---
name: "how"
description: "Use for \"how does X work\", code walkthroughs before changing something, and placement / ownership / layering questions (\"where should this live\", \"which package owns this\", \"is this the right layer\"). Explains subsystem architecture, runtime flow, onboarding mental models. Use why for motivation."
---
> Codex host: Follow [CODEX.md](../../CODEX.md) for applicability, permissions, and supported tools. Read only the references needed for the current action.

# How

Explore the codebase to answer "how does X work?" questions. Produce architectural explanations at the level of a senior engineer onboarding onto a subsystem, enough to build a working mental model, not so much that it reads like annotated source code.

The Primary investigates and explains directly by default. An explicit user request may authorize scoped read-only explorers. Resolve their roles, models, and efforts through `MODELS.md` and supported tool arguments through `CODEX.md`.

## 1. Assess complexity

If the scope is ambiguous, state your interpretation and explore. The user can redirect.

- **Simple** (a single module, a small utility, a narrow question such as "how does function X work"): no explorers. The Primary explores and explains in one pass, built from `references/explainer-prompt.md` without the explorer-findings section. Go to step 4.
- **Complex** (a subsystem spanning multiple files or services, a cross-cutting feature, a full architectural overview): go to step 2.

When in doubt, take the simple path.

## 2. Explore (complex only)

Decompose the question into 2 to 4 angles, each a distinct slice of the subsystem. The Primary collects the evidence for each angle. If the user explicitly requested parallel explorers, give each authorized explorer `references/explorer-prompt.md` with its angle filled in.

## 3. Synthesize (complex only)

The Primary synthesizes the collected evidence using `references/explainer-prompt.md`. Include any authorized explorer findings. A separate explainer is optional only within explicit user authorization.

## 4. Present

Present the supported explanation, its evidence, and its gaps. The Primary owns synthesis and final acceptance.

## Output format

The explanation uses the sections defined in `references/explainer-prompt.md`, dropping any that do not apply: Overview, Key Concepts, How It Works, Where Things Live, Gotchas.