@mariozechner/pi-coding-agent/docs/extensions.md

Coding agent CLI with read, bash, edit, write tools and session management

> 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)
  - [Resource Events](#resource-events)
  - [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 "@mariozechner/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 |
|---------|---------|
| `@mariozechner/pi-coding-agent` | Extension types (`ExtensionAPI`, `ExtensionContext`, events) |
| `@sinclair/typebox` | Schema definitions for tool parameters |
| `@mariozechner/pi-ai` | AI utilities (`StringEnum` for Google-compatible enums) |
| `@mariozechner/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.

For distributed pi packages installed with `pi install` (npm or git), runtime deps must be in `dependencies`. Package installation uses production installs (`npm install --omit=dev`), so `devDependencies` are not available at runtime.

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

## Writing an Extension

An extension exports a default factory function that receives `ExtensionAPI`. The factory can be synchronous or asynchronous:

```typescript
import type { ExtensionAPI } from "@mariozechner/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.

If the factory returns a `Promise`, pi awaits it before continuing startup. That means async initialization completes before `session_start`, before `resources_discover`, and before provider registrations queued via `pi.registerProvider()` are flushed.

### Async factory functions

Use an async factory for one-time startup work such as fetching remote configuration or dynamically discovering available models.

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

export default async function (pi: ExtensionAPI) {
  const response = await fetch("http://localhost:1234/v1/models");
  const payload = (await response.json()) as {
    data: Array<{
      id: string;
      name?: string;
      context_window?: number;
      max_tokens?: number;
    }>;
  };

  pi.registerProvider("local-openai", {
    baseUrl: "http://localhost:1234/v1",
    apiKey: "LOCAL_OPENAI_API_KEY",
    api: "openai-completions",
    models: payload.data.map((model) => ({
      id: model.id,
      name: model.name ?? model.id,
      reasoning: false,
      input: ["text"],
      cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
      contextWindow: model.context_window ?? 128000,
      maxTokens: model.max_tokens ?? 4096,
    })),
  });
}
```

This pattern makes the fetched models available during normal startup and to `pi --list-models`.

### 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 { reason: "startup" }
  └─► resources_discover { reason: "startup" }
      │
      ▼
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)            │       │
  │   ├─► before_provider_request (can inspect or replace payload)
  │   ├─► after_provider_response (status + headers, before stream consume)
  │   │                                            │       │
  │   │   LLM responds, may call tools:            │       │
  │   │     ├─► tool_execution_start               │       │
  │   │     ├─► tool_call (can block)              │       │
  │   │     ├─► tool_execution_update              │       │
  │   │     ├─► tool_result (can modify)           │       │
  │   │     └─► tool_execution_end                 │       │
  │   │                                            │       │
  │   └─► turn_end                                 │       │
  │                                                        │
  └─► agent_end                                            │
                                                           │
user sends another prompt ◄────────────────────────────────┘

/new (new session) or /resume (switch session)
  ├─► session_before_switch (can cancel)
  ├─► session_shutdown
  ├─► session_start { reason: "new" | "resume", previousSessionFile? }
  └─► resources_discover { reason: "startup" }

/fork or /clone
  ├─► session_before_fork (can cancel)
  ├─► session_shutdown
  ├─► session_start { reason: "fork", previousSessionFile }
  └─► resources_discover { reason: "startup" }

/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, SIGHUP, SIGTERM)
  └─► session_shutdown
```

### Resource Events

#### resources_discover

Fired after `session_start` so extensions can contribute additional skill, prompt, and theme paths.
The startup path uses `reason: "startup"`. Reload uses `reason: "reload"`.

```typescript
pi.on("resources_discover", async (event, _ctx) => {
  // event.cwd - current working directory
  // event.reason - "startup" | "reload"
  return {
    skillPaths: ["/path/to/skills"],
    promptPaths: ["/path/to/prompts"],
    themePaths: ["/path/to/themes"],
  };
});
```

### Session Events

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

#### session_start

Fired when a session is started, loaded, or reloaded.

