document-feature

Implementation documentation that explains how a software feature works, why it was designed that way, and what to consider before changing it.

In plain words
What is it for?
Writing and maintaining reference docs for implemented features, including their assumptions, trade-offs, and potential pitfalls.
Why use it?
It gives developers the context they need to modify existing code safely, without relying on old conversations or guessing at design decisions.

Skill for Claude CodeCodex

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 skills/qa-vault/codelore/document-feature
Any agent
npx skills add qa-vault/codelore --skill document-feature
Clone the repo
git clone --depth 1 https://github.com/qa-vault/codelore

Made for: Claude Code, Codex.

Per session 159 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 2,891 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
Origin original 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.

ModelPer sessionOnce invoked
Fable 5 $0.00159 $0.02891
Opus 5 $0.00079 $0.01445
Sonnet 5 $0.00032 $0.00578
Haiku 4.5 $0.00016 $0.00289

Measured yesterday against content hash c50c1fa4259d, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

document-feature 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.

skills/document-feature/SKILL.md · 265 lines

How it starts

The opening of the file, as written. The whole thing — 265 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Implementation Documentation Skill

Create and maintain reference documentation for implemented features and functionality. Each document answers the questions a developer (human or AI) would ask before touching the code: What does this do? Why was it done this way? What are the gotchas?

This is NOT a changelog or session log. It doesn't track when things happened or who did them. It describes how things work right now and why.

What Makes a Good Implementation Doc

Think of it like a briefing document. An AI agent picks up a task that says "add rate limiting to the auth endpoints." Before writing a line of code, it reads the auth implementation doc and learns: the auth module uses JWT with symmetric HS256, tokens are verified via middleware on each request, the token store is in-memory (no Redis yet), and the original design chose simplicity over horizontal scalability. Now the agent can make an informed decision about where and how to add rate limiting without breaking assumptions.

A good implementation doc has these qualities:

  • Self-contained. A reader with zero prior context can understand the feature.
  • Honest about trade-offs. Documents what was chosen AND what was sacrificed.
  • Actionable. Contains enough detail that someone can modify the feature safely.
  • Current. Reflects the code as it exists right now, not a historical record.
  • Concise. Covers what matters, skips what doesn't. No padding.

Step 0: Discover Existing Documentation

Before writing anything, scan the project for existing docs. The goal is consistency — new documents should look like they belong.

Discovery Procedure

  1. Find the docs directory. Look for docs/, doc/, documentation/, or docs at the project root. Check README.md for links to documentation. If nothing exists, ask the user where docs should live.

  2. Study existing docs. If implementation docs already exist, open 2–3 of them and extract the conventions:

    • File naming pattern (kebab-case? UPPERCASE? numbered?)
    • Document structure (what sections do they use? what order?)
    • Frontmatter (YAML? which fields?)
    • Formatting style (prose vs bullets, heading levels, code reference style)
    • Level of detail (high-level overviews vs deep implementation details?)
    • How they cross-reference other docs or code

Read the full file on GitHub · 265 lines

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.

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 · 265 lines · 159 tokens per session scan A c50c1fa4259d

Subscribe to this mod's changes

document-feature is a skill published in the GitHub repository qa-vault/codelore (2 stars, last pushed 4mo ago), licensed MIT. It adds 159 tokens to every session and 2,891 once invoked, about $0.0008 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 2026-08-31.

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