MCPInstall

existing-repo

Analyze existing repositories, maintain structure, setup guardrails and best practices

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Install

Works with every agent the skills CLI supports.

npx skills add https://github.com/alinaqi/maggy --skill existing-repo
  1. 01

    Install

    Run the command in your project. The CLI asks which agents to install for.

  2. 02

    Describe the task

    Ask your agent for something this skill covers. It loads existing-repo automatically when relevant.

  3. 03

    Review the result

    The agent follows the SKILL.md steps, scripts and references bundled with the skill.

SKILL.md

The instructions your agent reads when the skill is triggered.

Existing Repository Skill

For working with existing codebases - analyze structure, respect conventions, and set up proper guardrails without breaking anything.

Sources: Husky | lint-staged | pre-commit | commitlint


Core Principle

Understand before modifying. Existing repos have conventions, patterns, and history. Your job is to work within them, not reorganize them.


Phase 1: Repository Analysis

ALWAYS run this analysis first when joining an existing repo.

1.1 Basic Detection

# Check git status
git remote -v 2>/dev/null
git branch -a 2>/dev/null
git log --oneline -5 2>/dev/null

# Check for existing configs
ls -la .* 2>/dev/null | head -20
ls *.json *.toml *.yaml *.yml 2>/dev/null

1.2 Tech Stack Detection

# JavaScript/TypeScript
ls package.json tsconfig.json 2>/dev/null

# Python
ls pyproject.toml setup.py requirements*.txt 2>/dev/null

# Mobile
ls pubspec.yaml 2>/dev/null          # Flutter
ls android/build.gradle 2>/dev/null   # Android
ls ios/*.xcodeproj 2>/dev/null        # iOS

# Other
ls Cargo.toml 2>/dev/null             # Rust
ls go.mod 2>/dev/null                 # Go
ls Gemfile 2>/dev/null                # Ruby

1.3 Repo Structure Type

PatternDetectionMeaning
Monorepopackages/, apps/, workspaces in package.jsonMultiple projects, shared tooling
Full-Stack Monolithfrontend/ + backend/ in same repoSingle team, tightly coupled
Separate ConcernsOnly frontend OR backend codeSplit repos, separate deploys
MicroservicesMultiple service-* or domain dirsDistributed architecture
# Detect repo structure type
if [ -d "packages" ] || [ -d "apps" ]; then
    echo "MONOREPO detected"
elif [ -d "frontend" ] && [ -d "backend" ]; then
    echo "FULL-STACK MONOLITH detected"
elif [ -d "src" ] || [ -d "app" ]; then
    # Check if it's frontend or backend
    grep -q "react\|vue\|angular" package.json 2>/dev/null && echo "FRONTEND detected"
    grep -q "fastapi\|express\|django" package.json pyproject.toml 2>/dev/null && echo "BACKEND detected"
fi

1.4 Directory Mapping

# Get directory structure (max 3 levels)
find . -type d -maxdepth 3 \
    -not -path "*/node_modules/*" \
    -not -path "*/.git/*" \
    -not -path "*/venv/*" \
    -not -path "*/__pycache__/*" \
    -not -path "*/dist/*" \
    -not -path "*/build/*" \
    2>/dev/null | head -50

# Identify key directories
for dir in src app lib core services api routes components pages hooks utils models; do
    [ -d "$dir" ] && echo "Found: $dir/"
done

1.5 Entry Points

# Find main entry points
ls index.ts index.js main.ts main.py app.py server.ts server.js 2>/dev/null
cat package.json 2>/dev/null | grep -A1 '"main"'
cat pyproject.toml 2>/dev/null | grep -A1 'scripts'

Phase 2: Convention Detection

Identify and document existing patterns before making changes.

2.1 Code Style

# Check for formatters
ls .prettierrc* .editorconfig .eslintrc* biome.json 2>/dev/null  # JS/TS
ls pyproject.toml | xargs grep -l "ruff\|black\|isort" 2>/dev/null  # Python

# Check indent style from existing files
head -20 src/**/*.ts 2>/dev/null | grep "^\s" | head -1  # tabs vs spaces

2.2 Testing Setup

# JS/TS testing
grep -l "jest\|vitest\|mocha\|playwright" package.json 2>/dev/null
ls jest.config.* vitest.config.* playwright.config.* 2>/dev/null

