarize`: Whether to generate a summary of the abandoned branch
- `customInstructions`: Custom instructions for the summarizer
- `replaceInstructions`: If true, `customInstructions` replaces the default prompt instead of being appended
- `label`: Label to attach to the branch summary entry (or target entry if not summarizing)

### ctx.reload()

Run the same reload flow as `/reload`.

```typescript
pi.registerCommand("reload-runtime", {
  description: "Reload extensions, skills, prompts, and themes",
  handler: async (_args, ctx) => {
    await ctx.reload();
    return;
  },
});
```

Important behavior:
- `await ctx.reload()` emits `session_shutdown` for the current extension runtime
- It then reloads resources and emits `session_start` (and `resources_discover` with reason `"reload"`) for the new runtime
- The currently running command handler still continues in the old call frame
- Code after `await ctx.reload()` still runs from the pre-reload version
- Code after `await ctx.reload()` must not assume old in-memory extension state is still valid
- After the handler returns, future commands/events/tool calls use the new extension version

For predictable behavior, treat reload as terminal for that handler (`await ctx.reload(); return;`).

Tools run with `ExtensionContext`, so they cannot call `ctx.reload()` directly. Use a command as the reload entrypoint, then expose a tool that queues that command as a follow-up user message.

Example tool the LLM can call to trigger reload:

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

export default function (pi: ExtensionAPI) {
  pi.registerCommand("reload-runtime", {
    description: "Reload extensions, skills, prompts, and themes",
    handler: async (_args, ctx) => {
      await ctx.reload();
      return;
    },
  });

  pi.registerTool({
    name: "reload_runtime",
    label: "Reload Runtime",
    description: "Reload extensions, skills, prompts, and themes",
    parameters: Type.Object({}),
    async execute() {
      pi.sendUserMessage("/reload-runtime", { deliverAs: "followUp" });
      return {
        content: [{ type: "text", text: "Queued /reload-runtime as a follow-up command." }],
      };
    },
  });
}
```

## ExtensionAPI Methods

### pi.on(event, handler)

Subscribe to events. See [Events](#events) for event types and return values.

### pi.registerTool(definition)

Register a custom tool callable by the LLM. See [Custom Tools](#custom-tools) for full details.

```typescript
import { Type } from "@sinclair/typebox";
import { StringEnum } from "@stonega/pi-ai";

pi.registerTool({
  name: "my_tool",
  label: "My Tool",
  description: "What this tool does",
  parameters: Type.Object({
    action: StringEnum(["list", "add"] as const),
    text: Type.Optional(Type.String()),
  }),

  async execute(toolCallId, params, signal, onUpdate, ctx) {
    // Stream progress
    onUpdate?.({ content: [{ type: "text", text: "Working..." }] });

    return {
      content: [{ type: "text", text: "Done" }],
      details: { result: "..." },
    };
  },

  // Optional: Custom rendering
  renderCall(args, theme) { ... },
  renderResult(result, options, theme) { ... },
});
```

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

Inject a custom message into the session.

```typescript
pi.sendMessage({
  customType: "my-extension",
  content: "Message text",
  display: true,
  details: { ... },
}, {
  triggerTurn: true,
  deliverAs: "steer",
});
```

**Options:**
- `deliverAs` - Delivery mode:
  - `"steer"` (default) - Interrupts streaming. Delivered after current tool finishes, remaining tools skipped.
  - `"followUp"` - Waits for agent to finish. Delivered only when agent has no more tool calls.
  - `"nextTurn"` - Queued for next user prompt. Does not interrupt or trigger anything.
- `triggerTurn: true` - If agent is idle, trigger an LLM response immediately. Only applies to `"steer"` and `"followUp"` modes (ignored for `"nextTurn"`).

### pi.sendUserMessage(content, options?)

Send a user message to the agent. Unlike `sendMessage()` which sends custom messages, this sends an actual user message that appears as if typed by the user. Always triggers a turn.

```typescript
// Simple text message
pi.sendUserMessage("What is 2+2?");

// With content array (text + images)
pi.sendUserMessage([
  { type: "text", text: "Describe this image:" },
  { type: "image", source: { type: "base64", mediaType: "image/png", data: "..." } },
]);

// During streaming - must specify delivery mode
pi.sendUserMessage("Focus on error handling", { deliverAs: "steer" });
pi.sendUserMessage("And then summarize", { deliverAs: "followUp" });
```

**Options:**
- `deliverAs` - Required when agent is streaming:
  - `"steer"` - Interrupts after current tool, remaining tools skipped
  - `"followUp"` - Waits for agent to finish all tools

When not streaming, the message is sent immediately and triggers a new turn. When streaming without `deliverAs`, throws an error.

See [send-user-message.ts](../examples/extensions/send-user-message.ts) for a complete example.

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

Persist extension state (does NOT participate in LLM context).

