Codebase Explainer

Produce a durable onboarding artifact — the document past-you wishes you'd written before stepping away from this repo for six months. Not a chat answer that disappears: a committed ONBOARDING.md with a tight "read this first" list, system overview, dependency map, startup flow, auth flow, and the important files. Every claim cited with file:line.

When to use this skill

• "explain this codebase" / "explain the codebase" / "what does this repo do"
• "onboard me" / "give me a tour" / "tour this repo" / "where do I start"
• "I haven't looked at this in months" / "I forgot how this works"
• "codebase overview" / "read this first" / "create an onboarding doc"
• /codebase-explainer / /onboard

Do not auto-trigger for:

• Architecture refactor suggestions → improve-codebase-architecture
• Feature implementation → task-executor
• Bug triage → diagnose
• A specific diff review → code-review
• Session hand-off (different artifact, session-scoped) → handoff

Workflow

1. Detect the lay of the land (no sub-agents yet)

Read root-level signals directly to scope the exploration:

• Manifests — package.json, pyproject.toml, Cargo.toml, go.mod, *.csproj, *.sln, Gemfile, composer.json, pom.xml, build.gradle*.
• Entry-point hints — next.config.*, vite.config.*, tsconfig.json, Procfile, Dockerfile, docker-compose*.yml, main.*, Program.cs, manage.py.
• Existing docs — README.md, CONTEXT.md, docs/, any ONBOARDING.md already present.
• Workspace shape — monorepo (pnpm-workspace.yaml, lerna.json, nx.json, turbo.json) vs. single project.

If an ONBOARDING.md already exists, this is refresh mode: read it first, then look for drift (new top-level deps, moved entry points, new/removed routes, auth changes) and update in place — don't rewrite from scratch.

2. Run Explore sub-agents in parallel

Spawn subagent_type=Explore calls in a single message so they run concurrently. Brief each agent self-containedly — it has no conversation context. Default fan-out:

• System overview — what this project actually does, key domains, top-level modules, public-facing surfaces (HTTP routes / CLI commands / exported APIs).
• Dependency map — top-level prod deps only (skip dev/test/types). For each: what it does in this codebase (not generic), and the load-bearing call site (file:line). If a dep is genuinely non-obvious (niche library, internal fork, name that doesn't reveal its purpose), the agent MAY check context7 (or any docs-MCP server available in the environment) for a one-line "what is this library" — context7__resolve-library-id → context7__query-docs. Skip the lookup for well-known libraries (React, Express, Prisma, etc.) — the generic description is what we're trying to avoid; what matters is how it's used here. The "how it's used here" line requires a call site the agent actually opened. If no call site is found, the row reads "declared but no usage found" — that's a finding (candidate for removal), not a gap to paper over with a guess from the package name.
• Startup flow — entry point → bootstrap → config loading → server start. A numbered sequence with file:line per step.
• Auth flow — login/session/token handling, middleware/guards, user model, where authorization decisions are made. This section must be a traced code path — a middleware/guard/session check the agent read this session, cited file:line — never an inference from an auth library appearing in the manifest. If none, the agent must say "no auth detected" explicitly AND state what was searched (e.g. "grepped for middleware, session, jwt, [Authorize], guards — no hits") so Step 3 doesn't fabricate one.
• Important files — the 5–15 files a new contributor must know about, each with a one-line "why it matters." A file earns its slot by being load-bearing, and the agent names the evidence per file: imported widely (roughly how many importers), owns a core domain concept, or is an entry point. "Looks important" is not evidence.

Each agent reports back in ≤300 words with file:line citations. Don't ask Explore to read entire files — ask for the specific answer plus citations.

3. Synthesize ONBOARDING.md

Write the artifact to docs/ONBOARDING.md if a docs/ directory exists, otherwise ONBOARDING.md at repo root.

No-fabrication gate: every claim in the file traces to a probe run this session — a file an agent read, a grep it ran. If a claim can't name its probe, it's a hypothesis: verify it or omit it. Never fill a section by inference from framework conventions ("Next.js apps usually…").