```typescript
pi.on("session_start", async (event, ctx) => {
  // event.reason - "startup" | "reload" | "new" | "resume" | "fork"
  // event.previousSessionFile - present for "new", "resume", and "fork"
  ctx.ui.notify(`Session: ${ctx.sessionManager.getSessionFile() ?? "ephemeral"}`, "info");
});
```

#### session_before_switch

Fired before 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 };
  }
});
```

After a successful switch or new-session action, pi emits `session_shutdown` for the old extension instance, reloads and rebinds extensions for the new session, then emits `session_start` with `reason: "new" | "resume"` and `previousSessionFile`.
Do cleanup work in `session_shutdown`, then reestablish any in-memory state in `session_start`.

#### session_before_fork

Fired when forking via `/fork` or cloning via `/clone`.

```typescript
pi.on("session_before_fork", async (event, ctx) => {
  // event.entryId - ID of the selected entry
  // event.position - "before" for /fork, "at" for /clone
  return { cancel: true }; // Cancel fork/clone
  // OR
  return { skipConversationRestore: true }; // Reserved for future conversation restore control
});
```

After a successful fork or clone, pi emits `session_shutdown` for the old extension instance, reloads and rebinds extensions for the new session, then emits `session_start` with `reason: "fork"` and `previousSessionFile`.
Do cleanup work in `session_shutdown`, then reestablish any in-memory state in `session_start`.

#### 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 before an extension runtime is torn down.

```typescript
pi.on("session_shutdown", async (event, ctx) => {
  // event.reason - "quit" | "reload" | "new" | "resume" | "fork"
  // event.targetSessionFile - destination session for session replacement flows
  // 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
  // event.systemPromptOptions - structured options used to build the system prompt
  //   .customPrompt - any custom system prompt (from --system-prompt, SYSTEM.md, or custom templates)
  //   .selectedTools - tools currently active in the prompt
  //   .toolSnippets - one-line descriptions for each tool
  //   .promptGuidelines - custom guideline bullets
  //   .appendSystemPrompt - text from --append-system-prompt flags
  //   .cwd - working directory
  //   .contextFiles - AGENTS.md files and other loaded context files
  //   .skills - loaded skills

  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...",
  };
});
```

The `systemPromptOptions` field gives extensions access to the same structured data Pi uses to build the system prompt. This lets you inspect what Pi has loaded — custom prompts, guidelines, tool snippets, context files, skills — without re-discovering resources or re-parsing flags. Use it when your extension needs to make deep, informed changes to the system prompt while respecting user-provided configuration.

#### 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.

In parallel tool mode:
- `tool_execution_start` is emitted in assistant source order during the preflight phase
- `tool_execution_update` events may interleave across tools
- `tool_execution_end` is emitted in assistant source order, matching final tool result message order

```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 };
});
```

#### before_provider_request

Fired after the provider-specific payload is built, right before the request is sent. Handlers run in extension load order. Returning `undefined` keeps the payload unchanged. Returning any other value replaces the payload for later handlers and for the actual request.

```typescript
pi.on("before_provider_request", (event, ctx) => {
  console.log(JSON.stringify(event.payload, null, 2));

  // Optional: replace payload
  // return { ...event.payload, temperature: 0 };
});
```

This is mainly useful for debugging provider serialization and cache behavior.

#### after_provider_response

Fired after an HTTP response is received and before its stream body is consumed. Handlers run in extension load order.

```typescript
pi.on("after_provider_response", (event, ctx) => {
  // event.status - HTTP status code
  // event.headers - normalized response headers
  if (event.status === 429) {
    console.log("rate limited", event.headers["retry-after"]);
  }
});
```

Header availability depends on provider and transport. Providers that abstract HTTP responses may not expose headers.

### 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 after `tool_execution_start`, before the tool executes. **Can block.** Use `isToolCallEventType` to narrow and get typed inputs.

