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

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.

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/ for working implementations.

Table of Contents

• Quick Start
• Extension Locations
• Available Imports
• Writing an Extension
  • Extension Styles
• Events
  • Lifecycle Overview
  • Session Events
  • Agent Events
  • Tool Events
• ExtensionContext
• ExtensionCommandContext
• ExtensionAPI Methods
• State Management
• Custom Tools
• Custom UI
• Error Handling
• Mode Behavior

Quick Start

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

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, onUpdate, ctx, signal) {
      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:

pi -e ./my-extension.ts

Extension Locations

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:

{
  "extensions": ["/path/to/extension.ts", "/path/to/extension/dir"]
}

Discovery rules:

1. Direct files: extensions/*.ts or *.js → loaded directly
2. Subdirectory with index: extensions/myext/index.ts → loaded as single extension
3. Subdirectory with package.json: extensions/myext/package.json with "pi" field → loads declared paths

~/.pi/agent/extensions/
├── simple.ts                      # Direct file (auto-discovered)
├── my-tool/
│   └── index.ts                   # Subdirectory with index (auto-discovered)
└── my-extension-pack/
    ├── package.json               # Declares multiple extensions
    ├── node_modules/              # Dependencies installed here
    └── src/
        ├── safety-gates.ts        # First extension
        └── custom-tools.ts        # Second extension

// my-extension-pack/package.json
{
  "name": "my-extension-pack",
  "dependencies": {
    "zod": "^3.0.0"
  },
  "pi": {
    "extensions": ["./src/safety-gates.ts", "./src/custom-tools.ts"]
  }
}

The package.json approach enables:

• Multiple extensions from one package
• Third-party npm dependencies (resolved via jiti)
• Nested source structure (no depth limit within the package)
• Deployment to and installation from npm

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.

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

Writing an Extension

An extension exports a default function that receives ExtensionAPI:

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
  });

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

Extensions are loaded via 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

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

/new (new session) 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

session_start

Fired on initial session load.

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

Examples: claude-rules.ts, custom-header.ts, file-trigger.ts, status-line.ts, todo.ts, tools.ts

session_before_switch / session_switch

Fired when starting a new session (/new) or switching sessions (/resume).

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
});

Examples: confirm-destructive.ts, dirty-repo-guard.ts, status-line.ts, todo.ts

session_before_fork / session_fork

Fired when forking via /fork.

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
});

Examples: confirm-destructive.ts, dirty-repo-guard.ts, git-checkpoint.ts, todo.ts, tools.ts

session_before_compact / session_compact

Fired on compaction. See compaction.md for details.

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
});

Examples: custom-compaction.ts

session_before_tree / session_tree

Fired on /tree navigation.

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
});

Examples: todo.ts, tools.ts

session_shutdown

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

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

Examples: auto-commit-on-exit.ts

Agent Events

before_agent_start

Fired after user submits prompt, before agent loop. Can inject a message and/or modify the system prompt.

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...",
  };
});

Examples: claude-rules.ts, pirate.ts, plan-mode/index.ts, preset.ts, ssh.ts

agent_start / agent_end

Fired once per user prompt.

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

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

Examples: chalk-logger.ts, git-checkpoint.ts, plan-mode/index.ts

turn_start / turn_end

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

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

pi.on("turn_end", async (event, ctx) => {
  // event.turnIndex, event.message, event.toolResults
});

Examples: git-checkpoint.ts, plan-mode/index.ts, status-line.ts

context

Fired before each LLM call. Modify messages non-destructively.

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

Examples: plan-mode/index.ts

Model Events

model_select

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

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.

Examples: model-status.ts

Tool Events

tool_call

Fired before tool executes. Can block.

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

  if (shouldBlock(event)) {
    return { block: true, reason: "Not allowed" };
  }
});

Examples: chalk-logger.ts, permission-gate.ts, plan-mode/index.ts, protected-paths.ts

tool_result

Fired after tool executes. Can modify result.

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
  }

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

Examples: git-checkpoint.ts, plan-mode/index.ts

User Bash Events

user_bash

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

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 } };
});

Examples: ssh.ts, interactive-shell.ts

ExtensionContext

Every handler receives ctx: ExtensionContext:

ctx.ui

UI methods for user interaction. See Custom UI for full details.

ctx.hasUI

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

ctx.cwd

Current working directory.

ctx.sessionManager

Read-only access to session state:

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).

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

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:

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:

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.branch(entryId)

Branch from a specific entry:

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

ctx.navigateTree(targetId, options?)

Navigate to a different point in the session tree:

const result = await ctx.navigateTree("entry-id-456", {
  summarize: true,
});

ExtensionAPI Methods

pi.on(event, handler)

Subscribe to events. See Events for event types and return values.

pi.registerTool(definition)

Register a custom tool callable by the LLM. See Custom Tools for full details.

import { Type } from "@sinclair/typebox";
import { StringEnum } from "@mariozechner/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, onUpdate, ctx, signal) {
    // 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) { ... },
});

Examples: hello.ts, question.ts, questionnaire.ts, todo.ts, truncated-tool.ts

pi.sendMessage(message, options?)

