---
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."
disable-model-invocation: true
---

# Why

Investigate the motivation and intent behind code.

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

Read [host capabilities](../../references/host-capabilities.md), [model roles](../../references/model-roles.md), and the [orchestration contract](../../references/orchestration-contract.md) before assigning investigators. Bind operations to tools exposed in this session and observe results. Preserve the role labels `why investigators` and `why synthesizer`; select supported models under the model-role contract. Record requested and actual models only when observed, otherwise use `unknown`. Invocation metadata above is retained without assuming host enforcement.

## 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

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 a design rationale, a tradeoff, a motivating edge case, an external constraint, dead code, or a broad history 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, cursor location, 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 spawning investigators, anchor the investigation 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.

```bash
# Blame target lines for last-touch commits
git blame -L <start>,<end> <file>

# Full file history, with patches, through renames
git log --follow -p -- <file>

# Last N commits touching the file, PR numbers visible
git log --oneline -20 -- <file>

# Extract PR numbers from a commit message
git log -1 --format=%B <commit>
```

Pull PR bodies and discussion via `gh` for any substantive commits:

```bash
gh pr view <number> --json title,body,author,createdAt,mergedAt,labels,closingIssuesReferences,comments,reviews
```

Capture this as seed context (file paths, symbols, commits, PR numbers, linked ticket IDs). Pass it to the investigators.

## Step 3. Spawn Parallel Investigators (default posture)

**Default to the full parallel investigation.**

### Discovery

Before spawning investigators, inspect the session’s exposed tools and connector descriptions. Do not assume a host-specific MCP directory exists. Discover read operations, authorization and observed access for each category. Source-specific API names in the playbooks are examples; bind their purpose to the actual tool schema before use.

Map each available MCP to one evidence category:

1. Source control history
2. Issue / ticket tracker
3. Long-form documents
4. Real-time team chat
5. Infrastructure observability
6. Error / exception tracking
7. Product analytics warehouse

Try authorized local git history and available `gh` or source-control connector reads. Neither execution nor authenticated PR access is guaranteed. Keep local history and shared PR records distinct, and report missing access explicitly. For the other six, classify using the MCP name, server instructions, tool names, and resource descriptors. If an MCP could fit more than one category, choose the one matching its primary evidence. Record ambiguous cases in the coverage map.

Aim for a complete **coverage map**. Document unavailable, denied, failed and searched-empty categories separately. An unavailable source is not a null search result.

Launch all matching investigators in a single message so they run concurrently. Don't ask one agent to cover multiple MCPs.

Investigator configuration follows the orchestration contract. Each investigator owns one source, uses the `why investigators` role, and needs authorized source reads. A host mode that strips connectors cannot serve a connector-backed investigation. Keep investigators read-only in behavior: do not write files, send messages, change tickets, authenticate accounts, or modify external state. If independent workers are unavailable, perform separately scoped sequential category passes and synthesis, label reduced capability, and never claim worker independence. If no authorized source reads remain, report blocked.

Each investigator gets:
1. The base prompt from `references/investigator-prompt.md`
2. The category playbook `references/sources/<source>.md` for the selected MCP, adapted from the examples in `references/source-playbook.md`
3. The cross-cutting `references/sources/incident-postmortem.md` **if the target code looks defensive** (null checks, retry logic, timeout handling, rate limiting, feature flags, egress guards, OOM handlers)
4. The code anchor from Step 2 (file paths, symbols, commit hashes, PR numbers, ticket IDs)
5. The user's original question

### Investigator roster. One per available evidence category

Spawn one investigator per category that has a matching MCP. Each owns exactly one tool or MCP.

Each entry names the category and the kind of "why" it uniquely surfaces. Use it to know what to expect back, how to name a gap when a category returns empty, and (only in the rare provably-irrelevant case) to justify a skip.

1. **Source control investigator**. Git history, `gh` for PRs, code comments, tests. Always attempt this category. Missing git, execution or PR access is a gap. Best at surfacing *implementation-time rationale captured during review*.

2. **Issue / ticket tracker investigator** (e.g. Linear, Jira, GitHub Issues, Plane, Shortcut MCP). Best at surfacing *the product or business forcing function*. Strongest when the why is external to engineering.

