Rajeev Pentyala – Technical Blog on Power Platform, Azure and AI

Coding agents like GitHub Copilot CLI, Claude Code, and Cursor tend to write more code than a task needs. Say we ask the agent to add a date of birth field to a signup form. It may install a date picker library, write a wrapper component, and add a stylesheet.

The browser already has a date input that does the same job in one line.

Ponytail is an open source skill that makes the coding agent check the simple option first.

This post walks through a quick setup of Ponytail with GitHub Copilot CLI.

What is Ponytail?

Ponytail is a skill you install into your coding agent. It adds one rule to every coding request. Write only what the task actually needs.

• The persona is a senior developer. The skill states it plainly. Lazy means efficient, not careless.
• It works with many agents including GitHub Copilot CLI, Claude Code, Codex, and OpenCode.
• It is MIT licensed and free to use.
• It runs on coding requests. It is not a general writing mode for emails or chat.

Why It Helps

• Agents tend to reach for a library or a custom component when a built in feature already does the job.
• Ponytail checks the cheaper options first, in a fixed order, before it writes anything new.
• The result is a smaller change to review, which also means less to test and maintain.

Key Concept: Less code is the outcome, not the goal. The skill asks what the task needs. The code ends up small because the extra parts were never required.

Prerequisites

• GitHub Copilot CLI installed and signed in.
• A project folder to test against. Any small web project works.
• For Claude Code and Codex only, node on your PATH. Those plugins run two lifecycle hooks. Copilot CLI does not need this.

Setup Steps

Lets walk through the install in two short commands.

Install the Ponytail plugin

• Open GitHub Copilot CLI and run the marketplace command first:

/plugin marketplace add DietrichGebert/ponytail

• Then install the plugin in a second, separate prompt:

/plugin install ponytail@ponytail

• Send these as two separate prompts. Combining them into one does not work.

Note: A plugin and a skill are not the same thing. The plugin is the package that installs Ponytail into Copilot CLI. The skill is the instruction file that tells the agent how to behave.

Choose the intensity level

• Ponytail has three levels. full is the default, so you can skip this step to start.
• Copilot CLI puts plugin commands under the plugin name. Set a level like this:

/ponytail:ponytail ultra

• The three levels behave as follows:
  • lite builds what you asked for, then names the simpler option in one line. You pick.
  • full applies the full rule set. This is the default and the right starting point.
  • ultra writes the smallest version and questions the requirement itself. New users may find this too aggressive.

How Ponytail Decides What to Write

Now we have Ponytail installed. Lets look at how it picks a solution.

Before writing code, the agent walks a list and stops at the first option that fully meets the requirement.

1. Does this need to exist?   -> no: skip it (YAGNI)
2. Already in this codebase?  -> reuse it, don't rewrite
3. Standard library does it?  -> use it
4. Native platform feature?   -> use it
5. Installed dependency?      -> use it
6. Can it be one line?        -> one line
7. Only then: the minimum code that works

A few terms in that list, in plain words:

• YAGNI stands for “You Aren’t Gonna Need It”. Do not build something no one asked for yet.
• Standard library means the features that ship with the programming language itself.
• Native platform feature means something the browser, framework, or operating system already provides.
• Dependency means an external package the project pulls in.

Important: Ponytail reduces the solution, not the investigation. The skill is explicit that it reads the code and traces the real flow first. A small change in the wrong place is just a second bug.

Test It with a Simple Example

Now lets try a request that agents commonly over build. Ask your agent for a date of birth field on a signup form.

Without Ponytail

• The agent installs a date picker library such as flatpickr.
• It then writes a wrapper component and adds a stylesheet.
• On a benchmark run of this task, the change came to 404 lines.

With Ponytail

• The agent reaches step 4 and stops there. The browser already has a date input.
• The same benchmark task came to 23 lines in total.
• The core of the change is a single HTML element:

<!-- ponytail: browser has one -->
<input type="date">

• Notice the comment. Ponytail leaves a ponytail: note whenever a shortcut has a known limit. It records what the current choice assumes and what to do if you outgrow it.

Note: This example was picked because it is a clear case of over building. A custom date picker with brand styling or an unusual calendar may still justify a library. Ponytail is meant to check the simple option first, not to forbid the complex one.

What Ponytail Will Not Simplify

This is the part that makes the skill usable on real projects. Some things are never removed to save lines.

• Input validation at trust boundaries.
• Error handling that prevents data loss.
• Security measures.
• Accessibility basics.
• Anything you explicitly asked for.

If you ask for the complete version, Ponytail builds it and does not argue the point again.

How to Override or Stop It

Ponytail stays active on coding requests once loaded. You keep control in four ways.

• Switch to lite if you want the fuller build with a suggestion attached.
• Ask for the complete version in plain words. The skill treats an explicit request as final.
• Name the parts that must stay, such as a specific library or a validation rule.
• Turn it off by saying stop ponytail or normal mode.

Tip: Ponytail also applies to design decisions, not just to writing code. Its own description covers reviewing and designing code, and choosing libraries. Asking it to review an existing plan works well.

Summary

Ponytail is a small install with one clear job. It makes the agent check the cheap options before writing new code, and it leaves a note when it takes a shortcut. Start at full, try it on a form field, and read the diff it produces.

Full details, benchmark method, and examples are in the Ponytail repository.

🙂