Before `tool_call` runs, pi waits for previously emitted Agent events to finish draining through `AgentSession`. This means `ctx.sessionManager` is up to date through the current assistant tool-calling message.

In the default parallel tool execution mode, sibling tool calls from the same assistant message are preflighted sequentially, then executed concurrently. `tool_call` is not guaranteed to see sibling tool results from that same assistant message in `ctx.sessionManager`.

`event.input` is mutable. Mutate it in place to patch tool arguments before execution.

Behavior guarantees:
- Mutations to `event.input` affect the actual tool execution
- Later `tool_call` handlers see mutations made by earlier handlers
- No re-validation is performed after your mutation
- Return values from `tool_call` only control blocking via `{ block: true, reason?: string }`

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

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

  // Built-in tools: no type params needed
  if (isToolCallEventType("bash", event)) {
    // event.input is { command: string; timeout?: number }
    event.input.command = `source ~/.profile\n${event.input.command}`;

    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 "@mariozechner/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 execution finishes and before `tool_execution_end` plus the final tool result message events are emitted. **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

Use `ctx.signal` for nested async work inside the handler. This lets Esc cancel model calls, `fetch()`, and other abort-aware operations started by the extension.

```typescript
import { isBashToolResult } from "@mariozechner/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
  }

  const response = await fetch("https://example.com/summarize", {
    method: "POST",
    body: JSON.stringify({ content: event.content }),
    signal: ctx.signal,
  });

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

### User Bash Events

#### user_bash

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

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

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: Wrap pi's built-in local bash backend
  const local = createLocalBashOperations();
  return {
    operations: {
      exec(command, cwd, options) {
        return local.exec(`source ~/.profile\n${command}`, cwd, options);
      }
    }
  };

  // Option 3: 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

All handlers receive `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.

For `tool_call`, this state is synchronized through the current assistant message before handlers run. In parallel tool execution mode it is still not guaranteed to include sibling tool results from the same assistant message.

```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.signal

The current agent abort signal, or `undefined` when no agent turn is active.

Use this for abort-aware nested work started by extension handlers, for example:
- `fetch(..., { signal: ctx.signal })`
- model calls that accept `signal`
- file or process helpers that accept `AbortSignal`

`ctx.signal` is typically defined during active turn events such as `tool_call`, `tool_result`, `message_update`, and `turn_end`.
It is usually `undefined` in idle or non-turn contexts such as session events, extension commands, and shortcuts fired while pi is idle.

```typescript
pi.on("tool_result", async (event, ctx) => {
  const response = await fetch("https://example.com/api", {
    method: "POST",
    body: JSON.stringify(event),
    signal: ctx.signal,
  });

  const data = await response.json();
  return { details: data };
});
```

### 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, options?)

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
}

const cloneResult = await ctx.fork("entry-id-456", { position: "at" });
if (!cloneResult.cancelled) {
  // New session contains the active path through entry-id-456
}
```

Options:
- `position`: `"before"` (default) forks before the selected user message, restoring that prompt into the editor
- `position`: `"at"` duplicates the active path through the selected entry without restoring editor text

### 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.switchSession(sessionPath)

Switch to a different session file:

```typescript
const result = await ctx.switchSession("/path/to/session.jsonl");
if (result.cancelled) {
  // An extension cancelled the switch via session_before_switch
}
```

To discover available sessions, use the static `SessionManager.list()` or `SessionManager.listAll()` methods:

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

pi.registerCommand("switch", {
  description: "Switch to another session",
  handler: async (args, ctx) => {
    const sessions = await SessionManager.list(ctx.cwd);
    if (sessions.length === 0) return;
    const choice = await ctx.ui.select(
      "Pick session:",
      sessions.map(s => s.file),
    );
    if (choice) {
      await ctx.switchSession(choice);
    }
  },
});
```

### 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` with `reason: "reload"` and `resources_discover` with reason `"reload"`
- 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 "@mariozechner/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.

