Patterns for robust error handling across TypeScript, Python, and Go. Covers typed errors, error boundaries, retries, circuit breakers, and user-facing error messages.
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What the file says
# Error Handling Patterns
Consistent, robust error handling patterns for production applications.
## When to Activate
- Designing error types or exception hierarchies for a new module or service
- Adding retry logic or circuit breakers for unreliable external dependencies
- Reviewing API endpoints for missing error handling
- Implementing user-facing error messages and feedback
- Debugging cascading failures or silent error swallowing
## Core Principles
1. **Fail fast and loudly** — surface errors at the boundary where they occur; don't bury them
2. **Typed errors over string messages** — errors are first-class values with structure
3. **User messages ≠developer messages** — show friendly text to users, log full context server-side
4. **Never swallow errors silently** — every `catch` block must either handle, re-throw, or log
5. **Errors are part of your API contract** — document every error code a client may receive
## TypeScript / JavaScript
### Typed Error Classes
```typescript
// Define an error hierarchy for your domain
export class AppError extends Error {
constructor(
message: string,
public readonly code: string,
…
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