```typescript
pi.appendEntry("my-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-state") {
      // Reconstruct from entry.data
    }
  }
});
```

### pi.setSessionName(name)

Set the session display name (shown in session selector instead of first message).

```typescript
pi.setSessionName("Refactor auth module");
```

### pi.getSessionName()

Get the current session name, if set.

```typescript
const name = pi.getSessionName();
if (name) {
  console.log(`Session: ${name}`);
}
```

### pi.setLabel(entryId, label)

Set or clear a label on an entry. Labels are user-defined markers for bookmarking and navigation (shown in `/tree` selector).

```typescript
// Set a label
pi.setLabel(entryId, "checkpoint-before-refactor");

// Clear a label
pi.setLabel(entryId, undefined);

// Read labels via sessionManager
const label = ctx.sessionManager.getLabel(entryId);
```

Labels persist in the session and survive restarts. Use them to mark important points (turns, checkpoints) in the conversation tree.

### pi.registerCommand(name, options)

Register a command.

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

Optional: add argument auto-completion for `/command ...`:

```typescript
import type { AutocompleteItem } from "@stonega/pi-tui";

pi.registerCommand("deploy", {
  description: "Deploy to an environment",
  getArgumentCompletions: (prefix: string): AutocompleteItem[] | null => {
    const envs = ["dev", "staging", "prod"];
    const items = envs.map((e) => ({ value: e, label: e }));
    const filtered = items.filter((i) => i.value.startsWith(prefix));
    return filtered.length > 0 ? filtered : null;
  },
  handler: async (args, ctx) => {
    ctx.ui.notify(`Deploying: ${args}`, "info");
  },
});
```

### pi.getCommands()

Get the slash commands available for invocation via `prompt` in the current session. Includes extension commands, prompt templates, and skill commands.
The list matches the RPC `get_commands` ordering: extensions first, then templates, then skills.

```typescript
const commands = pi.getCommands();
const bySource = commands.filter((command) => command.source === "extension");
```

Each entry has this shape:

```typescript
{
  name: string; // Command name without the leading slash
  description?: string;
  source: "extension" | "prompt" | "skill";
  location?: "user" | "project" | "path"; // For templates and skills
  path?: string; // Files backing templates, skills, and extensions
}
```

Built-in interactive commands (like `/model` and `/settings`) are not included here. They are handled only in interactive
mode and would not execute if sent via `prompt`.

### pi.registerMessageRenderer(customType, renderer)

Register a custom TUI renderer for messages with your `customType`. See [Custom UI](#custom-ui).

### pi.registerShortcut(shortcut, options)

Register a keyboard shortcut. See [keybindings.md](keybindings.md) for the shortcut format and built-in keybindings.

```typescript
pi.registerShortcut("ctrl+shift+p", {
  description: "Toggle plan mode",
  handler: async (ctx) => {
    ctx.ui.notify("Toggled!");
  },
});
```

### pi.registerFlag(name, options)

Register a CLI flag.

```typescript
pi.registerFlag("plan", {
  description: "Start in plan mode",
  type: "boolean",
  default: false,
});

// Check value
if (pi.getFlag("--plan")) {
  // Plan mode enabled
}
```

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

Execute a shell command.

```typescript
const result = await pi.exec("git", ["status"], { signal, timeout: 5000 });
// result.stdout, result.stderr, result.code, result.killed
```

### pi.getActiveTools() / pi.getAllTools() / pi.setActiveTools(names)

Manage active tools.

```typescript
const active = pi.getActiveTools();  // ["read", "bash", "edit", "write"]
const all = pi.getAllTools();        // [{ name: "read", description: "Read file contents..." }, ...]
const names = all.map(t => t.name);  // Just names if needed
pi.setActiveTools(["read", "bash"]); // Switch to read-only
```

### pi.setModel(model)

Set the current model. Returns `false` if no API key is available for the model. See [models.md](models.md) for configuring custom models.

```typescript
const model = ctx.modelRegistry.find("anthropic", "claude-sonnet-4-5");
if (model) {
  const success = await pi.setModel(model);
  if (!success) {
    ctx.ui.notify("No API key for this model", "error");
  }
}
```

### pi.getThinkingLevel() / pi.setThinkingLevel(level)

Get or set the thinking level. Level is clamped to model capabilities (non-reasoning models always use "off").

```typescript
const current = pi.getThinkingLevel();  // "off" | "minimal" | "low" | "medium" | "high" | "xhigh"
pi.setThinkingLevel("high");
```

### pi.events

Shared event bus for communication between extensions:

```typescript
pi.events.on("my:event", (data) => { ... });
pi.events.emit("my:event", { ... });
```

### pi.registerProvider(name, config)

Register or override a model provider dynamically. Useful for proxies, custom endpoints, or team-wide model configurations.

