pi-extension-builder

Pi Extension Builder

Freshness: Fetch live docs when implementing event handlers, stateful tools, file-mutating tools, or custom rendering: fetch_content https://raw.githubusercontent.com/badlogic/pi-mono/refs/heads/main/packages/coding-agent/docs/extensions.md

---

Pre-conditions

1. Check for existing extensions before creating a new one:
   • Global: ~/.pi/agent/extensions/
   • Project-local: .pi/extensions/
2. Confirm with the user: global or project-local scope?
3. Are npm dependencies needed? → determines extension structure below.

---

What are you building?

Goal	Tool
Custom tool for the LLM	pi.registerTool()
Slash command /mycommand	pi.registerCommand()
Block a tool call	pi.on("tool_call") → return { block: true, reason? }
Modify tool call arguments	pi.on("tool_call") → mutate event.input in place (return only blocks)
Modify tool output	pi.on("tool_result") → return { content?, details?, isError? }
Intercept user input	pi.on("input") → return { action: "transform", text: ... }
Inject context before each turn	pi.on("before_agent_start") → return { message?, systemPrompt? }
User interaction (confirm, select, input)	ctx.ui.*
Custom TUI rendering	renderCall / renderResult
Register dynamic skills/prompts/themes	pi.on("resources_discover") → return { skillPaths?, promptPaths?, themePaths? }
Override built-in tool (bash, read, write…)	pi.registerTool({ name: "bash", ... }) — same name replaces built-in
Persist state across sessions	pi.appendEntry() / details pattern
Share via npm/git	package.json with pi key

---

Extension structure

Single file (simple extensions):

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

Directory (multiple modules):

~/.pi/agent/extensions/my-extension/
├── index.ts
└── helpers.ts

Package (needs npm dependencies):

~/.pi/agent/extensions/my-extension/
├── package.json    # with "pi" key
├── node_modules/
└── src/index.ts

package.json for a package:

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

---

Extension skeleton

import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
import {
  truncateHead,
  DEFAULT_MAX_BYTES,
  DEFAULT_MAX_LINES,
} from "@mariozechner/pi-coding-agent";
import { Type } from "@sinclair/typebox";
import { StringEnum } from "@mariozechner/pi-ai"; // required for Google model compatibility

export default function (pi: ExtensionAPI) {
  pi.registerTool({
    name: "my_tool",
    label: "My Tool",
    description: "What this tool does (shown to LLM)",
    promptSnippet:
      "One-liner for Available tools section — omit and LLM won't discover this tool",
    parameters: Type.Object({
      action: StringEnum(["list", "add"] as const),
    }),
    async execute(toolCallId, params, signal, onUpdate, ctx) {
      if (signal?.aborted)
        return { content: [{ type: "text", text: "Cancelled" }] };

      // throw to signal failure — returning any value always succeeds (isError: false)
      const { stdout } = await pi.exec("git", ["status"], { signal });

      const truncation = truncateHead(stdout, {
        maxLines: DEFAULT_MAX_LINES,
        maxBytes: DEFAULT_MAX_BYTES,
      });
      return { content: [{ type: "text", text: truncation.content }] };
    },
  });

  pi.on("session_shutdown", async () => {
    // close connections, file watchers, timers
  });
}

---

Workflow

1. Define the goal — tool, command, event handler, or a combination
2. Choose scope — global vs project-local
3. Choose structure — single file / directory / package
4. Scaffold — export default function (pi: ExtensionAPI) { ... }
5. Register capabilities — registerTool, registerCommand, pi.on
6. Handle state — decide if details pattern is needed for reconstruction
7. Test — pi -e ./extension.ts or /reload after placing in auto-discovery path
8. Package if needed — add package.json with pi key for sharing

---

Critical pitfalls

• Google compatibility: use StringEnum(["a", "b"] as const), never Type.Enum() or union types
• Error signaling: throw new Error(...) from execute() to report failure — returning any value always succeeds (isError: false)
• Output truncation: use truncateHead/truncateTail from @mariozechner/pi-coding-agent for output over 50KB/2000 lines — untruncated output breaks context
• promptSnippet: without it, the LLM won't see the tool in the Available tools section and won't discover it
• tool_call mutation: return { block: true } to block, mutate event.input in place to modify args — return value does NOT modify arguments
• Non-interactive modes: always check ctx.hasUI before calling UI methods
• State and branches: store state in tool result details; reconstruct from ctx.sessionManager.getBranch() in BOTH session_start AND session_tree (tree navigation fires session_tree, not session_start)
• File-mutating tools: wrap with withFileMutationQueue() — tool calls run in parallel, without the queue concurrent edits to the same file will clobber each other
• sendUserMessage during streaming: must pass { deliverAs: "steer" | "followUp" } — calling without it while agent is streaming throws
• Cleanup: always register session_shutdown for connections, file watchers, timers
• Reload is terminal: code after await ctx.reload() is not guaranteed to run — treat it as the last statement

---

Placement and scope

Path	Scope	Hot-reload
~/.pi/agent/extensions/*.ts	Global	Yes (/reload)
~/.pi/agent/extensions/*/index.ts	Global (subdirectory)	Yes
.pi/extensions/*.ts	Project-local	Yes
pi -e ./path.ts	One-time test	No

---

Available imports (no npm install needed)

Package	Purpose
@mariozechner/pi-coding-agent	ExtensionAPI, event types, type guards, truncateHead, truncateTail, withFileMutationQueue
@sinclair/typebox	Type.Object, Type.String, etc. for parameters
@mariozechner/pi-ai	StringEnum for Google-compatible enums
@mariozechner/pi-tui	Text, Box, List, Input, Select