Codebase Map for Fast Onboarding
A one-screen map of any repo: how to run it, real entry points, one request traced end to end, and where the landmines are.
Before & after
Same prompt, same agent — the only difference is the skill.
I can definitely help with that. You'd want to start by looking at the project structure — what's in the top-level directories. Then check the main entry point, usually something like index.js or main.go. Look at the dependencies to understand what frameworks or libraries are in use. Find the key files where the core logic lives. Read the README if there is one. Check if there are any tests to understand how things work. This could take a while depending on how large the codebase is. How big is the project? What language is it written in? Are there any specific features or modules you're most interested in understanding? That might help me guide you better.
CODEBASE MAP — Quick Understanding PROJECT STRUCTURE: /src — application code (1,200 lines) /tests — unit + integration tests (600 lines) /lib — shared utilities and helpers /config — environment and settings /docs — setup and architecture notes KEY ENTRY POINTS: Main: src/index.ts (initializes app, loads config) API: src/server.ts (Express routes, port 3000) Worker: src/jobs/processor.ts (background task queue) DEPENDENCIES (top 5): Express 4.18 — HTTP server and routing PostgreSQL 13 — persistence layer BullMQ — job queue for async work TypeScript 5 — type safety Jest — testing framework CORE MODULES: 1. Authentication (src/auth/) — JWT tokens, session management 2. Data models (src/models/) — User, Task, Project entities 3. API handlers (src/routes/) — REST endpoints 4. Job queue (src/jobs/) — async notification, reporting tasks 5. Middleware (src/middleware/) — logging, error handling, auth guards DATA FLOW: Client request → Express route → auth middleware → business logic → database query → response COMPILE & RUN: npm run dev (localhost:3000) npm test (Jest suite) npm run build (production bundle) KEY FILES TO READ FIRST: 1. src/index.ts (2 min read) 2. src/server.ts (3 min read) 3. src/models/ overview (5 min read) 4. docs/ARCHITECTURE.md (10 min read) ESTIMATED ONBOARDING TIME: 20-30 minutes to production readiness
About this skill
You just opened a repo you have never seen. The README is two years stale, the folder names lie, and you need to ship a change by Friday. Asking Claude to explain it usually gets you a directory listing with adjectives. This skill reads the repo the way a senior engineer does on day one and hands you a one-screen map you can act on.
What it does
- Pulls run, test, and build commands from CI config and lockfiles, and flags where the README disagrees
- Finds real entry points by type: HTTP routes, CLI commands, workers, cron jobs, library exports, frontend routes
- Traces one real request or command end to end with file paths, so you see how the layers actually connect
- Separates core code from generated, vendored, legacy, and dead code, and uses git churn to show where work really happens
- Lists external seams (database, queues, third-party APIs, env vars) and landmines like half-finished migrations or two competing patterns
- Labels every claim as read in code or inferred from names, so you know what to trust
What you get
A compact codebase map: what the system is, how to run it, entry points, a traced golden path, core vs ignorable modules, external dependencies, landmines, and the three files to read next. Deeper dives on any area on request.
Best for
Developers joining a team, contractors dropped into a client repo, reviewers auditing an unfamiliar service, and anyone about to make their first change in code they did not write.
SKILL.md preview
---
name: codebase-map-fast-onboarding
description: Produces a one-screen, evidence-labeled map of an unfamiliar codebase by reading manifests and CI first, locating real entry points, tracing one request end to end, and separating core code from noise and landmines. Use when the user says "help me understand this repo", "where do I even start in this codebase", "map this project for me", "I just joined and need to get up to speed", "how does a request flow through this app", "the README is useless, how do I run this", "explain this monorepo's structure", or "I need to change X but can't find where it lives".
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# Codebase Map for Fast Onboarding
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