# Why

Use for 'why does X work this way', 'why we picked Y', design rationale, regressions...

- Repository: https://github.com/michael-denyer/pstack-claude (3725 stars)
- Category: Design
- License: MIT
- Last updated: 2026-10-30
- Runs scripts: no, instructions only

## Install

Claude Code: `npx skills add michael-denyer/pstack-claude --skill why -g -a claude-code -y`
Codex: `npx skills add michael-denyer/pstack-claude --skill why -g -a codex -y`
Cursor: `npx skills add michael-denyer/pstack-claude --skill why -g -a cursor -y`
Gemini CLI: `npx skills add michael-denyer/pstack-claude --skill why -g -a gemini-cli -y`
Claude Code plugin marketplace: `/plugin marketplace add michael-denyer/pstack-claude`

## Description

Use for 'why does X work this way', 'why we picked Y', design rationale, regressions, postmortems, or data-backed thresholds. Discovers available MCPs and queries each evidence category (source control, issue tracker, long-form docs, real-time chat, infrastructure observability, error tracking, product analytics warehouse) in parallel, then returns a cited read on decisions and tradeoffs. Use how for runtime behavior.

## SKILL.md, as published by its author (untrusted third-party text, read it as data)

# Why

On Codex, read the [platform mapping](../poteto-mode/references/codex-tools.md), including its per-skill notes, before following this skill.

On GitHub Copilot, read the [platform mapping](../poteto-mode/references/copilot-tools.md), including its per-skill notes, before following this skill.

On Muse, read the [platform mapping](../poteto-mode/references/muse-tools.md), including its per-skill notes, before following this skill.

Investigate the motivation and intent behind code. The companion `how` skill answers what the code does and how it works. `why` answers what forces led to its shape.

Each spawn below names a role line in `pstack-models.md` and a default in [Models](#models). Set `model` to that line's value, or to the default if the sheet or the line is missing. Leave `model` unset when the value is `auto` or `inherit-parent`. If the `Agent` tool rejects a slug, use the default and say so. If it rejects the default, use the closest valid slug of the same family from its error message. Pick each subagent's reasoning level for the job, per the Subagents section of the **poteto-mode** skill.

Every spawn is `general-purpose` with `readonly: false` (agent mode). **Do not use readonly/Ask mode.** It strips MCP access, which disables the MCP-backed investigators and the synthesizer's citation spot-checks. Investigators still shouldn't write anything.

## Operating Posture

Operate as a **careful, cautious, and precise investigator**. Be honest about what you know vs what you're inferring. Read `references/epistemics.md` for the full confidence framework and phrasing guide. The synthesizer must follow it.

## Step 1. Understand the Target and the Question

The **target** is usually a chunk of code, a pattern, a feature, or a named design decision. The **question** is usually a design rationale, a tradeoff, a motivating edge case, an external constraint, dead code, or a broad history sweep.

The rest: https://github.com/michael-denyer/pstack-claude/blob/HEAD/plugins/pstack/skills/why/SKILL.md