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.
npx skills add fergupa/claude_plugins --skill feature-specgit clone --depth 1 https://github.com/fergupa/claude_pluginsWrote 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.
[](https://agentmods.dev/skills/fergupa/claude_plugins/feature-spec)<a href="https://agentmods.dev/skills/fergupa/claude_plugins/feature-spec"><img src="https://agentmods.dev/badge/skills/fergupa/claude_plugins/feature-spec.svg" alt="Measured on agentmods" height="20"></a>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 | $0.00052 | $0.01979 |
| Opus 5 | $0.00026 | $0.00989 |
| Sonnet 5 | $0.00010 | $0.00396 |
| Haiku 4.5 | $0.00005 | $0.00198 |
Grade A, and why
feature-spec 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 3d 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.
This is a copy
89% identical to write-spec — 106 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.
How it starts
The opening of the file, as written. The whole thing — 177 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Feature Spec Skill
You are an expert at writing product requirements documents (PRDs) and feature specifications. You help product managers define what to build, why, and how to measure success.
PRD Structure
A well-structured PRD follows this template:
1. Problem Statement
- Describe the user problem in 2-3 sentences
- Who experiences this problem and how often
- What is the cost of not solving it (user pain, business impact, competitive risk)
- Ground this in evidence: user research, support data, metrics, or customer feedback
2. Goals
- 3-5 specific, measurable outcomes this feature should achieve
- Each goal should answer: "How will we know this succeeded?"
- Distinguish between user goals (what users get) and business goals (what the company gets)
- Goals should be outcomes, not outputs ("reduce time to first value by 50%" not "build onboarding wizard")
3. Non-Goals
- 3-5 things this feature explicitly will NOT do
- Adjacent capabilities that are out of scope for this version
- For each non-goal, briefly explain why it is out of scope (not enough impact, too complex, separate initiative, premature)
- Non-goals prevent scope creep during implementation and set expectations with stakeholders
4. User Stories
Write user stories in standard format: "As a [user type], I want [capability] so that [benefit]"
Guidelines:
- The user type should be specific enough to be meaningful ("enterprise admin" not just "user")
- The capability should describe what they want to accomplish, not how
- The benefit should explain the "why" — what value does this deliver
- Include edge cases: error states, empty states, boundary conditions
- Include different user types if the feature serves multiple personas
- Order by priority — most important stories first
Example:
- "As a team admin, I want to configure SSO for my organization so that my team members can log in with their corporate credentials"
- "As a team member, I want to be automatically redirected to my company's SSO login so that I do not need to remember a separate password"
- "As a team admin, I want to see which members have logged in via SSO so that I can verify the rollout is working"
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.
- 3d ago First seen · 177 lines · 52 tokens per session scan A f5d5d24e062b
feature-spec is a skill published in the GitHub repository fergupa/claude_plugins (2 stars, last pushed 7mo ago), licensed Apache-2.0. It adds 52 tokens to every session and 1,979 once invoked, about $0.0003 per session on Opus 5. A static security scan graded it A with 0 findings. It is 89% identical to write-spec, differing in 106 lines, and is treated as a copy.
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