Codebase Onboarding

Systematically explore an unfamiliar codebase and produce an ARCHITECTURE.md that gives a developer everything they need to start contributing.

When to Use

• Joining a new team or project
• Picking up an open-source repo to contribute to
• Revisiting a codebase after a long absence
• Onboarding a teammate who needs a guided tour

Workflow

1. Survey the Repository Structure

Start with a high-level scan of the repo layout:

# Top-level structure
ls -la

# Directory tree (2-3 levels deep)
find . -maxdepth 3 -type d -not -path './.git*' -not -path './node_modules*' -not -path './.direnv*' -not -path './target*' -not -path './dist*' -not -path './.next*' | sort

# Count files by extension to identify primary languages
find . -type f -not -path './.git/*' -not -path './node_modules/*' | sed 's/.*\.//' | sort | uniq -c | sort -rn | head -20

# Repo size and scale
find . -type f -not -path './.git/*' -not -path './node_modules/*' | wc -l

2. Identify the Tech Stack

Read key configuration files to determine the stack:

Check for these files (in order of priority):

File	Reveals
package.json	Node.js ecosystem, dependencies, scripts
Cargo.toml	Rust crate, dependencies
go.mod	Go module, dependencies
pyproject.toml / setup.py / requirements.txt	Python project
flake.nix / default.nix	Nix-managed project
Makefile / Justfile	Build commands and workflows
docker-compose.yml / Dockerfile	Containerization
.github/workflows/*.yml	CI/CD pipeline
tsconfig.json	TypeScript configuration
.tool-versions / .mise.toml	Runtime version management

Read each config file that exists. Note:

• Primary language(s) and framework(s)
• Build tool and task runner
• Package manager
• Test framework
• Linter/formatter
• Key dependencies (the ones that define the architecture)

3. Find Entry Points

Locate where execution begins:

# Common entry points
find . -maxdepth 3 -name "main.*" -o -name "index.*" -o -name "app.*" -o -name "server.*" -o -name "cli.*" | grep -v node_modules | grep -v .git

# API routes / handlers
find . -maxdepth 4 -type d -name "routes" -o -name "handlers" -o -name "controllers" -o -name "api" | grep -v node_modules

# Binary / executable definitions
# (Check Cargo.toml [[bin]], package.json "bin", go main packages, etc.)

Read 2-3 entry point files to understand the application bootstrap.

4. Map the Architecture

Trace the major components by reading key files:

Identify components:

• What are the top-level directories and what does each own?
• Which directories represent distinct services, packages, or modules?
• Where does business logic live vs. infrastructure vs. presentation?

Trace data flow:

• Follow a request from entry point through the layers
• Identify the database/storage layer
• Map external service integrations
• Note message queues, event buses, or async patterns

Draw a Mermaid component diagram:

graph TD
    A[Entry Point] --> B[Router/Handler]
    B --> C[Service Layer]
    C --> D[Data Access]
    D --> E[(Database)]
    C --> F[External API]

Adapt this to the actual architecture found. Include the key components and their relationships.

5. Catalog Patterns and Conventions

Search for recurring patterns:

# Error handling patterns
rg "catch|Error|Result<|anyhow|thiserror|raise|except" --type-list
rg -l "error" --type [primary-lang] | head -5

# Test patterns
find . -type f -name "*test*" -o -name "*spec*" | head -10
# Read one test file to understand conventions

# Configuration patterns
rg -l "config|env|settings" --type [primary-lang] | head -5

Document:

• Error handling: How errors are created, propagated, and reported
• API patterns: REST, GraphQL, RPC conventions; request/response shapes
• Test conventions: File naming, test structure, fixtures, mocking approach
• State management: How application state is stored and accessed
• Configuration: How config values flow into the application

6. Document Development Workflow

Look for workflow clues:

# Check for documented commands
cat Makefile 2>/dev/null || cat Justfile 2>/dev/null || true

# Package manager scripts
cat package.json 2>/dev/null | jq '.scripts' 2>/dev/null || true

# README instructions
cat README.md 2>/dev/null | head -100 || true

Document:

• Build: How to compile or bundle
• Test: How to run the test suite
• Run locally: How to start the app for development
• Deploy: How deployments work (if discoverable from CI config)
• Key env vars: Required environment variables and what they control

7. Write ARCHITECTURE.md

Write the final document to the repo root. Use this structure:

# Architecture

> Auto-generated by codebase exploration. Last updated: [date]

## Overview
[2-3 sentence summary: what this project does, who it's for, and the core technology]

## Tech Stack
| Layer | Technology |
|-------|-----------|
| Language | [e.g., TypeScript 5.x] |
| Framework | [e.g., Next.js 14] |
| Database | [e.g., PostgreSQL via Prisma] |
| Testing | [e.g., Vitest + Playwright] |
| Build | [e.g., Turborepo + pnpm] |
| CI/CD | [e.g., GitHub Actions] |

## Architecture

[Mermaid diagram here]

### Component Descriptions
- **[Component A]**: [what it does, key files]
- **[Component B]**: [what it does, key files]

### Data Flow
[Describe how a typical request flows through the system]

## Key Abstractions
- **[Abstraction 1]**: [what it is, where it lives, why it exists]
- **[Abstraction 2]**: [what it is, where it lives, why it exists]

## Patterns & Conventions

### Error Handling
[How errors are handled]

### API Conventions
[Request/response patterns, naming conventions]

### Test Conventions
[File naming, test structure, how to write a new test]

### Configuration
[How config values are managed]

## Development Workflow

### Prerequisites
[What needs to be installed]

### Build
```bash
[build command]

Test

[test command]

Run Locally

[run command]

Deploy

[How deployment works, or "See CI configuration"]

Entry Points

Entry Point	Purpose	File
[Main]	[Application start]	src/main.ts
[CLI]	[Command-line interface]	src/cli.ts
[API]	[HTTP server]	src/server.ts

Directory Structure

[annotated tree of top-level directories]

## Quality Checklist

Before finishing, verify the ARCHITECTURE.md covers:

- [ ] Someone new could understand what this project does in 30 seconds
- [ ] The tech stack table is complete and accurate
- [ ] The Mermaid diagram reflects the actual component relationships
- [ ] At least 3 patterns/conventions are documented with examples
- [ ] Dev workflow commands have been verified (they exist in Makefile/package.json/etc.)
- [ ] Entry points are listed with file paths
- [ ] No placeholder text remains

## Example Invocation

/onboard

Run from the root of any repository. No arguments needed -- the skill explores the current working directory.