```typescript
// Register a new provider with custom models
pi.registerProvider("my-proxy", {
  baseUrl: "https://proxy.example.com",
  apiKey: "PROXY_API_KEY",  // env var name or literal
  api: "anthropic-messages",
  models: [
    {
      id: "claude-sonnet-4-20250514",
      name: "Claude 4 Sonnet (proxy)",
      reasoning: false,
      input: ["text", "image"],
      cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
      contextWindow: 200000,
      maxTokens: 16384
    }
  ]
});

// Override baseUrl for an existing provider (keeps all models)
pi.registerProvider("anthropic", {
  baseUrl: "https://proxy.example.com"
});

// Register provider with OAuth support for /login
pi.registerProvider("corporate-ai", {
  baseUrl: "https://ai.corp.com",
  api: "openai-responses",
  models: [...],
  oauth: {
    name: "Corporate AI (SSO)",
    async login(callbacks) {
      // Custom OAuth flow
      callbacks.onAuth({ url: "https://sso.corp.com/..." });
      const code = await callbacks.onPrompt({ message: "Enter code:" });
      return { refresh: code, access: code, expires: Date.now() + 3600000 };
    },
    async refreshToken(credentials) {
      // Refresh logic
      return credentials;
    },
    getApiKey(credentials) {
      return credentials.access;
    }
  }
});
```

**Config options:**
- `baseUrl` - API endpoint URL. Required when defining models.
- `apiKey` - API key or environment variable name. Required when defining models (unless `oauth` provided).
- `api` - API type: `"anthropic-messages"`, `"openai-completions"`, `"openai-responses"`, etc.
- `headers` - Custom headers to include in requests.
- `authHeader` - If true, adds `Authorization: Bearer` header automatically.
- `models` - Array of model definitions. If provided, replaces all existing models for this provider.
- `oauth` - OAuth provider config for `/login` support. When provided, the provider appears in the login menu.
- `streamSimple` - Custom streaming implementation for non-standard APIs.

See [custom-provider.md](custom-provider.md) for advanced topics: custom streaming APIs, OAuth details, model definition reference.

## State Management

Extensions with state should store it in tool result `details` for proper branching support:

```typescript
export default function (pi: ExtensionAPI) {
  let items: string[] = [];

  // Reconstruct state from session
  pi.on("session_start", async (_event, ctx) => {
    items = [];
    for (const entry of ctx.sessionManager.getBranch()) {
      if (entry.type === "message" && entry.message.role === "toolResult") {
        if (entry.message.toolName === "my_tool") {
          items = entry.message.details?.items ?? [];
        }
      }
    }
  });

  pi.registerTool({
    name: "my_tool",
    // ...
    async execute(toolCallId, params, signal, onUpdate, ctx) {
      items.push("new item");
      return {
        content: [{ type: "text", text: "Added" }],
        details: { items: [...items] },  // Store for reconstruction
      };
    },
  });
}
```

## Custom Tools

Register tools the LLM can call via `pi.registerTool()`. Tools appear in the system prompt and can have custom rendering.

Note: Some models are idiots and include the @ prefix in tool path arguments. Built-in tools strip a leading @ before resolving paths. If your custom tool accepts a path, normalize a leading @ as well.

### Tool Definition

```typescript
import { Type } from "@sinclair/typebox";
import { StringEnum } from "@stonega/pi-ai";
import { Text } from "@stonega/pi-tui";

pi.registerTool({
  name: "my_tool",
  label: "My Tool",
  description: "What this tool does (shown to LLM)",
  parameters: Type.Object({
    action: StringEnum(["list", "add"] as const),  // Use StringEnum for Google compatibility
    text: Type.Optional(Type.String()),
  }),

  async execute(toolCallId, params, signal, onUpdate, ctx) {
    // Check for cancellation
    if (signal?.aborted) {
      return { content: [{ type: "text", text: "Cancelled" }] };
    }

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

    // Run commands via pi.exec (captured from extension closure)
    const result = await pi.exec("some-command", [], { signal });

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

  // Optional: Custom rendering
  renderCall(args, theme) { ... },
  renderResult(result, options, theme) { ... },
});
```

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

### Overriding Built-in Tools

Extensions can override built-in tools (`read`, `bash`, `edit`, `write`, `grep`, `find`, `ls`) by registering a tool with the same name. Interactive mode displays a warning when this happens.