`pi.registerTool()` works both during extension load and after startup. You can call it inside `session_start`, command handlers, or other event handlers. New tools are refreshed immediately in the same session, so they appear in `pi.getAllTools()` and are callable by the LLM without `/reload`.

Use `pi.setActiveTools()` to enable or disable tools (including dynamically added tools) at runtime.

Use `promptSnippet` to opt a custom tool into a one-line entry in `Available tools`, and `promptGuidelines` to append tool-specific bullets to the default `Guidelines` section when the tool is active.

See [dynamic-tools.ts](../examples/extensions/dynamic-tools.ts) for a full example.

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

pi.registerTool({
  name: "my_tool",
  label: "My Tool",
  description: "What this tool does",
  promptSnippet: "Summarize or transform text according to action",
  promptGuidelines: ["Use this tool when the user asks to summarize previously generated text."],
  parameters: Type.Object({
    action: StringEnum(["list", "add"] as const),
    text: Type.Optional(Type.String()),
  }),
  prepareArguments(args) {
    // Optional compatibility shim. Runs before schema validation.
    // Return the current schema shape, for example to fold legacy fields
    // into the modern parameter object.
    return args;
  },

  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, context) { ... },
  renderResult(result, options, theme, context) { ... },
});
```

### 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) - Queues the message while streaming. Delivered after the current assistant turn finishes executing its tool calls, before the next LLM call.
  - `"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"` - Queues the message for delivery after the current assistant turn finishes executing its tool calls
  - `"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.

If multiple extensions register the same command name, pi keeps them all and assigns numeric invocation suffixes in load order, for example `/review:1` and `/review:2`.

```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 "@mariozechner/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");
const userScoped = commands.filter((command) => command.sourceInfo.scope === "user");
```

Each entry has this shape:

```typescript
{
  name: string; // Invokable command name without the leading slash. May be suffixed like "review:1"
  description?: string;
  source: "extension" | "prompt" | "skill";
  sourceInfo: {
    path: string;
    source: string;
    scope: "user" | "project" | "temporary";
    origin: "package" | "top-level";
    baseDir?: string;
  };
}
```

Use `sourceInfo` as the canonical provenance field. Do not infer ownership from command names or from ad hoc path parsing.

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. This works for both built-in tools and dynamically registered tools.

```typescript
const active = pi.getActiveTools();
const all = pi.getAllTools();
// [{
//   name: "read",
//   description: "Read file contents...",
//   parameters: ..., 
//   sourceInfo: { path: "<builtin:read>", source: "builtin", scope: "temporary", origin: "top-level" }
// }, ...]
const names = all.map(t => t.name);
const builtinTools = all.filter((t) => t.sourceInfo.source === "builtin");
const extensionTools = all.filter((t) => t.sourceInfo.source !== "builtin" && t.sourceInfo.source !== "sdk");
pi.setActiveTools(["read", "bash"]); // Switch to read-only
```

`pi.getAllTools()` returns `name`, `description`, `parameters`, and `sourceInfo`.

Typical `sourceInfo.source` values:
- `builtin` for built-in tools
- `sdk` for tools passed via `createAgentSession({ customTools })`
- extension source metadata for tools registered by extensions

### 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.

Calls made during the extension factory function are queued and applied once the runner initialises. Calls made after that — for example from a command handler following a user setup flow — take effect immediately without requiring a `/reload`.

If you need to discover models from a remote endpoint, prefer an async extension factory over deferring the fetch to `session_start`. pi waits for the factory before startup continues, so the registered models are available immediately, including to `pi --list-models`.

```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.

### pi.unregisterProvider(name)

Remove a previously registered provider and its models. Built-in models that were overridden by the provider are restored. Has no effect if the provider was not registered.

Like `registerProvider`, this takes effect immediately when called after the initial load phase, so a `/reload` is not required.

```typescript
pi.registerCommand("my-setup-teardown", {
  description: "Remove the custom proxy provider",
  handler: async (_args, _ctx) => {
    pi.unregisterProvider("my-proxy");
  },
});
```

## State M