A review step that asks a separate Rubber Duck Reviewer agent to critique a plan or implementation. Rubber-duck reviewing means explaining or examining work from an independent perspective to expose problems.

Use it before or after substantial engineering work, such as architectural decisions, database changes, security work, or a self-contained implementation unit.

It provides an additional critique before work continues, which can reveal flaws in complex designs, security-sensitive changes, or multi-file implementations.

rubber-duck — Skill for Claude Code and Codex by jim60105

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.

Any agent

npx skills add jim60105/copilot-prompt --skill rubber-duck

Clone the repo

git clone --depth 1 https://github.com/jim60105/copilot-prompt

Made for: Claude Code, Codex.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

README.md

[![agentmods](https://agentmods.dev/badge/skills/jim60105/copilot-prompt/rubber-duck/github.svg)](https://agentmods.dev/skills/jim60105/copilot-prompt/rubber-duck)

Your own site

<a href="https://agentmods.dev/skills/jim60105/copilot-prompt/rubber-duck"><img src="https://agentmods.dev/badge/skills/jim60105/copilot-prompt/rubber-duck/github.svg" alt="Measured on agentmods" height="20"></a>

Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

Your own site · 80×15

<a href="https://agentmods.dev/skills/jim60105/copilot-prompt/rubber-duck"><img src="https://agentmods.dev/badge/skills/jim60105/copilot-prompt/rubber-duck.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 118 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 1,295 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe.

Origin unknown No closer match found 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.

Model	Per session	Once invoked
Fable 5.1	$0.00118	$0.01295
Opus 5	$0.00059	$0.00647
Sonnet 5	$0.00024	$0.00259
Haiku 4.5	$0.00012	$0.00129

Measured 11d ago against content hash e55f4a661656, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-10, from the pricing page.

Security

Grade A, and why

rubber-duck 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 11d ago.

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.

skills/rubber-duck/SKILL.md · 125 lines

The source is not reproduced here

Licensed GFDL-1.3

The repository is licensed GFDL-1.3, which this catalogue does not treat as permission to reproduce the file. Read it at the source.

Read it on GitHub

Files

What ships with it

1 file beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.

references/rubber-duck-system-prompt.md 7.4 KB

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.

11d ago First seen · 125 lines · 118 tokens per session scan A e55f4a661656

Subscribe to this mod's changes

rubber-duck is a skill published in the GitHub repository jim60105/copilot-prompt (21 stars, last pushed 3d ago), licensed GFDL-1.3. It adds 118 tokens to every session and 1,295 once invoked, about $0.0006 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.

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rubber-duck