Pi Extension Development

Build Pi coding-agent extensions with the current ExtensionAPI. Use the Pi documentation and examples that come with your installed Pi package as the canonical source when in doubt. Typical locations are the package docs/, examples/extensions/, and generated type definitions for the installed version.

When implementing or debugging an extension, read the relevant docs/examples for your installed Pi version first and prefer current examples over memory.

Current API Quick Start

// ~/.pi/agent/extensions/my-extension.ts
import type { ExtensionAPI } from "@earendil-works/pi-coding-agent";
import { Type } from "typebox";

export default function (pi: ExtensionAPI) {
  pi.registerCommand("hello", {
    description: "Say hello",
    handler: async (args, ctx) => {
      // args is the raw string after /hello, not an object.
      ctx.ui.notify(`Hello ${args.trim() || "world"}!`, "info");
    },
  });

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

Extension Locations

Auto-discovered extensions:

Hot-reload auto-discovered extensions with /reload.

Current Command API

Commands are registered with handler, and the first parameter is always a string containing the raw command tail.

pi.registerCommand("goal", {
  description: "Start a goal",
  getArgumentCompletions: async (prefix) => [{ value: "status", label: "status" }],
  handler: async (args, ctx) => {
    const input = args.trim();
    if (!input) {
      ctx.ui.notify("Usage: /goal <objective>", "warning");
      return;
    }
    // ...
  },
});

Migration pitfall: old object args

Do not write new commands like this:

// Stale/incorrect for current Pi
pi.registerCommand("goal", {
  args: [{ name: "objective" }],
  execute: async (args, ctx) => args.objective,
});

Current Pi does not pass schema-shaped command objects. If compatibility with old code is useful, parse defensively:

function parseCommandInput(args: unknown): string {
  if (typeof args === "string") return args.trim();
  const objective = (args as { objective?: unknown } | null)?.objective;
  return typeof objective === "string" ? objective.trim() : "";
}

Current Tool API

Tools use execute(toolCallId, params, signal, onUpdate, ctx) and return content blocks.

import { Type } from "typebox";
import { StringEnum } from "@earendil-works/pi-ai";

pi.registerTool({
  name: "convert",
  label: "Convert",
  description: "Convert text between formats",
  promptSnippet: "Convert text between json, yaml, and toml",
  parameters: Type.Object({
    format: StringEnum(["json", "yaml", "toml"] as const),
    text: Type.String(),
  }),
  async execute(toolCallId, params, signal, onUpdate, ctx) {
    onUpdate?.({ content: [{ type: "text", text: "Converting..." }] });
    if (signal?.aborted) return { content: [{ type: "text", text: "Cancelled" }] };
    return { content: [{ type: "text", text: params.text }], details: { format: params.format } };
  },
});

Use StringEnum from @earendil-works/pi-ai for string enums, especially for Google-compatible schemas.

Common Events

Current event names include:

pi.on("session_start", async (event, ctx) => {});
pi.on("session_before_compact", async (event, ctx) => {});
pi.on("session_compact", async (event, ctx) => {});
pi.on("session_shutdown", async (event, ctx) => {});
pi.on("before_agent_start", async (event, ctx) => {
  return { systemPrompt: event.systemPrompt + "\n\nExtra instructions" };
});
pi.on("agent_end", async (event, ctx) => {});     // once per user/extension prompt
pi.on("turn_end", async (event, ctx) => {});      // can fire multiple times inside one prompt
pi.on("tool_call", async (event, ctx) => {});
pi.on("tool_result", async (event, ctx) => {});
pi.on("input", async (event, ctx) => ({ action: "continue" }));

Use agent_end when you want one action after a complete agent run. Use turn_end only when you explicitly need every internal LLM/tool round; otherwise you can accidentally queue duplicate follow-up messages.

UI Methods

ctx.ui.notify("Saved", "info");       // "info" | "warning" | "error"
const ok = await ctx.ui.confirm("Delete?", "This cannot be undone");
const choice = await ctx.ui.select("Pick one", ["A", "B"]);
const text = await ctx.ui.editor("Edit text", "initial value");
ctx.ui.setStatus("my-ext", "running");
ctx.ui.setWidget("my-ext", ["line 1", "line 2"]);

Do not use unsupported notify types such as "success"; use "info" unless the message is warning/error.