```bash
# Extension's read tool replaces built-in read
pi -e ./tool-override.ts
```

Alternatively, use `--no-tools` to start without any built-in tools:
```bash
# No built-in tools, only extension tools
pi --no-tools -e ./my-extension.ts
```

See [examples/extensions/tool-override.ts](../examples/extensions/tool-override.ts) for a complete example that overrides `read` with logging and access control.

**Rendering:** If your override doesn't provide custom `renderCall`/`renderResult` functions, the built-in renderer is used automatically (syntax highlighting, diffs, etc.). This lets you wrap built-in tools for logging or access control without reimplementing the UI.

**Your implementation must match the exact result shape**, including the `details` type. The UI and session logic depend on these shapes for rendering and state tracking.

Built-in tool implementations:
- [read.ts](https://github.com/badlogic/pi-mono/blob/main/packages/coding-agent/src/core/tools/read.ts) - `ReadToolDetails`
- [bash.ts](https://github.com/badlogic/pi-mono/blob/main/packages/coding-agent/src/core/tools/bash.ts) - `BashToolDetails`
- [edit.ts](https://github.com/badlogic/pi-mono/blob/main/packages/coding-agent/src/core/tools/edit.ts)
- [write.ts](https://github.com/badlogic/pi-mono/blob/main/packages/coding-agent/src/core/tools/write.ts)
- [grep.ts](https://github.com/badlogic/pi-mono/blob/main/packages/coding-agent/src/core/tools/grep.ts) - `GrepToolDetails`
- [find.ts](https://github.com/badlogic/pi-mono/blob/main/packages/coding-agent/src/core/tools/find.ts) - `FindToolDetails`
- [ls.ts](https://github.com/badlogic/pi-mono/blob/main/packages/coding-agent/src/core/tools/ls.ts) - `LsToolDetails`

### Remote Execution

Built-in tools support pluggable operations for delegating to remote systems (SSH, containers, etc.):

```typescript
import { createReadTool, createBashTool, type ReadOperations } from "@stonega/pi-coding-agent";

// Create tool with custom operations
const remoteRead = createReadTool(cwd, {
  operations: {
    readFile: (path) => sshExec(remote, `cat ${path}`),
    access: (path) => sshExec(remote, `test -r ${path}`).then(() => {}),
  }
});

// Register, checking flag at execution time
pi.registerTool({
  ...remoteRead,
  async execute(id, params, signal, onUpdate, _ctx) {
    const ssh = getSshConfig();
    if (ssh) {
      const tool = createReadTool(cwd, { operations: createRemoteOps(ssh) });
      return tool.execute(id, params, signal, onUpdate);
    }
    return localRead.execute(id, params, signal, onUpdate);
  },
});
```

**Operations interfaces:** `ReadOperations`, `WriteOperations`, `EditOperations`, `BashOperations`, `LsOperations`, `GrepOperations`, `FindOperations`

The bash tool also supports a spawn hook to adjust the command, cwd, or env before execution:

```typescript
import { createBashTool } from "@stonega/pi-coding-agent";

const bashTool = createBashTool(cwd, {
  spawnHook: ({ command, cwd, env }) => ({
    command: `source ~/.profile\n${command}`,
    cwd: `/mnt/sandbox${cwd}`,
    env: { ...env, CI: "1" },
  }),
});
```

See [examples/extensions/ssh.ts](../examples/extensions/ssh.ts) for a complete SSH example with `--ssh` flag.

### Output Truncation

**Tools MUST truncate their output** to avoid overwhelming the LLM context. Large outputs can cause:
- Context overflow errors (prompt too long)
- Compaction failures
- Degraded model performance

The built-in limit is **50KB** (~10k tokens) and **2000 lines**, whichever is hit first. Use the exported truncation utilities:

```typescript
import {
  truncateHead,      // Keep first N lines/bytes (good for file reads, search results)
  truncateTail,      // Keep last N lines/bytes (good for logs, command output)
  truncateLine,      // Truncate a single line to maxBytes with ellipsis
  formatSize,        // Human-readable size (e.g., "50KB", "1.5MB")
  DEFAULT_MAX_BYTES, // 50KB
  DEFAULT_MAX_LINES, // 2000
} from "@stonega/pi-coding-agent";

async execute(toolCallId, params, signal, onUpdate, ctx) {
  const output = await runCommand();

  // Apply truncation
  const truncation = truncateHead(output, {
    maxLines: DEFAULT_MAX_LINES,
    maxBytes: DEFAULT_MAX_BYTES,
  });

  let result = truncation.content;

  if (truncation.truncated) {
    // Write full output to temp file
    const tempFile = writeTempFile(output);

