Examples

Example: Caveman

The smallest real plugin — one middleware hook that changes how every agent writes.

Official plugin · one systemPrompt middleware · no permissions · 15 lines

Caveman makes agents answer dramatically shorter — fragments, no filler — while keeping code, commands, errors and URLs byte-exact. It's inspired by the open-source caveman prompt, and it shows how little a plugin needs to be.

The package

caveman/
  readme.md
  index.ts

---
name: Caveman
description: An output-brevity skill — appends terseness guidance so the model produces short, fragment-based answers while keeping code, commands, and URLs byte-exact.
version: 1.0
---

# Caveman
…
## Permissions

None. The middleware only rewrites the system prompt it is handed — no `host.*` calls, no network egress.

// plugins/caveman/index.ts
const GUIDANCE =
  "\n\n## Caveman mode (output-brevity skill)\n" +
  "Rewrite your responses to be dramatically shorter while keeping technical accuracy.\n" +
  "- Cut filler, pleasantries, and explanatory padding.\n" +
  "- Use sentence fragments and minimal grammar.\n" +
  "- Keep code, commands, error messages, and URLs byte-for-byte exact; never abbreviate them.\n" +
  "- Keep the user's language (French input stays French).\n" +
  "Say the least that fully answers. Example: 'New ref each render. Wrap in useMemo.' not a paragraph.";

globalThis.middleware.systemPrompt = async (context) => {
  return context.systemPrompt + GUIDANCE;
};

Walkthrough

1. GUIDANCE is a constant. Everything the plugin does is decided at the top level, so publish-time introspection sees it plainly.
2. globalThis.middleware.systemPrompt = … — assigning this one registry key is the whole declaration. At publish the platform reads it back and records one capability: Middleware · systemPrompt.
3. The handler appends and returns. It receives the system prompt this turn is about to use and returns the new one. It doesn't need tools (there's nothing to cancel on this hook) or any other context.
4. No host calls → no permissions. The Browse page's What it can reach is empty. That is a strong promise to an installer: this code can't reach the network, files, agents, or anything else.

What happens at run time

Every turn of every agent on the installing account: the platform composes the system prompt, runs the systemPrompt chain (Caveman, plus any other installed plugins' hooks, in order), and sends the result to the model. Caveman has 5 seconds; it needs microseconds. If it ever failed, the prompt would pass through unchanged.

Variations

• My Dude (also official) is the same shape with an instruction whose effect is exactly checkable — every reply ends with "my dude". Useful as a probe that middleware reaches the model.
• Ponytail (official) appends a "smallest correct solution" ladder for coding agents.
• Make it configurable with a setting — see the Quickstart.
• Apply it only to some agents by checking context.agentId or context.sessionKind.