feature-dev

A guided process for building a new software feature with careful codebase exploration, architecture design, and OpenSpec documentation. OpenSpec is a structured record of requirements, design choices, and implementation tasks.

In plain words
What is it for?
Use it to investigate the codebase, clarify ambiguous behavior, design the architecture, document decisions, and implement a feature with its work tracked through OpenSpec.
Why use it?
It reduces the risk of coding against a mistaken understanding of the existing system or leaving important decisions undocumented. The work is organized into phases so it can be reviewed and resumed.

Command

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 commands/mbertani/opsx-feature-dev/feature-dev
Clone the repo
git clone --depth 1 https://github.com/mbertani/opsx-feature-dev
Per session 44 Only the description is in the session, so the agent can decide to use it. The body loads when it is invoked.
When invoked 2,020 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 $0.00044 $0.02020
Opus 5 $0.00022 $0.01010
Sonnet 5 $0.00009 $0.00404
Haiku 4.5 $0.00004 $0.00202

Measured yesterday against content hash f0062817d307, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

feature-dev 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.

commands/feature-dev.md · 255 lines

How it starts

The opening of the file, as written. The whole thing — 255 lines — stays where its author put it; the contents beside it link to each section on GitHub.

OpenSpec Feature Development

You are helping a developer implement a new feature using a systematic workflow that combines deep codebase understanding with OpenSpec's artifact-driven documentation. Every phase produces or consumes OpenSpec artifacts so the work is traceable and resumable.

Core Principles

  • Ask clarifying questions: Identify all ambiguities, edge cases, and underspecified behaviors. Ask specific, concrete questions rather than making assumptions. Wait for user answers before proceeding with implementation. Ask questions early (after understanding the codebase, before designing architecture).
  • Understand before acting: Read and comprehend existing code patterns first.
  • Read files identified by agents: When launching agents, ask them to return lists of the most important files to read. After agents complete, read those files to build detailed context before proceeding.
  • Simple and elegant: Prioritize readable, maintainable, architecturally sound code.
  • Document through OpenSpec: All decisions, designs, and tasks flow through OpenSpec artifacts.
  • Use TodoWrite: Track all progress throughout the workflow.

Phase 1: Discovery

Goal: Understand what needs to be built and create the OpenSpec change.

Initial request: $ARGUMENTS

Actions:

  1. Create a todo list with all 7 phases.
  2. If the feature is unclear, use the AskUserQuestion tool to ask:
    • What problem are they solving?
    • What should the feature do?
    • Any constraints or requirements?
  3. Derive a kebab-case change name from the description (e.g., "add user authentication" -> add-user-auth).
  4. Create the OpenSpec change:
    openspec new change "<name>"
    
  5. Get the schema and artifact structure:
    openspec status --change "<name>" --json
    
  6. Summarize your understanding and confirm with the user before proceeding.

Phase 2: Codebase Exploration

Goal: Understand relevant existing code and patterns at both high and low levels.

Read the full file on GitHub · 255 lines

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 · 255 lines · 44 tokens per session scan A f0062817d307

Subscribe to this mod's changes

feature-dev is a command published in the GitHub repository mbertani/opsx-feature-dev (4 stars, last pushed 4mo ago), licensed MIT. It adds 44 tokens to every session and 2,020 once invoked, about $0.0002 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.