    // Inform the LLM where to find complete output
    result += `\n\n[Output truncated: ${truncation.outputLines} of ${truncation.totalLines} lines`;
    result += ` (${formatSize(truncation.outputBytes)} of ${formatSize(truncation.totalBytes)}).`;
    result += ` Full output saved to: ${tempFile}]`;
  }

  return { content: [{ type: "text", text: result }] };
}
```

**Key points:**
- Use `truncateHead` for content where the beginning matters (search results, file reads)
- Use `truncateTail` for content where the end matters (logs, command output)
- Always inform the LLM when output is truncated and where to find the full version
- Document the truncation limits in your tool's description

See [examples/extensions/truncated-tool.ts](../examples/extensions/truncated-tool.ts) for a complete example wrapping `rg` (ripgrep) with proper truncation.

### Multiple Tools

One extension can register multiple tools with shared state:

```typescript
export default function (pi: ExtensionAPI) {
  let connection = null;

  pi.registerTool({ name: "db_connect", ... });
  pi.registerTool({ name: "db_query", ... });
  pi.registerTool({ name: "db_close", ... });

  pi.on("session_shutdown", async () => {
    connection?.close();
  });
}
```

### Custom Rendering

Tools can provide `renderCall` and `renderResult` for custom TUI display. See [tui.md](tui.md) for the full component API and [tool-execution.ts](https://github.com/badlogic/pi-mono/blob/main/packages/coding-agent/src/modes/interactive/components/tool-execution.ts) for how built-in tools render.

Tool output is wrapped in a `Box` that handles padding and background. Your render methods return `Component` instances (typically `Text`).

#### renderCall

Renders the tool call (before/during execution):

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

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);  // 0,0 padding - Box handles it
}
```

#### renderResult

Renders the tool result:

```typescript
renderResult(result, { expanded, isPartial }, theme) {
  // Handle streaming
  if (isPartial) {
    return new Text(theme.fg("warning", "Processing..."), 0, 0);
  }

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

  // Normal result - support expanded view (Ctrl+O)
  let text = theme.fg("success", "✓ Done");
  if (expanded && result.details?.items) {
    for (const item of result.details.items) {
      text += "\n  " + theme.fg("dim", item);
    }
  }
  return new Text(text, 0, 0);
}
```

#### Keybinding Hints

Use `keyHint()` to display keybinding hints that respect user's keybinding configuration:

```typescript
import { keyHint } from "@stonega/pi-coding-agent";

renderResult(result, { expanded }, theme) {
  let text = theme.fg("success", "✓ Done");
  if (!expanded) {
    text += ` (${keyHint("expandTools", "to expand")})`;
  }
  return new Text(text, 0, 0);
}
```

Available functions:
- `keyHint(action, description)` - Editor actions (e.g., `"expandTools"`, `"selectConfirm"`)
- `appKeyHint(keybindings, action, description)` - App actions (requires `KeybindingsManager`)
- `editorKey(action)` - Get raw key string for editor action
- `rawKeyHint(key, description)` - Format a raw key string

#### Best Practices

- Use `Text` with padding `(0, 0)` - the Box handles padding
- Use `\n` for multi-line content
- Handle `isPartial` for streaming progress
- Support `expanded` for detail on demand
- Keep default view compact

#### Fallback

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

## Custom UI

Extensions can interact with users via `ctx.ui` methods and customize how messages/tools render.

**For custom components, see [tui.md](tui.md)** which has copy-paste patterns for:
- Selection dialogs (SelectList)
- Async operations with cancel (BorderedLoader)
- Settings toggles (SettingsList)
- Status indicators (setStatus)
- Working message during streaming (setWorkingMessage)
- Widgets above/below editor (setWidget)
- Custom footers (setFooter)

### Dialogs

```typescript
// Select from options
const choice = await ctx.ui.select("Pick one:", ["A", "B", "C"]);

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

// Text input
const name = await ctx.ui.input("Name:", "placeholder");

// Multi-line editor
const text = await ctx.ui.editor("Edit:", "prefilled text");

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

#### Timed Dialogs with Countdown

Dialogs support a `timeout` option that auto-dismisses with a live countdown display:

```typescript
// Dialog shows "Title (5s)" → "Title (4s)" → ... → auto-dismisses at 0
const confirmed = await ctx.ui.confirm(
  "Timed Confirmation",
  "This dialog will auto-cancel in 5 seconds. Confirm?",
  { timeout: 5000 }
);

if (confirmed) {
  // User confirmed
} else {
  // User cancelled or timed out
}
```

**Return values on timeout:**
- `select()` returns `undefined`
- `confirm()` returns `false`
- `input()` returns `undefined`

#### Manual Dismissal with AbortSignal

For more control (e.g., to distinguish timeout from user cancel), use `AbortSignal`:

