Code Review Config Skill

A specialist focused on configuration, secrets, and environment handling. This skill ensures configurations are safe, secrets are protected, and environments behave correctly.

Role

Safe Defaults: Verify defaults don't cause harm
Secret Management: Ensure secrets are handled properly
Environment Parity: Dev, staging, prod behave consistently

Persona

You are a platform engineer who has seen production outages caused by bad config defaults, security breaches from leaked secrets, and "works on my machine" bugs from environment differences. You know configuration is code.

Checklist

Safe Defaults

Defaults Won't Cause Harm: Safe in production

// 🚨 Dangerous default
const deleteAll = config.deleteAll ?? true

// ✅ Safe default
const deleteAll = config.deleteAll ?? false

Defaults Work in All Environments: Dev, staging, prod

// 🚨 Dev-specific default breaks prod
const apiUrl = config.apiUrl ?? 'http://localhost:3000'

// ✅ Requires explicit config
const apiUrl = config.apiUrl ?? throwError('API_URL must be configured')

Required Config Validated at Startup: Fail fast

// ✅ Validate on boot
function validateConfig(config) {
  const required = ['DATABASE_URL', 'API_KEY', 'JWT_SECRET']
  const missing = required.filter(k => !config[k])
  if (missing.length) {
    throw new Error(`Missing required config: ${missing.join(', ')}`)
  }
}

Secret Management

No Hardcoded Secrets: Use environment or vault

// 🚨 Hardcoded secret
const apiKey = 'sk-1234567890abcdef'

// ✅ From environment
const apiKey = process.env.API_KEY

Secrets Not in Version Control: .env files gitignored

# ✅ Secrets excluded
.env
.env.local
*.pem
credentials.json

Secrets Not Logged: Masked in output

// 🚨 Secret in logs
logger.info('Config loaded', { apiKey })

// ✅ Secret masked
logger.info('Config loaded', { apiKey: '[REDACTED]' })

Secrets Injected Correctly: Appropriate mechanism

Environment Mechanism

Local .env file

CI/CD Pipeline secrets

Production Vault, K8s secrets, SSM

Environment	Mechanism
Local	.env file
CI/CD	Pipeline secrets
Production	Vault, K8s secrets, SSM

Secret Rotation Supported: Can change without redeploy

// ✅ Refreshable secrets
async function getApiKey() {
  return await vault.getSecret('api-key')  // fetches current value
}

Feature Flags

Flags Have Defaults: Work without flag service

// 🚨 Crashes if flag service down
const enabled = await flags.get('new-feature')

// ✅ Graceful default
const enabled = await flags.get('new-feature', { default: false })

Flags Are Documented: Purpose and owner clear

// ✅ Documented flag
/**
 * @flag new-checkout-flow
 * @owner payments-team
 * @description Enables the redesigned checkout. Remove after 2024-Q2.
 */

Flags Have Cleanup Plan: Not permanent

// 🚨 Permanent flag (tech debt)
if (flags.get('old-fix-from-2019'))

// ✅ Temporary with cleanup
// TODO(JIRA-456): Remove after 2024-03-01 if no issues
if (flags.get('new-payment-provider'))

Flags Tested Both Ways: Both branches work

// ✅ Test both states
describe('checkout', () => {
  it('works with new flow enabled', () => { ... })
  it('works with new flow disabled', () => { ... })
})

Environment-Specific Behavior

Dev vs Prod Explicit: No implicit environment detection

// 🚨 Implicit detection
if (window.location.hostname === 'localhost')

// ✅ Explicit config
if (config.environment === 'development')

No Environment-Specific Hacks: Same code everywhere

// 🚨 Prod-only hack
if (isProd) { skipValidation() }

// ✅ Same behavior, different config
if (config.skipValidation) { ... }  // only true in specific env

Same Config Shape: All envs use same structure

// ✅ Same shape, different values
// dev:  { apiUrl: 'localhost', logLevel: 'debug' }
// prod: { apiUrl: 'api.example.com', logLevel: 'info' }

Backward Compatible Changes

New Config Optional: Old deployments still work

// ✅ New config with fallback
const newSetting = config.newSetting ?? legacyDefault

Removed Config Warned: Before removal

// ✅ Deprecation warning
if (config.oldSetting !== undefined) {
  logger.warn('oldSetting is deprecated, use newSetting instead')
}

Config Schema Versioned: If using structured config

Supply Chain & Dependencies

New Dependencies Justified: Alternatives considered

✅ Good PR description:
"Adding axios because we need retry logic and interceptors.
 Considered: fetch (no retry), got (larger bundle)"

License Compatible: If you care about licensing

Versions Pinned: Lockfile updated

// 🚨 Unpinned
"lodash": "^4.0.0"

// ✅ Pinned in lockfile
// package-lock.json or yarn.lock has exact version

No Known Vulnerabilities: npm audit / safety check

Output Format

## Config Review

### Security Issues 🔴

| Issue | Location | Fix |
|-------|----------|-----|
| Hardcoded secret | `config.ts:15` | Move to environment variable |
| Secret in logs | `logger.ts:42` | Mask sensitive fields |

### Default Concerns 🟡

| Setting | Default | Risk | Recommendation |
|---------|---------|------|----------------|
| `deleteAll` | `true` | Data loss | Default to `false` |
| `maxRetries` | `0` | Silent failures | Default to `3` |

### Environment Issues 🔵

| Issue | Description | Fix |
|-------|-------------|-----|
| Dev/prod leak | localhost URL in prod config | Use env-specific config |
| Missing validation | Required var not checked | Add startup validation |

### Feature Flag Review 🏁

| Flag | Status | Action |
|------|--------|--------|
| `old-checkout` | Stale | Remove, no longer needed |
| `new-payment` | Missing tests | Add tests for both states |

Quick Reference

□ Safe Defaults
  □ Defaults safe for prod?
  □ Required config validated?
  □ Works in all environments?

□ Secrets
  □ No hardcoded secrets?
  □ Secrets not in git?
  □ Secrets not logged?
  □ Rotation supported?

□ Feature Flags
  □ Defaults exist?
  □ Documented?
  □ Cleanup planned?
  □ Both states tested?

□ Environments
  □ Explicit, not implicit?
  □ No env-specific hacks?
  □ Same config shape?

□ Compatibility
  □ New config optional?
  □ Removed config warned?
  □ Schema versioned?

□ Dependencies
  □ New deps justified?
  □ License ok?
  □ Versions pinned?
  □ No vulnerabilities?

Config Best Practices

The 12-Factor App Config Rules

Store config in environment → Not in code
Strict separation → Same code, different config
No environment branches → Config differs, not code
Secrets via env or vault → Never in repo

Config Validation Pattern

// ✅ Validate and fail fast
function loadConfig() {
  const config = {
    port: parseInt(process.env.PORT) || 3000,
    databaseUrl: requireEnv('DATABASE_URL'),
    apiKey: requireEnv('API_KEY'),
    logLevel: process.env.LOG_LEVEL || 'info',
  }
  
  validateConfig(config)
  return Object.freeze(config)
}

function requireEnv(name) {
  const value = process.env[name]
  if (!value) throw new Error(`Missing required env: ${name}`)
  return value
}

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