3. **Long-form documents investigator** (e.g. Notion, Confluence, Google Docs, Coda MCP). Best at surfacing *long-form design rationale*. Where the why is written out before it becomes code.

4. **Real-time team chat investigator** (e.g. Slack, Discord, Microsoft Teams, Mattermost MCP). Best at surfacing *real-time deliberation that never reached a doc*. Especially important when the source control, ticket, and doc paper trail is thin.

5. **Infrastructure observability investigator** (e.g. Datadog, New Relic, Honeycomb, Grafana, Splunk MCP). Infra/runtime view. Best at surfacing *infrastructure and runtime reality that motivated the code*. Strongest when the target reacts to an infra signal (timeouts, retries, rate limits, circuit breakers).

6. **Error / exception tracking investigator** (e.g. Sentry, Rollbar, Bugsnag, Airbrake MCP). Best at surfacing *the specific exceptions and error trajectories that motivated defensive or corrective code*. Strongest for catch blocks, null guards, type checks, retries, and other defenses.

7. **Product analytics warehouse investigator** (e.g. Databricks, Snowflake, BigQuery, ClickHouse, dbt, Redshift MCP). Product/data view. Best at surfacing *product and data reality that shaped the code*. Strongest for flag-gated code, experiment-driven ships, data migrations, and "where did this number come from" questions.

### When to skip an investigator

Only skip with an **explicit, written justification** that goes in the final "Sources Consulted" section. Two valid reasons:

- **No MCP is available for that category** in this environment. Flag this as a gap, not a choice. Example: "Real-time team chat skipped. No matching MCP available, so the conversational record was not searchable."
- **The source is provably irrelevant**, not just "probably irrelevant." A high bar. Example: "Error / exception tracking skipped. Target is a build-time script with no runtime code path."

If your scope assessment suggests a single-commit trivial target where the PR description already contains the complete answer, you may answer inline **only after** confirming all seven available category searches would be redundant. Say so explicitly. This should be rare.

## Step 4. Synthesize

Spawn one synthesizer through an observed worker binding, using the `why synthesizer` role and authorized source reads for citation checks. Use sequential synthesis with an explicit reduced-capability label when workers are unavailable. The synthesizer must not write files or modify external state.

The synthesizer gets:
1. The investigator findings, including any null results and any categories skipped with justification
2. The code anchor from Step 2 (file paths, symbols, commit hashes, PR numbers, ticket IDs)
3. The user's original question
4. The epistemics framework from `references/epistemics.md`
5. The synthesizer prompt template from `references/synthesizer-prompt.md`

## Step 5. Present

Take the synthesizer's output and present it to the user. You may lightly edit for clarity or add context from the conversation, but **do not rewrite the confidence language**.

## Output Format

The output structure is the one in `references/synthesizer-prompt.md`: The Question, The Code in Question, What We Found, What We Can Reasonably Infer, Competing Hypotheses, What We Don't Know, Sources Consulted, Confidence Summary. Adapt as needed, but keep the confidence separation intact, and keep Sources Consulted as one line per investigator, including the ones that returned nothing or were skipped, with the reason.

After the Sources Consulted block, if the user's `why` question is a precursor to actually changing this code, convert the lineage findings into a Preserve / Change / Avoid / Risk constraint set suitable for planning the change.

## Common Failure Modes to Avoid

- **Recency bias**. Assuming the most recent commit is authoritative. The current shape is often the accretion of many earlier decisions. Trace back.

Keep private source visibility intact. Retrieve only authorized relevant content, avoid raw telemetry or personal-data dumps, and do not redistribute restricted records beyond the authorized audience. External messages and writes require separate explicit authorization.

## Reference Files

- `references/epistemics.md`. Confidence tiers and phrasing guide. The synthesizer must follow it.
- `references/investigator-prompt.md`. Base prompt template for investigator subagents.
- `references/source-playbook.md`. Index pointing at the category playbooks below.
- `references/sources/*.md`. One self-contained example playbook per category, plus cross-cutting `incident-postmortem.md`. Give an investigator the single file that matches its category and adapt it to the available MCP.
- `references/synthesizer-prompt.md`. Prompt template for the synthesizer subagent, including the output format.