```typescript
const controller = new AbortController();
const timeoutId = setTimeout(() => controller.abort(), 5000);

const confirmed = await ctx.ui.confirm(
  "Timed Confirmation",
  "This dialog will auto-cancel in 5 seconds. Confirm?",
  { signal: controller.signal }
);

clearTimeout(timeoutId);

if (confirmed) {
  // User confirmed
} else if (controller.signal.aborted) {
  // Dialog timed out
} else {
  // User cancelled (pressed Escape or selected "No")
}
```

See [examples/extensions/timed-confirm.ts](../examples/extensions/timed-confirm.ts) for complete examples.

### Widgets, Status, and Footer

```typescript
// Status in footer (persistent until cleared)
ctx.ui.setStatus("my-ext", "Processing...");
ctx.ui.setStatus("my-ext", undefined);  // Clear

// Working message (shown during streaming)
ctx.ui.setWorkingMessage("Thinking deeply...");
ctx.ui.setWorkingMessage();  // Restore default

// Widget above editor (default)
ctx.ui.setWidget("my-widget", ["Line 1", "Line 2"]);
// Widget below editor
ctx.ui.setWidget("my-widget", ["Line 1", "Line 2"], { placement: "belowEditor" });
ctx.ui.setWidget("my-widget", (tui, theme) => new Text(theme.fg("accent", "Custom"), 0, 0));
ctx.ui.setWidget("my-widget", undefined);  // Clear

// Custom footer (replaces built-in footer entirely)
ctx.ui.setFooter((tui, theme) => ({
  render(width) { return [theme.fg("dim", "Custom footer")]; },
  invalidate() {},
}));
ctx.ui.setFooter(undefined);  // Restore built-in footer

// Terminal title
ctx.ui.setTitle("pi - my-project");

// Editor text
ctx.ui.setEditorText("Prefill text");
const current = ctx.ui.getEditorText();

// Paste into editor (triggers paste handling, including collapse for large content)
ctx.ui.pasteToEditor("pasted content");

// Tool output expansion
const wasExpanded = ctx.ui.getToolsExpanded();
ctx.ui.setToolsExpanded(true);
ctx.ui.setToolsExpanded(wasExpanded);

// Custom editor (vim mode, emacs mode, etc.)
ctx.ui.setEditorComponent((tui, theme, keybindings) => new VimEditor(tui, theme, keybindings));
ctx.ui.setEditorComponent(undefined);  // Restore default editor

// Theme management (see themes.md for creating themes)
const themes = ctx.ui.getAllThemes();  // [{ name: "dark", path: "/..." | undefined }, ...]
const lightTheme = ctx.ui.getTheme("light");  // Load without switching
const result = ctx.ui.setTheme("light");  // Switch by name
if (!result.success) {
  ctx.ui.notify(`Failed: ${result.error}`, "error");
}
ctx.ui.setTheme(lightTheme!);  // Or switch by Theme object
ctx.ui.theme.fg("accent", "styled text");  // Access current theme
```

### Custom Components

For complex UI, use `ctx.ui.custom()`. This temporarily replaces the editor with your component until `done()` is called:

```typescript
import { Text, Component } from "@stonega/pi-tui";

const result = await ctx.ui.custom<boolean>((tui, theme, keybindings, done) => {
  const text = new Text("Press Enter to confirm, Escape to cancel", 1, 1);

  text.onKey = (key) => {
    if (key === "return") done(true);
    if (key === "escape") done(false);
    return true;
  };

  return text;
});

if (result) {
  // User pressed Enter
}
```

The callback receives:
- `tui` - TUI instance (for screen dimensions, focus management)
- `theme` - Current theme for styling
- `keybindings` - App keybinding manager (for checking shortcuts)
- `done(value)` - Call to close component and return value

See [tui.md](tui.md) for the full component API.

#### Overlay Mode (Experimental)

Pass `{ overlay: true }` to render the component as a floating modal on top of existing content, without clearing the screen:

```typescript
const result = await ctx.ui.custom<string | null>(
  (tui, theme, keybindings, done) => new MyOverlayComponent({ onClose: done }),
  { overlay: true }
);
```

For advanced positioning (anchors, margins, percentages, responsive visibility), pass `overlayOptions`. Use `onHandle` to control visibility programmatically:

```typescript
const result = await ctx.ui.custom<string | null>(
  (tui, theme, keybindings, done) => new MyOverlayComponent({ onClose: done }),
  {
    overlay: true,
    overlayOptions: { anchor: "top-right", width: "50%", margin: 2 },
    onHandle: (handle) => { /* handle.setHidden(true/false) */ }
  }
);
```

See [tui.md](tui.md) for the full `OverlayOptions` API and [overlay-qa-tests.ts](../examples/extensions/overlay-qa-tests.ts) for examples.

### Custom Editor

Replace the main input editor with a custom implementation (vim mode, emacs mode, etc.):