Sending Messages

User messages

pi.sendUserMessage("Continue the task");
pi.sendUserMessage("Follow up after current run", { deliverAs: "followUp" });

sendUserMessage() sends an actual user message and always triggers a turn. If the agent is streaming, provide deliverAs: "steer" | "followUp".

Custom messages

pi.sendMessage(
  {
    customType: "my-extension",
    content: "Context for the model",
    display: true,          // boolean, not a label string
    details: { label: "Optional render metadata" },
  },
  { triggerTurn: true, deliverAs: "followUp" },
);

Custom messages participate in context. For state that should not be sent to the model, use pi.appendEntry() instead.

Session Persistence

Persist state as custom entries:

pi.appendEntry("my-state", { enabled: true, count: 3 });

pi.on("session_start", (_event, ctx) => {
  const entries = ctx.sessionManager.getBranch();
  const latest = [...entries].reverse().find(
    (e) => e.type === "custom" && "customType" in e && e.customType === "my-state",
  );
});

Custom entries are not part of LLM context. Custom messages are.

Compaction Hooks

Use current compaction events for long-running extensions:

pi.on("session_before_compact", async (event, ctx) => {
  // event.preparation has messagesToSummarize, previousSummary, tokensBefore, firstKeptEntryId, etc.
  pi.appendEntry("my-state", currentState);
});

pi.on("session_compact", async (event, ctx) => {
  pi.sendMessage(
    {
      customType: "my-context-restored",
      content: "Context compacted. Re-read key files before continuing.",
      display: true,
    },
    { deliverAs: "followUp", triggerTurn: true },
  );
});

You can trigger compaction programmatically from event/command contexts with:

ctx.compact({
  customInstructions: "Focus on active goal state and modified files",
  onComplete: () => ctx.ui.notify("Compaction complete", "info"),
  onError: (err) => ctx.ui.notify(`Compaction failed: ${err.message}`, "error"),
});

Context and Runtime Helpers

Useful current helpers:

ctx.sessionManager.getEntries();
ctx.sessionManager.getBranch();
ctx.sessionManager.getSessionFile();
ctx.getContextUsage();
ctx.getSystemPrompt();
ctx.isIdle();
ctx.hasPendingMessages();
ctx.abort();
ctx.shutdown();

Command contexts additionally support session replacement and reload helpers:

await ctx.reload();
await ctx.newSession({ withSession: async (newCtx) => newCtx.sendUserMessage("Continue") });
await ctx.switchSession("/path/to/session.jsonl", { withSession: async (newCtx) => {} });

After ctx.reload(), ctx.newSession(), ctx.fork(), or ctx.switchSession(), do not use captured old pi or old ctx session-bound objects. Use the replacement callback context where provided.

Debugging Checklist

1. Read current docs/types, especially types.d.ts, before assuming an API shape.
2. Confirm the extension is discovered: top-level .ts or directory index.ts under an extension folder.
3. Use /reload after editing an auto-discovered extension.
4. For command bugs, verify handler(args, ctx) treats args as a string.
5. For repeated autonomous actions, prefer agent_end over turn_end unless you need each internal model turn.
6. For compaction bugs, inspect session JSONL for compaction, custom, and custom_message/custom message entries.
7. Use PI_OFFLINE=1 pi --no-session --no-tools --no-skills -p '/your-command status' for lightweight command-load smoke tests when the command does not need an LLM.

Migration Notes from Older Examples

Older extension snippets may show APIs such as:

• @mariozechner/pi-coding-agent imports — aliases still often work, but prefer @earendil-works/pi-coding-agent in new skills/docs.
• registerCommand(... execute: ...) — current API uses handler.
• command args: [{ name }] metadata and args.objective — current command args are raw strings.
• pi.registerKeyboardShortcut / pi.addWidget / pi.addOverlay — current APIs use registerShortcut, ctx.ui.setWidget, and ctx.ui.custom(..., { overlay: true }).
• tool execute(params, context) returning plain strings — current tools use execute(toolCallId, params, signal, onUpdate, ctx) and content blocks.

When existing code uses a stale pattern, fix the root API mismatch rather than adding symptom workarounds.