codebase-onboarding

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• SKILL.md

SKILL.md (first lines)

# Codebase Onboarding

Systematically analyze an unfamiliar codebase and produce a structured onboarding guide. Designed for developers joining a new project or setting up Gemini CLI in an existing repo for the first time.

## When to Use

- First time opening a project with Gemini CLI
- Joining a new team or repository
- User asks "help me understand this codebase"
- User asks to generate a GEMINI.md for a project
- User says "onboard me" or "walk me through this repo"

## How It Works

### Phase 1: Reconnaissance

Gather raw signals about the project without reading every file. Run these checks in parallel:

```
1. Package manifest detection
   → package.json, go.mod, Cargo.toml, pyproject.toml, pom.xml, build.gradle,
     Gemfile, composer.json, mix.exs, pubspec.yaml

2. Framework fingerprinting
   → next.config.*, nuxt.config.*, angular.json, vite.config.*,
     django settings, flask app factory, fastapi main, rails config

3. Entry point identification
   → main.*, index.*, app.*, server.*, cmd/, src/main/

4. Directory structure snapshot
   → Top 2 levels of the directory tree, ignoring node_modules, vendor,
     .git, dist, build, __pycache__, .next

5. Config and tooling detection
   → .eslintrc*, .prettierrc*, tsconfig.json, Makefile, Dockerfile,
     docker-compose*, .github/workflows/, .env.example, CI configs

6. Test structure detection
   → tests/, test/, __tests__/, *_test.go, *.spec.ts, *.test.js,
     pytest.ini, jest.config.*, vitest.config.*
```

### Phase 2: Architecture Mapping

From the reconnaissance data, identify:

**Tech Stack**
- Language(s) and version constraints
- Framework(s) and major libraries
- Database(s) and ORMs
- Build tools and bundlers
- CI/CD platform

**Architecture Pattern**
- Monolith, monorepo, microservices, or serverless
- Frontend/backend split or full-stack
- API style: REST, GraphQL, gRPC, tRPC

**Key Directories**
Map the top-level directories to their purpose:

<!-- Example for a React project — replace with detected directories -->
```
src/components/  → React UI components
src/api/         → API route handlers
src/lib/         → Shared utilities
src/db/          → Database models and migrations
tests/           → Test suites
scripts/         → Build and deployment scripts
```

**Data Flow**
Trace one request from entry to response:
- Where does a request enter? (router, handler, controller)
- How is it validated? (middleware, schemas, guards)
- Where is business logic? (services, models, use cases)
- How does it reach the database? (ORM, raw queries, repositories)

### Phase 3: Convention Detection

Identify patterns the codebase already follows:

**Naming Conventions**
- File naming: kebab-case, camelCase, PascalCase, snake_case
- Component/class naming patterns
- Test file naming: `*.test.ts`, `*.spec.ts`, `*_test.go`

**Code Patterns**
- Error handling style: try/catch, Result types, error codes
- Dependency injection or direct imports
- State management approach
- Async patterns: callbacks, promises, async/await, channels

**Git Conventions**
- Branch naming from recent branches
- Commit message style from recent commits
- PR workflow (squash, merge, rebase)
- If the repo has no commits yet or only a shallow history (e.g. `git clone --depth 1`), skip this section and note "Git history unavailable or too shallow to detect conventions"

### Phase 4: Generate Onboarding Artifacts

Produce two outputs:

#### Output 1: Onboarding Guide

```markdown
# Onboarding Guide: [Project Name]

## Overview
[2-3 sentences: what this project does and who it serves]

## Tech Stack
<!-- Example for a Next.js project — replace with detected stack -->
| Layer | Technology | Version |
|-------|-----------|---------|
| Language | TypeScript | 5.x |
| Framework | Next.js | 14.x |
| Database | PostgreSQL | 16 |

From Jamkris/everything-gemini-code, MIT.

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