# Python testing
grep -l "pytest\|unittest" pyproject.toml 2>/dev/null
ls pytest.ini conftest.py 2>/dev/null

# Test directories
ls -d tests/ test/ __tests__/ spec/ 2>/dev/null

2.3 CI/CD Setup

# Check existing workflows
ls -la .github/workflows/ 2>/dev/null
ls .gitlab-ci.yml Jenkinsfile .circleci/ 2>/dev/null

# Check deploy configs
ls vercel.json render.yaml fly.toml railway.json Dockerfile 2>/dev/null

2.4 Documentation Style

# Find README pattern
head -30 README.md 2>/dev/null

# Find existing docs
ls -la docs/ documentation/ wiki/ 2>/dev/null
ls CONTRIBUTING.md CHANGELOG.md 2>/dev/null

Phase 3: Guardrails Audit

Check what guardrails exist and what's missing.

3.1 Pre-commit Hooks Status

# Check for hook managers
ls .husky/ 2>/dev/null && echo "Husky installed"
ls .pre-commit-config.yaml 2>/dev/null && echo "pre-commit framework installed"
ls .git/hooks/pre-commit 2>/dev/null && echo "Manual pre-commit hook exists"

# Check what hooks run
cat .husky/pre-commit 2>/dev/null
cat .pre-commit-config.yaml 2>/dev/null

3.2 Linting Status

# JS/TS linting
grep -q "eslint" package.json && echo "ESLint configured"
grep -q "biome" package.json && echo "Biome configured"
ls .eslintrc* biome.json 2>/dev/null

# Python linting
grep -q "ruff" pyproject.toml && echo "Ruff configured"
grep -q "flake8" pyproject.toml setup.cfg && echo "Flake8 configured"

3.3 Type Checking Status

# TypeScript
ls tsconfig.json 2>/dev/null && echo "TypeScript configured"
grep "strict" tsconfig.json 2>/dev/null

# Python type checking
grep -q "mypy" pyproject.toml && echo "mypy configured"
grep -q "pyright" pyproject.toml && echo "pyright configured"
ls py.typed 2>/dev/null

3.4 Commit Message Enforcement

# commitlint
ls commitlint.config.* 2>/dev/null && echo "commitlint configured"
cat .husky/commit-msg 2>/dev/null
grep "conventional" package.json 2>/dev/null

3.5 Security Scanning

# Check for security tools
grep -q "detect-secrets\|trufflehog" .pre-commit-config.yaml package.json 2>/dev/null
ls .github/workflows/*.yml | xargs grep -l "security\|audit" 2>/dev/null

Phase 4: Guardrails Setup

Only add missing guardrails. Never overwrite existing configurations.

4.1 JavaScript/TypeScript Projects

Husky + lint-staged (if not present)

# Check if already installed
if [ ! -d ".husky" ]; then
    # Install Husky
    npm install -D husky lint-staged
    npx husky init

    # Create pre-commit hook
    echo 'npx lint-staged' > .husky/pre-commit
    chmod +x .husky/pre-commit
fi

lint-staged config (add to package.json if missing):

{
  "lint-staged": {
    "*.{ts,tsx,js,jsx}": [
      "eslint --fix",
      "prettier --write"
    ],
    "*.{json,md,yml,yaml}": [
      "prettier --write"
    ]
  }
}

ESLint (if not present)

# Check if eslint exists
if ! grep -q "eslint" package.json; then
    npm install -D eslint @typescript-eslint/parser @typescript-eslint/eslint-plugin
fi

eslint.config.js (ESLint 9+ flat config):

import eslint from '@eslint/js'
import tseslint from 'typescript-eslint'

export default tseslint.config(
  eslint.configs.recommended,
  ...tseslint.configs.recommended,
  {
    rules: {
      '@typescript-eslint/no-unused-vars': ['error', { argsIgnorePattern: '^_' }],
      '@typescript-eslint/explicit-function-return-type': 'off',
      'no-console': ['warn', { allow: ['warn', 'error'] }]
    }
  },
  {
    ignores: ['dist/', 'node_modules/', 'coverage/']
  }
)

Prettier (if not present)

if ! grep -q "prettier" package.json; then
    npm install -D prettier
fi

.prettierrc (respect existing style or use sensible defaults):