```typescript
import { CustomEditor, type ExtensionAPI } from "@stonega/pi-coding-agent";
import { matchesKey } from "@stonega/pi-tui";

class VimEditor extends CustomEditor {
  private mode: "normal" | "insert" = "insert";

  handleInput(data: string): void {
    if (matchesKey(data, "escape") && this.mode === "insert") {
      this.mode = "normal";
      return;
    }
    if (this.mode === "normal" && data === "i") {
      this.mode = "insert";
      return;
    }
    super.handleInput(data);  // App keybindings + text editing
  }
}

export default function (pi: ExtensionAPI) {
  pi.on("session_start", (_event, ctx) => {
    ctx.ui.setEditorComponent((_tui, theme, keybindings) =>
      new VimEditor(theme, keybindings)
    );
  });
}
```

**Key points:**
- Extend `CustomEditor` (not base `Editor`) to get app keybindings (escape to abort, ctrl+d, model switching)
- Call `super.handleInput(data)` for keys you don't handle
- Factory receives `theme` and `keybindings` from the app
- Pass `undefined` to restore default: `ctx.ui.setEditorComponent(undefined)`

See [tui.md](tui.md) Pattern 7 for a complete example with mode indicator.

### Message Rendering

Register a custom renderer for messages with your `customType`:

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

pi.registerMessageRenderer("my-extension", (message, options, theme) => {
  const { expanded } = options;
  let text = theme.fg("accent", `[${message.customType}] `);
  text += message.content;

  if (expanded && message.details) {
    text += "\n" + theme.fg("dim", JSON.stringify(message.details, null, 2));
  }

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

Messages are sent via `pi.sendMessage()`:

```typescript
pi.sendMessage({
  customType: "my-extension",  // Matches registerMessageRenderer
  content: "Status update",
  display: true,               // Show in TUI
  details: { ... },            // Available in renderer
});
```

### Theme Colors

All render functions receive a `theme` object. See [themes.md](themes.md) for creating custom themes and the full color palette.

```typescript
// Foreground colors
theme.fg("toolTitle", text)   // Tool names
theme.fg("accent", text)      // Highlights
theme.fg("success", text)     // Success (green)
theme.fg("error", text)       // Errors (red)
theme.fg("warning", text)     // Warnings (yellow)
theme.fg("muted", text)       // Secondary text
theme.fg("dim", text)         // Tertiary text

// Text styles
theme.bold(text)
theme.italic(text)
theme.strikethrough(text)
```

For syntax highlighting in custom tool renderers:

```typescript
import { highlightCode, getLanguageFromPath } from "@stonega/pi-coding-agent";

// Highlight code with explicit language
const highlighted = highlightCode("const x = 1;", "typescript", theme);

