omp Hooks and Extensions

Disambiguation — three different hook systems

This machine runs three unrelated things called "hooks". Confirm which one the task means before editing anything:

System	Lives in	Shape
omp hooks/extensions (this skill)	~/.omp/agent/extensions/, <cwd>/.omp/extensions/	TS/JS module, default-exported factory
Claude Code hooks	~/.claude/settings.json hooks	JSON event → shell command
Shared harness guard	~/.agents/harness/hooks/	Normalized cross-harness deny policy

Use ExtensionAPI, not HookAPI

HookAPI is the legacy API. The runtime now initializes the extension runner: --hook is an alias for --extension, discovered .ts/.js hook factories load as extension modules, and tools are wrapped by ExtensionToolWrapper rather than HookToolWrapper.

ExtensionAPI is a superset — same event model, plus extension-only events (tool_execution_*, input, user_bash, user_python, mcp_notification, before_provider_request, session_stop, …), tool/command/renderer/provider registration.

Write new work against ExtensionAPI. Reach for HookAPI only when maintaining an existing hook module.

import type { ExtensionAPI } from "@oh-my-pi/pi-coding-agent";

export default function myExtension(pi: ExtensionAPI) {
  pi.on("tool_call", async (event, ctx) => {
    if (event.toolName !== "bash") return;
    if (!/\brm\s+-rf\s+\//.test(String(event.input.command ?? ""))) return;
    return { block: true, reason: "rm -rf / blocked by policy" };
  });
}

The default export must be a function (not a class). It may return a promise; the loader awaits it.

The two contracts you will use most

Block or rewrite a tool call — tool_call

Returns { block?, reason?, input? }.

• Any handler returning block: true stops execution immediately (first block short-circuits). reason becomes the error text the model sees.
• A handler that throws also blocks — the wrapper fails closed. Do not let incidental exceptions through unless blocking is what you want.
• Returning input replaces the raw execution arguments. Handlers do not see each other's revisions (last non-blocking return wins), it is ignored when block is true, and it never applies to computer calls.

Patch a tool result — tool_result

Returns { content?, details?, isError? }.

• Ordering differs by API: under ExtensionAPI this is middleware-style — each handler sees prior handlers' modifications. Under HookAPI every handler sees the original event and the last return wins.
• isError is typed but HookToolWrapper does not apply it; on tool failure the original error is rethrown after handlers run.
• tool_result still fires with isError: true when the tool failed.

Reshape context — context

Returns { messages? }, fires before each LLM call, chained (each handler receives the previous handler's output). Return nothing to pass through.

Where modules live and what wins

Load order, deduped by absolute path, first seen wins:

1. Native auto-discovered — <cwd>/.omp/extensions, ~/.omp/agent/extensions
2. Discovered .ts/.js hook-capability factories
3. Installed plugin entries (package.json omp.extensions)
4. Explicit configured paths — CLI -e/--extension/--hook first, then settings extensions

A module both auto-discovered and explicitly configured loads once, at the auto-discovered position.

Config (user ~/.omp/agent/config.yml, project <cwd>/.omp/config.yml):

extensions:
  - ~/my-exts/safety.ts
  - ./local/ext-pack
disabledExtensions:
  - extension-module:foo # derived from entry path: /x/foo.ts -> foo

Disable controls: --no-extensions (explicit -e/--hook paths still load), disabledExtensions by capability id. Full precedence, profile paths, and directory-resolution rules are in references/configuration.md.

Pitfalls that actually bite

• Runtime actions are unavailable during load. Calling pi.sendMessage() from the factory throws ExtensionRuntimeNotInitializedError. Register in the factory; act from events, commands, and tools.
• Extensions are not sandboxed — they share the process. A raw setInterval/setTimeout/detached-promise callback that throws escapes handler dispatch, becomes an uncaughtException, and the postmortem handler tears down the whole session. Use ctx.setInterval / ctx.setTimeout / ctx.clearTimer: same signatures, contained failures, unref'd, and cleared on session_shutdown.
• appendEntry customType is a global namespace. Use a reverse-domain key (com.example.my-ext.state) and avoid core-reserved values.
• Gitignore applies unevenly. Native auto-discovery globs with gitignore: true, hidden: false; explicit configured-directory scanning uses readdir and applies no gitignore filtering.
• Directory scanning is one level deep. No recursion past a single subdirectory; index.ts wins over index.js.
• Command names colliding with built-ins are skipped with diagnostics, and reserved shortcuts (ctrl+c, escape, enter, …) are ignored.
• Guard every interactive call — ctx.hasUI is false in headless, print, and subagent modes.

References

• references/event-catalog.md — every event, when it fires, what it returns
• references/configuration.md — discovery roots, profiles, precedence, disable ids, layout examples

Authoritative upstream docs ship with the installed omp and are version-matched; read them directly rather than trusting the website (which is a client-rendered SPA and returns no content to a fetch):

• omp://extensions.md — ExtensionAPI reference
• omp://extension-loading.md — discovery and loading
• omp://hooks.md — legacy hook subsystem internals
• omp://skills/authoring-hooks.md — authoring guide
• omp://skills/examples/safety-hook/README.md — worked example