code-explainer

name	code-explainer
description	Use when you need to explain any piece of code for handoff, onboarding, or knowledge transfer — produces a dual-audience explanation (user-facing and modifier-facing) plus the fragile part and key assumption. Triggers: "explain this code", "what does this do", "help me understand", "onboard someone to", "document this", "explain for handoff", "code walkthrough".

Code Explainer

Iron Law

TWO LEVELS EVERY TIME — NEVER COLLAPSE THEM INTO ONE

Level 1 is for someone who uses the code. Level 2 is for someone who changes it. They need different information. Mixing them makes both audiences worse off.

When to Use

• Inheriting code you did not write
• Onboarding a new developer to an existing module
• Preparing to modify something you haven't touched in months
• Handing off ownership to another team
• Writing documentation from code

The Four Outputs (All Mandatory)

Level 1 — For the User (what it does)

Explain what this code accomplishes in plain language. No jargon. A non-technical stakeholder should be able to read this and understand:

• What problem the code solves
• What inputs it needs
• What it produces or causes to happen

Write this as if you're explaining to a product manager, not a developer.

Level 2 — For the Modifier (why it is built this way)

Explain the architectural decisions. A senior engineer about to touch this code needs to know:

• Why is it structured this way? (not just what it does)
• What would break if you changed X?
• What are the dependencies — internal and external?
• Where are the edge cases? What inputs cause unexpected behavior?
• What assumptions are baked in that aren't obvious from reading?
• What would a senior engineer want to verify before touching this?

Do not dumb this down. Level 2 readers are technical.

The Fragile Part (1 sentence, mandatory)

The section of this code most likely to break under unexpected conditions — unusual inputs, concurrent access, external failures, edge-case data. Name the specific location (function, line, block), not a general area.

The Assumption (1 sentence, mandatory)

The key assumption baked into this code that, if wrong, causes it to fail. This is not an edge case — it's a design premise. Examples: "assumes the caller always passes a non-null user object", "assumes the upstream API returns items in chronological order", "assumes this runs in a single-threaded context".

Process

1. Read the code — do not explain from filenames or function names alone. Read the actual implementation.
2. Trace execution — follow the real data flow, not the intended data flow. Note where they differ.
3. Find the structural decisions — look for unusual choices: why a loop instead of a map, why a flag instead of a type, why synchronous instead of async.
4. Identify the fragile section — look for: unchecked array access, external calls without retry/timeout, regex on unvalidated input, shared mutable state, implicit ordering.
5. Name the assumption — ask: "What must be true for this to work correctly that the code doesn't verify?"
6. Write Level 1 first — forces you to understand what it actually does before explaining why.
7. Write Level 2 — now add the structural reasoning. If you can't explain why, say so explicitly.

Output Format

## [Function / Module / File Name]

### Level 1 — What It Does
[Plain language, no jargon. 2-5 sentences. Non-technical reader can follow this.]

### Level 2 — Why It Is Built This Way
[Architectural reasoning. Dependencies. What breaks if you change X. Edge cases. Implicit contracts.]

### The Fragile Part
[One sentence. Specific location. Why it breaks under unexpected conditions.]

### The Assumption
[One sentence. The design premise that, if wrong, causes failure.]

Rules

• Level 1 must be readable by someone who does not code. Test this by re-reading and asking: "would a PM understand this?"
• Level 2 must be useful to a developer who knows the language. Do not over-simplify.
• The fragile part and the assumption are never optional. Every piece of code has both. If you cannot find them, you have not read the code carefully enough.
• If you genuinely cannot determine the assumption, say: "I haven't verified this — would need to trace the callers."
• Do not explain what the code says — explain what it means and why it is that way.

Common Mistakes to Avoid

Mistake	What to do instead
Restating the code in prose ("it loops through the list and…")	Explain the intent and the contract
Vague fragile part ("error handling could be improved")	Name the specific line or block and the specific failure mode
Generic assumption ("assumes valid input")	Name the specific invariant the code relies on
Collapsing Level 1 and Level 2	Keep them separate — different readers, different needs
Explaining a function in isolation	Check what it calls and what calls it before writing Level 2

Related

For debugging broken code: use systematic-debugging skill. For reviewing code for quality issues: use code-reviewer skill. For onboarding to an entire codebase: use codebase-onboarding skill.