// Auto-detect language from file path
const lang = getLanguageFromPath("/path/to/file.rs");  // "rust"
const highlighted = highlightCode(code, lang, theme);
```

## Error Handling

- Extension errors are logged, agent continues
- `tool_call` errors block the tool (fail-safe)
- Tool `execute` errors are reported to the LLM with `isError: true`

## Mode Behavior

| Mode | UI Methods | Notes |
|------|-----------|-------|
| Interactive | Full TUI | Normal operation |
| RPC (`--mode rpc`) | JSON protocol | Host handles UI, see [rpc.md](rpc.md) |
| JSON (`--mode json`) | No-op | Event stream to stdout, see [json.md](json.md) |
| Print (`-p`) | No-op | Extensions run but can't prompt |

In non-interactive modes, check `ctx.hasUI` before using UI methods.

## Examples Reference

All examples in [examples/extensions/](../examples/extensions/).

| Example | Description | Key APIs |
|---------|-------------|----------|
| **Tools** |||
| `hello.ts` | Minimal tool registration | `registerTool` |
| `question.ts` | Tool with user interaction | `registerTool`, `ui.select` |
| `questionnaire.ts` | Multi-step wizard tool | `registerTool`, `ui.custom` |
| `todo.ts` | Stateful tool with persistence | `registerTool`, `appendEntry`, `renderResult`, session events |
| `truncated-tool.ts` | Output truncation example | `registerTool`, `truncateHead` |
| `tool-override.ts` | Override built-in read tool | `registerTool` (same name as built-in) |
| **Commands** |||
| `pirate.ts` | Modify system prompt per-turn | `registerCommand`, `before_agent_start` |
| `summarize.ts` | Conversation summary command | `registerCommand`, `ui.custom` |
| `handoff.ts` | Cross-provider model handoff | `registerCommand`, `ui.editor`, `ui.custom` |
| `qna.ts` | Q&A with custom UI | `registerCommand`, `ui.custom`, `setEditorText` |
| `send-user-message.ts` | Inject user messages | `registerCommand`, `sendUserMessage` |
| `reload-runtime.ts` | Reload command and LLM tool handoff | `registerCommand`, `ctx.reload()`, `sendUserMessage` |
| `shutdown-command.ts` | Graceful shutdown command | `registerCommand`, `shutdown()` |
| **Events & Gates** |||
| `permission-gate.ts` | Block dangerous commands | `on("tool_call")`, `ui.confirm` |
| `protected-paths.ts` | Block writes to specific paths | `on("tool_call")` |
| `confirm-destructive.ts` | Confirm session changes | `on("session_before_switch")`, `on("session_before_fork")` |
| `dirty-repo-guard.ts` | Warn on dirty git repo | `on("session_before_*")`, `exec` |
| `input-transform.ts` | Transform user input | `on("input")` |
| `model-status.ts` | React to model changes | `on("model_select")`, `setStatus` |
| `system-prompt-header.ts` | Display system prompt info | `on("agent_start")`, `getSystemPrompt` |
| `claude-rules.ts` | Load rules from files | `on("session_start")`, `on("before_agent_start")` |
| `file-trigger.ts` | File watcher triggers messages | `sendMessage` |
| **Compaction & Sessions** |||
| `custom-compaction.ts` | Custom compaction summary | `on("session_before_compact")` |
| `trigger-compact.ts` | Trigger compaction manually | `compact()` |
| `git-checkpoint.ts` | Git stash on turns | `on("turn_end")`, `on("session_fork")`, `exec` |
| `auto-commit-on-exit.ts` | Commit on shutdown | `on("session_shutdown")`, `exec` |
| **UI Components** |||
| `status-line.ts` | Footer status indicator | `setStatus`, session events |
| `custom-footer.ts` | Replace footer entirely | `registerCommand`, `setFooter` |
| `custom-header.ts` | Replace startup header | `on("session_start")`, `setHeader` |
| `modal-editor.ts` | Vim-style modal editor | `setEditorComponent`, `CustomEditor` |
| `rainbow-editor.ts` | Custom editor styling | `setEditorComponent` |
| `widget-placement.ts` | Widget above/below editor | `setWidget` |
| `overlay-test.ts` | Overlay components | `ui.custom` with overlay options |
| `overlay-qa-tests.ts` | Comprehensive overlay tests | `ui.custom`, all overlay options |
| `notify.ts` | Simple notifications | `ui.notify` |
| `timed-confirm.ts` | Dialogs with timeout | `ui.confirm` with timeout/signal |
| `mac-system-theme.ts` | Auto-switch theme | `setTheme`, `exec` |
| **Complex Extensions** |||
| `plan-mode/` | Full plan mode implementation | All event types, `registerCommand`, `registerShortcut`, `registerFlag`, `setStatus`, `setWidget`, `sendMessage`, `setActiveTools` |
| `preset.ts` | Saveable presets (model, tools, thinking) | `registerCommand`, `registerShortcut`, `registerFlag`, `setModel`, `setActiveTools`, `setThinkingLevel`, `appendEntry` |
| `tools.ts` | Toggle tools on/off UI | `registerCommand`, `setActiveTools`, `SettingsList`, session events |
| **Remote & Sandbox** |||
| `ssh.ts` | SSH remote execution | `registerFlag`, `on("user_bash")`, `on("before_agent_start")`, tool operations |
| `interactive-shell.ts` | Persistent shell session | `on("user_bash")` |
| `sandbox/` | Sandboxed tool execution | Tool operations |
| `subagent/` | Spawn sub-agents | `registerTool`, `exec` |
| **Games** |||
| `snake.ts` | Snake game | `registerCommand`, `ui.custom`, keyboard handling |
| `space-invaders.ts` | Space Invaders game | `registerCommand`, `ui.custom` |
| `doom-overlay/` | Doom in overlay | `ui.custom` with overlay |
| **Providers** |||
| `custom-provider-anthropic/` | Custom Anthropic proxy | `registerProvider` |
| `custom-provider-gitlab-duo/` | GitLab Duo integration | `registerProvider` with OAuth |
| **Messages & Communication** |||
| `message-renderer.ts` | Custom message rendering | `registerMessageRenderer`, `sendMessage` |
| `event-bus.ts` | Inter-extension events | `pi.events` |
| **Session Metadata** |||
| `session-name.ts` | Name sessions for selector | `setSessionName`, `getSessionName` |
| `bookmark.ts` | Bookmark entries for /tree | `setLabel` |
| **Misc** |||
| `antigravity-image-gen.ts` | Image generation tool | `registerTool`, Google Antigravity |
| `inline-bash.ts` | Inline bash in tool calls | `on("tool_call")` |
| `bash-spawn-hook.ts` | Adjust bash command, cwd, and env before execution | `createBashTool`, `spawnHook` |
| `with-deps/` | Extension with npm dependencies | Package structure with `package.json` |