Sections, in this order:

1. Read this first — 3–7 bullets max. The minimum a fresh contributor needs to not be dangerous.
2. System overview — 1–3 paragraphs. What it does, who it serves, the dominant architectural shape.
3. Startup flow — numbered, with file:line per step.
4. Auth flow — same shape, or a one-liner "No authentication layer — this is X (searched: middleware, session/token handling, guards)" if the agent confirmed none.
5. Dependency map — table: Dependency | What it does here | Key call site.
6. Important files — bulleted list with file:line and a one-line "why it matters."
7. Project glossary (optional) — only if CONTEXT.md exists or domain terms surfaced repeatedly during exploration.
8. Footer — one line: "Generated by /codebase-explainer. Re-run to refresh."

No timestamps in the file body (dates rot). The git history is the timestamp.

4. Render a condensed summary inline

After writing the file, render in chat: the "Read this first" bullets verbatim, plus a one-sentence pointer to the file path. Do not re-paste the full document — the file is the artifact.

5. Offer the share step (optional)

If making this teammate-accessible would help, mention ShareOnboardingGuide — it uploads ONBOARDING.md and returns a link. Offer once; never auto-invoke.

Examples

Example 1: First pass on a six-month-old project

User: "I haven't touched this repo since November — onboard me"

Claude:

1. Reads package.json, next.config.ts, prisma/schema.prisma, existing README.md. Notes: Next.js App Router + NextAuth + Prisma + Postgres.
2. Spawns five Explore agents in parallel (overview, deps, startup, auth, important files) in one message.
3. Synthesizes → writes ONBOARDING.md with startup flow rooted at app/layout.tsx:12 → lib/auth.ts:34 and auth flow citing the NextAuth authOptions and the requireSession() helper.
4. Prints the "Read this first" bullets and the file path. Offers ShareOnboardingGuide.

Example 2: Refresh against an existing onboarding doc

User: "/codebase-explainer — refresh the onboarding doc, I added Stripe and a new admin route last sprint"

Claude:

1. Reads existing docs/ONBOARDING.md. Spawns Explore agents scoped to "what's changed": new top-level deps, new routes under app/**/route.ts, auth changes.
2. Updates the dependency map (Stripe row added with lib/payments/stripe.ts:18 citation), appends the admin route to startup flow, leaves untouched sections alone.
3. Reports a diff-style summary in chat: "+1 dep, +1 route, auth unchanged."

Anti-patterns

• ❌ Generic framework descriptions ("This is a Next.js app."). Call out what makes THIS codebase non-obvious — the custom session helper, the funky background job runner, the legacy module that traps newcomers.
• ❌ Listing every dependency including dev/test/types. Prod + top-level only.
• ❌ "Important files" with 40 entries. 5–15 is the budget. If you can't pick, you don't understand the codebase yet — Explore more.
• ❌ Inventing an auth flow when there isn't one. If the agent reports no auth, the section says "No authentication layer" plus what was searched. That's load-bearing information for a future reader.
• ❌ Auth described from the manifest instead of the code. ❌ "Uses NextAuth for authentication" because next-auth sits in package.json vs. ✅ "Requests hit middleware.ts:8 → auth() from lib/auth.ts:12; unauthenticated users redirect at middleware.ts:19." Trace the path or write "none detected (searched: …)".
• ❌ Papering over a dependency with no found call site. "Declared but no usage found" goes in the table verbatim — it's a finding.
• ❌ Important files picked by vibes. Each entry names its evidence: import count, domain concept owned, or entry-point role.
• ❌ Reading whole source files into the main context. Sub-agents return summaries with file:line citations — trust them.
• ❌ Claims without file:line citations. A claim the next reader can't verify rots fast.
• ❌ Hard-coded dates ("as of November 2025") in the file body. Git history is the timestamp.
• ❌ Rewriting an existing ONBOARDING.md from scratch when the user asked to refresh.
• ✅ Tight "read this first," cited everywhere, explicit about absences, refreshable.

Notes

• Composes with improve-codebase-architecture: an onboarding pass often surfaces shallow-module clusters worth a deepening review. Hand the user a pointer; don't propose refactors here.
• Composes with handoff: ONBOARDING.md is project-level orientation (durable across many sessions); a handoff is session-level state. Different artifacts, both useful.
• ShareOnboardingGuide (when available in the environment) uploads ONBOARDING.md and returns a teammate-shareable link. Optional; user-initiated.