crash-safety

Crash & Error Resilience Audit

Find risky unwraps, missing guards, and unhandled errors. Add safe fallbacks and user-friendly error states.

When to use this skill

• Before App Store submission
• After adding new async/error-prone code
• When reviewing PRs with optional handling
• After crash reports from TestFlight

Quick Reference: Risk Levels

Pattern	Risk	Fix
fatalError()	🔴 Critical	Remove or guard with #if DEBUG
! force unwrap	🔴 High	Use guard let, if let, or ??
try!	🔴 High	Use do/catch or try? with fallback
preconditionFailure()	🔴 High	Remove from production paths
Implicit unwrap (Type!)	🟡 Medium	Convert to optional with safe access
Missing @MainActor	🟡 Medium	Add for UI-updating code

---

Step 1: Fatal Crash Patterns

1.1 fatalError / preconditionFailure / assertionFailure

# These crash in production!
rg --type swift -g '!*Tests*' -g '!*UITests*' 'fatalError\(|preconditionFailure\(|assertionFailure\(' YourApp/

Fix: Remove or wrap in #if DEBUG:

#if DEBUG
fatalError("Unimplemented")
#else
return defaultValue
#endif

1.2 Force Unwraps (!)

# Force unwraps - tighter pattern to reduce false positives
rg --type swift -g '!*Tests*' -g '!*UITests*' '[a-zA-Z_][a-zA-Z0-9_]*\s*!' YourApp/ | grep -v '!=' | grep -v '//' | head -40

Exceptions (OK to skip):

• @IBOutlet (UIKit pattern)
• NSImage(named:)! with known system images
• URL(string: "https://...")! with hardcoded valid URLs
• Code inside #Preview { } blocks
• Code inside *Tests*/ directories

1.3 Force Try (try!)

rg --type swift -g '!*Tests*' -g '!*UITests*' 'try!' YourApp/

Note: try! is acceptable in tests and previews where failure should crash during development.

Fix: Replace with do/catch or try? with default:

// ❌ Crashes on failure
let data = try! encoder.encode(object)

// ✅ Safe with error handling
let data = try? encoder.encode(object) ?? Data()