Inject a custom message into the session.

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").

Examples: file-trigger.ts, plan-mode/index.ts

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.

// 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 for a complete example.

pi.appendEntry(customType, data?)

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

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
    }
  }
});

Examples: plan-mode/index.ts, preset.ts, snake.ts, tools.ts

pi.setSessionName(name)

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

pi.setSessionName("Refactor auth module");

pi.getSessionName()

Get the current session name, if set.

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

pi.registerCommand(name, options)

Register a command.

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

Examples: custom-footer.ts, custom-header.ts, handoff.ts, pirate.ts, plan-mode/index.ts, preset.ts, qna.ts, send-user-message.ts, snake.ts, summarize.ts, todo.ts, tools.ts

pi.registerMessageRenderer(customType, renderer)

Register a custom TUI renderer for messages with your customType. See Custom UI.

pi.registerShortcut(shortcut, options)

Register a keyboard shortcut.

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

Examples: plan-mode/index.ts, preset.ts

pi.registerFlag(name, options)

Register a CLI flag.

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

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

Examples: plan-mode/index.ts, preset.ts

pi.exec(command, args, options?)

Execute a shell command.

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

Examples: auto-commit-on-exit.ts, dirty-repo-guard.ts, git-checkpoint.ts

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

Manage active tools.

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

Examples: plan-mode/index.ts, preset.ts, tools.ts

pi.setModel(model)

Set the current model. Returns false if no API key is available for the model.

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");
  }
}

Examples: preset.ts

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

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

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

Examples: preset.ts

pi.events

Shared event bus for communication between extensions:

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

State Management

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

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, onUpdate, ctx, signal) {
      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.

Tool Definition

import { Type } from "@sinclair/typebox";
import { StringEnum } from "@mariozechner/pi-ai";
import { Text } from "@mariozechner/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, onUpdate, ctx, signal) {
    // 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 @mariozechner/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.

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

Alternatively, use --no-tools to start without any built-in tools:

# No built-in tools, only extension tools
pi --no-tools -e ./my-extension.ts

See 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 - ReadToolDetails
• bash.ts - BashToolDetails
• edit.ts
• write.ts
• grep.ts - GrepToolDetails
• find.ts - FindToolDetails
• ls.ts - LsToolDetails

Remote Execution

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

import { createReadTool, createBashTool, type ReadOperations } from "@mariozechner/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, onUpdate, _ctx, signal) {
    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

See 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:

import {
  truncateHead,      // Keep first N lines/bytes (good for file reads, search results)
  truncateTail,      // Keep last N lines/bytes (good for logs, command output)
  formatSize,        // Human-readable size (e.g., "50KB", "1.5MB")
  DEFAULT_MAX_BYTES, // 50KB
  DEFAULT_MAX_LINES, // 2000
} from "@mariozechner/pi-coding-agent";

async execute(toolCallId, params, onUpdate, ctx, signal) {
  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 for a complete example wrapping rg (ripgrep) with proper truncation.

Multiple Tools

One extension can register multiple tools with shared state:

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 for the full component API.

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):

import { Text } from "@mariozechner/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:

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);
}

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 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 editor (setWidget)
• Custom footers (setFooter)

Dialogs

// 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"

Examples:

• ctx.ui.select(): confirm-destructive.ts, dirty-repo-guard.ts, git-checkpoint.ts, permission-gate.ts, plan-mode/index.ts, question.ts, questionnaire.ts
• ctx.ui.confirm(): confirm-destructive.ts
• ctx.ui.editor(): handoff.ts
• ctx.ui.setEditorText(): handoff.ts, qna.ts

Timed Dialogs with Countdown

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

// 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:

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 for complete examples.

Widgets, Status, and Footer

// 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 (string array or factory function)
ctx.ui.setWidget("my-widget", ["Line 1", "Line 2"]);
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();

// 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
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

Examples:

• ctx.ui.setStatus(): plan-mode/index.ts, preset.ts, status-line.ts
• ctx.ui.setWidget(): plan-mode/index.ts
• ctx.ui.setFooter(): custom-footer.ts
• ctx.ui.setHeader(): custom-header.ts
• ctx.ui.setEditorComponent(): modal-editor.ts
• ctx.ui.setTheme(): mac-system-theme.ts

Custom Components

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

import { Text, Component } from "@mariozechner/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 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:

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

Overlay components should define a width property to control their size. The overlay is centered by default. See overlay-test.ts for a complete example.

Examples: handoff.ts, plan-mode/index.ts, preset.ts, qna.ts, snake.ts, summarize.ts, todo.ts, tools.ts, overlay-test.ts

Custom Editor

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

import { CustomEditor, type ExtensionAPI } from "@mariozechner/pi-coding-agent";
import { matchesKey } from "@mariozechner/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 Pattern 7 for a complete example with mode indicator.

Examples: modal-editor.ts

Message Rendering

Register a custom renderer for messages with your customType:

import { Text } from "@mariozechner/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():

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:

// 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)

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	JSON protocol	Host handles UI
Print (-p)	No-op	Extensions run but can't prompt

In print mode, check ctx.hasUI before using UI methods.