{
  "semi": false,
  "singleQuote": true,
  "trailingComma": "es5",
  "tabWidth": 2,
  "printWidth": 100
}

commitlint (if not present)

if [ ! -f "commitlint.config.js" ]; then
    npm install -D @commitlint/cli @commitlint/config-conventional
    echo "npx commitlint --edit \$1" > .husky/commit-msg
    chmod +x .husky/commit-msg
fi

commitlint.config.js:

export default {
  extends: ['@commitlint/config-conventional'],
  rules: {
    'type-enum': [
      2,
      'always',
      ['feat', 'fix', 'docs', 'style', 'refactor', 'test', 'chore', 'ci', 'perf', 'revert']
    ],
    'subject-case': [2, 'always', 'lower-case'],
    'subject-max-length': [2, 'always', 72]
  }
}

4.2 Python Projects

pre-commit framework (if not present)

# Install pre-commit
if [ ! -f ".pre-commit-config.yaml" ]; then
    pip install pre-commit
    pre-commit install
fi

.pre-commit-config.yaml:

repos:
  # Ruff - linting and formatting (replaces black, isort, flake8)
  - repo: https://github.com/astral-sh/ruff-pre-commit
    rev: v0.14.13
    hooks:
      - id: ruff
        args: [--fix, --exit-non-zero-on-fix]
      - id: ruff-format

  # Type checking
  - repo: https://github.com/pre-commit/mirrors-mypy
    rev: v1.16.0
    hooks:
      - id: mypy
        additional_dependencies: [types-requests]
        args: [--ignore-missing-imports]

  # Security
  - repo: https://github.com/Yelp/detect-secrets
    rev: v1.5.0
    hooks:
      - id: detect-secrets
        args: ['--baseline', '.secrets.baseline']

  # General
  - repo: https://github.com/pre-commit/pre-commit-hooks
    rev: v5.0.0
    hooks:
      - id: trailing-whitespace
      - id: end-of-file-fixer
      - id: check-yaml
      - id: check-added-large-files
      - id: check-merge-conflict

  # Commit messages
  - repo: https://github.com/compilerla/conventional-pre-commit
    rev: v4.0.0
    hooks:
      - id: conventional-pre-commit
        stages: [commit-msg]

pyproject.toml additions (if not present)

[tool.ruff]
target-version = "py312"
line-length = 100

[tool.ruff.lint]
select = [
    "E",   # pycodestyle errors
    "F",   # pyflakes
    "I",   # isort
    "B",   # flake8-bugbear
    "UP",  # pyupgrade
    "S",   # flake8-bandit (security)
]
ignore = ["E501"]  # line length handled by formatter

[tool.mypy]
python_version = "3.12"
strict = true
ignore_missing_imports = true

[tool.pytest.ini_options]
testpaths = ["tests"]
addopts = "-v --cov=src --cov-report=term-missing --cov-fail-under=80"

4.3 Branch Protection (Document for User)

Recommend these GitHub branch protection rules:

## Recommended Branch Protection (main branch)

1. **Require pull request before merging**
   - Require 1 approval
   - Dismiss stale reviews on new commits

2. **Require status checks**
   - Lint
   - Type check
   - Tests
   - Security scan

3. **Require signed commits** (optional but recommended)

4. **Do not allow bypassing above settings**

Phase 5: Structure Preservation Rules

NEVER Do These

  • Don't reorganize directory structure - Work within existing patterns
  • Don't rename files for "consistency" - Match existing naming conventions
  • Don't add new patterns - Use patterns already in the codebase
  • Don't change import styles - Match existing (relative vs absolute, etc.)
  • Don't change formatting - Match existing style or use existing formatter config
  • Don't add new dependencies lightly - Check if equivalent exists

ALWAYS Do These

  • Read existing code first - Understand patterns before writing new code
  • Match existing conventions - Naming, structure, error handling
  • Use existing utilities - Don't reinvent what exists
  • Follow existing test patterns - Match test file naming and structure
  • Preserve existing configs - Only add, don't modify unless fixing bugs

Convention Detection Checklist

Before writing any code, identify:

ConventionExampleWhere to Check
NamingcamelCase vs snake_caseExisting file names
File structurefeature/ vs type/Directory layout
Export styledefault vs namedExisting modules
Error handlingthrow vs return ErrorExisting functions
Loggingconsole vs loggerExisting code
Testingdescribe/it vs test()Existing tests
CommentsJSDoc vs inlineExisting code

Phase 6: Analysis Report Template

After running analysis, generate this report:

