/why

name: why
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."
license: MIT. LICENSE.txt has complete terms
metadata:
    author: Lauren Tan
    github-repo: https://github.com/cursor/plugins
    github-path: pstack/skills/why
    github-ref: refs/heads/main
    github-tree-sha: 46125561306434d8a1d7745d540d8932ab0cd2a2

Why

Investigate the motivation and intent behind code. Why was it built this way? What edge cases were considered? What product, business, or operational constraints shaped the design? What alternatives were rejected, and why?

Companion to the `how` skill. `how` answers what the code does and how it works. `why` answers what forces led to its shape.

How this skill works

Historical context spreads across seven evidence categories: source control history, issue or ticket tracking, long-form documents, real-time team chat, infrastructure observability, error or exception tracking, and product analytics warehouses. You cannot predict from the question alone which one holds the answer, so the skill enumerates available MCPs at run time, maps each to a category, queries all seven in parallel, then synthesizes with explicit confidence calibration. Null results from searched categories are first-class evidence about how the decision was made; report them alongside positive findings. The default is coverage, not minimalism.

Operating Posture

Operate as a careful, cautious, precise investigator. Think like a detective piecing together a historical case from fragmentary records. When the record is thin, say so.

Concretely:

• **Evidence before narrative.** Collect the pieces first, then see what story they support. Never pick a story and recruit the evidence that fits it.
• **Precision over polish.** Prefer the exact quote and citation over a smooth paraphrase. A reader should be able to follow any claim back to its source and verify it in under a minute.
• **Consider what you haven't seen.** The evidence you find is a sample, not the whole truth. Before concluding, ask what you would expect to see if an alternative explanation were true, and whether you looked for it.
• **Name the gaps.** If a thread goes cold, a source isn't searchable, or a question has no answer, document the gap. Don't paper it over with an authoritative-sounding guess.
• **Hedge on purpose.** When evidence is indirect, your language should signal it ("appears to", "likely", "suggests"). Confidence-matching phrasing is a feature of the output, not a stylistic choice the synthesizer may override.
• **No shortcut by code-reading.** The code tells you what it does, rarely why it exists. Resist inferring intent from code shape.

This posture is the working method, not a disclaimer.

Core Epistemics

This skill builds a **patchwork understanding** from fragmented historical evidence. Tickets go stale. Chat threads get deleted. Commit messages lie. People change their minds between the PR description and the implementation. The original author may have left the company.

Be ruthlessly honest about what you know versus what you're inferring. The goal is not a satisfying story; it is to surface evidence, calibrate confidence, and let the user decide.

Principles:

• **Cite everything.** Every claim about intent should reference a specific commit hash, PR number, ticket ID, doc URL, chat permalink, or code comment. If you can't cite it, it's inference, not fact, and must be labeled as such.
• **Prefer "appears to" over "because".** Hedge when evidence is indirect. Reserve confident language for direct, explicit evidence.
• **Surface contradictions.** If two sources disagree, show both. Don't quietly pick the one that fits your narrative.
• **Acknowledge gaps.** If a question has no answer in any source you searched, say so. An honest "we couldn't find out why" beats a confident guess.
• **Multiple hypotheses are valid.** When the evidence fits several stories, present them all with the evidence for each. Let the user triangulate.
• **Beware rationalization.** Code that makes sense today may have been written for reasons that no longer apply, or for no good reason at all. Don't retrofit intent.

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

Parse what the user is asking. The **target** is usually a chunk of code, a pattern, a feature, or a named design decision. The **question** is usually one of:

• "Why was X designed this way?" Design rationale.
• "Why do we do X instead of Y?" Tradeoff or alternatives.
• "What edge cases motivated this?" Defensive reasoning.
• "What business or product constraint led to this?" External forcing function.
• "Why does this code still exist?" Dead-code territory.
• "What's the history of X?" Broad archaeological sweep.

If the target is vague ("why do we do it this way?" with no clear referent), make your best guess from conversation context (open files, recent edits, the code location under discussion, and what was just discussed). State your interpretation briefly so the user can redirect if you're off, then proceed.

Step 2. Establish the Code Anchor

Before running the investigations, anchor them in concrete code. You need:

• The relevant file path(s) and line range(s)
• The key symbols (function names, class names, constants)
• An initial commit list. The last few commits touching the target.
• PR numbers from merge commits (pattern `(#1234)` in the subject line)

Build this inline. It is cheap, and every repository specialist and MCP query needs it.