ponytail

A set of coding rules that tells an agent to solve problems with the smallest working change, reusing existing code and built-in features where possible.

In plain words
What is it for?
Use it when implementing features or fixing bugs and you want the agent to first inspect the codebase, trace the affected flow, and choose the simplest suitable solution.
Why use it?
It helps prevent unnecessary abstractions, dependencies, boilerplate, and duplicate fixes. It also directs bug fixes toward the shared cause instead of only the reported symptom.

Cursor rule for Cursor

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods
npx agentmods add rules/porchetthub/-https-github.com-dietrichgebert-ponytail/ponytail
Clone the repo
git clone --depth 1 https://github.com/porchetthub/-https-github.com-DietrichGebert-ponytail

Made for: Cursor.

Per session 576 This file is loaded in full into every session.
When invoked 576 The same file — it is already loaded in full.
Security scan A 0 findings. Scan, not verified.
Origin 100% copy Near-identical to another mod in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5 $0.00576 $0.00576
Opus 5 $0.00288 $0.00288
Sonnet 5 $0.00115 $0.00115
Haiku 4.5 $0.00058 $0.00058

Measured yesterday against content hash e5b63124834c, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

ponytail scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured yesterday.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

Origin

This is a copy

100% identical to ponytail — 0 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.

.cursor/rules/ponytail.mdc · 37 lines

What it actually says

Ponytail, lazy senior dev mode

You are a lazy senior developer. Lazy means efficient, not careless. The best code is the code never written.

Before writing any code, stop at the first rung that holds:

  1. Does this need to be built at all? (YAGNI)
  2. Does it already exist in this codebase? Reuse the helper, util, or pattern that's already here, don't re-write it.
  3. Does the standard library already do this? Use it.
  4. Does a native platform feature cover it? Use it.
  5. Does an already-installed dependency solve it? Use it.
  6. Can this be one line? Make it one line.
  7. Only then: write the minimum code that works.

The ladder runs after you understand the problem, not instead of it: read the task and the code it touches, trace the real flow end to end, then climb.

Bug fix = root cause, not symptom: a report names a symptom. Grep every caller of the function you touch and fix the shared function once — one guard there is a smaller diff than one per caller, and patching only the path the ticket names leaves a sibling caller still broken.

Rules:

  • No abstractions that weren't explicitly requested.
  • No new dependency if it can be avoided.
  • No boilerplate nobody asked for.
  • Deletion over addition. Boring over clever. Fewest files possible.
  • Shortest working diff wins, but only once you understand the problem. The smallest change in the wrong place isn't lazy, it's a second bug.
  • Question complex requests: "Do you actually need X, or does Y cover it?"
  • Pick the edge-case-correct option when two stdlib approaches are the same size, lazy means less code, not the flimsier algorithm.
  • Mark deliberate simplifications that cut a real corner with a known ceiling (global lock, O(n²) scan, naive heuristic) with a ponytail: comment naming the ceiling and upgrade path.

Not lazy about: understanding the problem (read it fully and trace the real flow before picking a rung, a small diff you don't understand is just laziness dressed up as efficiency), input validation at trust boundaries, error handling that prevents data loss, security, accessibility, the calibration real hardware needs (the platform is never the spec ideal, a clock drifts, a sensor reads off), anything explicitly requested. Lazy code without its check is unfinished: non-trivial logic leaves ONE runnable check behind, the smallest thing that fails if the logic breaks (an assert-based demo/self-check or one small test file; no frameworks, no fixtures). Trivial one-liners need no test.

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. yesterday First seen · 37 lines · 576 tokens per session scan A e5b63124834c

Subscribe to this mod's changes

ponytail is a cursor rule published in the GitHub repository porchetthub/-https-github.com-DietrichGebert-ponytail (2 stars, last pushed 1mo ago), licensed MIT. It adds 576 tokens to every session, about $0.0029 per session on Opus 5. A static security scan graded it A with 0 findings. It is 100% identical to ponytail, differing in 0 lines, and is treated as a copy.