# Repository Analysis Report

## Overview
- **Repo Type**: [Monorepo | Full-Stack | Frontend | Backend | Microservices]
- **Primary Language**: [TypeScript | Python | ...]
- **Framework**: [React | FastAPI | ...]
- **Age**: [X commits, Y contributors]

## Directory Structure

[tree output]


## Tech Stack
| Category | Technology | Config File |
|----------|------------|-------------|
| Language | TypeScript | tsconfig.json |
| Framework | React | - |
| Testing | Vitest | vitest.config.ts |
| Linting | ESLint | eslint.config.js |
| Formatting | Prettier | .prettierrc |

## Guardrails Status

### Present
- [x] ESLint configured
- [x] Prettier configured
- [x] TypeScript strict mode

### Missing (Recommended)
- [ ] Pre-commit hooks (Husky + lint-staged)
- [ ] Commit message validation (commitlint)
- [ ] Security scanning in CI

## Conventions Detected
| Pattern | Observed | Example |
|---------|----------|---------|
| Naming | camelCase | `getUserById.ts` |
| Imports | Absolute | `@/components/Button` |
| Testing | Colocated | `Button.test.tsx` |
| Exports | Named | `export { Button }` |

## Recommendations
1. Add Husky + lint-staged for pre-commit hooks
2. Add commitlint for conventional commits
3. Add security workflow to GitHub Actions

## Files to Review First
- `src/index.ts` - Main entry point
- `src/utils/` - Shared utilities
- `tests/setup.ts` - Test configuration

Gradual Implementation Strategy

Don't add all guardrails at once. Follow this timeline:

WeekFocusWhy
1Formatting (Prettier/Ruff)Non-breaking, easy wins
2Linting (ESLint/Ruff)Catches obvious issues
3Pre-commit hooksAutomates week 1-2
4Commit message validationTeam consistency
5Type checking strictnessCatches runtime errors
6Security scanningCatches vulnerabilities

Working with Separate Repos

When frontend and backend are in separate repos:

Frontend Repo Setup

# Clone and analyze
git clone [frontend-repo]
cd frontend

# Run analysis
# Expect: React/Vue/Angular, no backend code

# Add frontend-specific guardrails
# - Husky + lint-staged
# - ESLint + Prettier
# - Component testing (Vitest/Jest)

Backend Repo Setup

# Clone and analyze
git clone [backend-repo]
cd backend

# Run analysis
# Expect: FastAPI/Express/Django, no frontend code

# Add backend-specific guardrails
# - pre-commit framework
# - Ruff + mypy
# - API testing (pytest/Jest)

Cross-Repo Coordination

ConcernSolution
Shared typesGenerate from OpenAPI spec
API contractsContract testing (Pact)
DeploymentsCoordinate via CI/CD triggers
VersioningSemantic versioning on both

Anti-Patterns

  • Adding unused guardrails - Only add what the team will use
  • Strict rules on day 1 - Introduce gradually
  • Blocking on warnings - Start permissive, tighten over time
  • Ignoring existing patterns - Work with what exists
  • Over-engineering - Simple rules > complex systems
  • Skipping the analysis phase - Always understand before changing

Quick Reference: Detection Commands

# One-liner repo analysis
echo "=== Repo Type ===" && \
ls -d packages apps frontend backend 2>/dev/null || echo "Standard repo" && \
echo "=== Tech Stack ===" && \
ls *.json *.toml *.yaml 2>/dev/null && \
echo "=== Existing Guardrails ===" && \
ls .husky .pre-commit-config.yaml .eslintrc* 2>/dev/null || echo "None detected" && \
echo "=== Entry Points ===" && \
ls index.* main.* app.* server.* 2>/dev/null

FAQ

What does the existing-repo skill do?

Analyze existing repositories, maintain structure, setup guardrails and best practices

How do I install existing-repo?

Run npx skills add https://github.com/alinaqi/maggy --skill existing-repo in your project. The skills CLI detects the agents you use and copies the skill into the right folder. For Claude Code you can also place the skill folder at ~/.claude/skills/existing-repo/ (personal) or .claude/skills/existing-repo/ (project).

Which AI agents can use existing-repo?

Any agent that supports the open Agent Skills format (a folder with a SKILL.md file), including Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, Windsurf, Cline, OpenCode and more.

Is existing-repo safe to install?

No partner security audit is available yet. Skills can run commands and scripts, so read the SKILL.md and any bundled files before installing.

Who maintains existing-repo?

It is published by alinaqi in alinaqi/maggy. It has 285 recorded installs.

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