Explain Code

explain-code

Explain specific lines, functions, or files from a pull request with line-number references and synchronized before/after views for changed code.

When to Use It

• A reviewer does not understand what a block of code does
• Explaining a complex algorithm or pattern to a junior contributor
• Understanding a change in context before approving
• Documenting rationale for code that isn't self-explanatory

How to Launch It

In GitHub Copilot Chat:

/explain-code owner/repo#123 src/auth.ts:40-60

Or with natural language:

/explain-code owner/repo#123 "the JWT validation block"

Or for a whole file:

/explain-code owner/repo#123 src/auth.ts

What to Expect

1. Locate the target — Finds the specified lines, function, or file in the PR diff
2. Read context — Pulls in surrounding lines, imports, and related functions for context
3. Produce explanation — Plain-language walkthrough of what the code does, why, and any risks
4. Show line-numbered code — All code references include exact L-number format for traceability
5. Before/after comparison — For changed code, shows what the original code did and what changed

Explanation Format

## src/auth.ts:40-60 — JWT Token Validation

**What it does:**
This block extracts the Bearer token from the Authorization header,
decodes the JWT payload, and verifies the signature against the
server's secret key using the `jsonwebtoken` library.

**Line by line:**
- L42: Splits the Authorization header on " " to get the raw token
- L45: Calls `jwt.verify()` — this throws if invalid or expired
- L52: Attaches decoded payload to `req.user` for downstream handlers

**Important:**
- L47: `process.env.JWT_SECRET` is read here — if undefined, `jwt.verify()`
  accepts any token. Should throw at startup if secret is missing.

Before/After View

For changed code:

Before (main):                     After (this PR):
──────────────────────────────     ──────────────────────────────
const token = req.headers          const token = req.headers
  .authorization                     .authorization?.split(' ')[1]
const decoded = jwt.verify         const decoded = jwt.verify(
  (token, process.env.SECRET)        token,
                                     process.env.SECRET ?? throwMissing()
                                   )

Example Variations

/explain-code owner/repo#123 src/auth.ts:40-60
/explain-code #123 "the role check middleware"
/explain-code #123 src/utils.ts               # Explain whole file
/explain-code #123 L88                        # Single line

Connected Agents

Agent	Role
pr-review agent	Executes this prompt

Related Prompts

• review-pr — full review with all findings
• pr-comment — leave a specific comment on the code
• pr-report — generate a written review document