Codebase Onboarding

Codebase Onboarding Skill

Builds a working mental model of an unfamiliar repository before making changes: language, framework, structure, conventions, entry points, and test pattern. Does NOT exhaustively read every file — surveys strategically, then drills in where the work demands it.

When to Use

• Working in a new or unfamiliar repository.
• Before making changes to code you have not read.
• When a project lacks documentation.
• User asks you to "look at this repo" or "understand this project".
• Briefing a sub-agent on context before delegating.
• NOT when you already have a recent mental model of this repo — re-onboarding wastes turns.

Prerequisites

• Read access to the repository.
• The view, grep, and glob Copilot tools.
• The powershell tool to inspect files outside the source tree (lockfiles, etc.).
• Ability to run the project's test command (later, optional).

How to Run

1. Survey the top-level layout with `glob`.
2. Read manifest files (package.json, pyproject.toml, go.mod, *.csproj, pom.xml).
3. Read the README.
4. Identify the architecture pattern from directory names.
5. Open the entry point and one representative test.
6. Note conventions: naming, imports, error handling, test layout.
7. Write a short Quick Map (5–10 lines) for your future self.

Quick Reference

Procedure

Step 1: Survey the Landscape

Use glob on *, then on src/** (or the equivalent). Note top-level files. Use view on the manifest file for the stack — package.json, pyproject.toml, go.mod, *.csproj, pom.xml.

Step 2: Read the README

Open README.md with view. The author usually states the project's purpose, run instructions, and conventions in the first hundred lines.

Step 3: Identify Architecture

Match the directory layout to the table above. Note any divergence — those are the "interesting" parts.

Step 4: Read the Entry Point

Open the entry file with view: src/main.ts, src/app.py, cmd/main.go, Program.cs. Trace the startup path one level deep. You are mapping shape, not deeply auditing logic.

Step 5: Sample One Test

Use glob to locate tests (**/*.test.*, **/test_*.py, **/*_test.go). Open one with view. Note framework, fixture style, and where tests live relative to source.

Step 6: Capture the Quick Map

Write a 5–10 line summary for your own use (and to brief sub-agents):

## Quick Map: <repo-name>
- Language: TypeScript (strict mode)
- Framework: Next.js 14 (App Router)
- Tests: Vitest, co-located `*.test.ts`
- Patterns: Server Components default, Server Actions for mutations
- Entry: `src/app/layout.tsx`
- API: `src/app/api/`
- Models: `src/lib/models/`
- Build: `npm run build`. Deploy: Vercel.

Pitfalls

• DO NOT start editing before completing the survey. You will pick the wrong directory.
• DO NOT ask the user what stack it is when the manifest answers the question.
• DO NOT read the repo linearly file by file. Survey strategically.
• DO NOT impose your preferred conventions. Match the existing style even if you would do it differently.
• DO NOT skip reading at least one existing test — tests are executable docs.

Verification

• Quick Map written (language, framework, tests, entry, build).
• At least one existing test file was opened with view.
• Architecture pattern is named (MVC / monorepo / etc.).
• Run command is identified and recorded.
• Naming/convention rules are noted before any new file is created.