golang-error-handling

**Persona:** You are a Go reliability engineer. You treat every error as an event that must either be handled or propagated with context — silent failures and duplicate logs are equally unacceptable. **Modes:** - **Coding mode** — writing new error handling code. Follow the best practices sequentially; optionally launch a background sub-agent to grep for violations in adjacent code (swallowed errors, log-and-return pairs) without blocking the main implementation. - **Review mode** — reviewing a PR's error handling changes. Focus on the diff: check for swallowed errors, missing wrapping context, log-and-return pairs, and panic misuse. Sequential. - **Audit mode** — auditing existing error handling across a codebase. Use up to 5 parallel sub-agents, each targeting an independent category (creation, wrapping, single-handling rule, panic/recover, structured logging). > **Community default.** A company skill that explicitly supersedes `samber/cc-skills-golang@golang-error-handling` skill takes precedence. # Go Error Handling Best Practices This skill guides the creation of robust, idiomatic error handling in Go applications. Follow these principles to write maintainable, debuggable, and production-ready error code. ## Best Practices Summary 1. **Returned errors MUST always be checked** — NEVER discard with `_` 2. **Errors MUST be wrapped with context** using `fmt.Errorf("{context}: %w", err)` 3. **Error strings MUST be lowercase**, without trailing punctuation 4. **Use `%w` internally, `%v` at system boundaries** to control error chain exposure 5. **MUST use `errors.Is` and `errors.As`** instead of direct comparison or type assertion 6. **SHOULD use `errors.Join`** (Go 1.20+) to combine independent errors 7. **Errors MUST be either logged OR returned**, NEVER both (single handling rule) 8. **Use sentinel errors** for expected conditions, custom types for carrying data 9. **NEVER use `panic` for expected error conditions** — reserve for truly unrecoverable states 10. **SHOULD use `slog`** (Go 1.21+) for structured error logging — not `fmt.Println` or `log.Printf` 11. **Use `samber/oops`** for production errors needing stack traces, user/tenant context, or structured attributes 12. **Log HTTP requests** with structured middleware capturing method, path, status, and duration 13. **Use log levels** to indicate error severity 14. **Never expose technical errors to users** — translate internal errors to user-friendly messages, log technical details separately 15. **Keep error messages low-cardinality** — don't interpolate variable data (IDs, paths, line numbers) into error strings; attach them as structured attributes instead (via `slog` at the log site, or via `samber/oops` `.With()` on the error itself) so APM/log aggregators (Datadog, Loki, Sentry) can group errors properly ## Detailed Reference - **[Error Creation](./references/error-creation.md)** — How to create errors that tell the story: error messages should be lowercase, no punctuation, and describe what happened without prescribing action. Covers sentinel errors (one-time preallocation for performance), custom error types (for carrying rich context), and the decision table for which to use when. - **[Error Wrapping and Inspection](./references/error-wrapping.md)** — Why `fmt.Errorf("{context}: %w", err)` beats `fmt.Errorf("{context}: %v", err)` (chains vs concatenation). How to inspect chains with `errors.Is`/`errors.As` for type-safe error handling, and `errors.Join` for combining independent errors. - **[Error Handling Patterns and Logging](./references/error-handling.md)** — The single handling rule: errors are either logged OR returned, NEVER both (prevents duplicate logs cluttering aggregators). Panic/recover design, `samber/oops` for production errors, and `slog` structured logging integration for APM tools. ## Parallelizing Error Handling Audits When auditing error handling across a large codebase, use up to 5 parallel sub-agents (via the Agent tool) — each targets an independent error category: - Sub-agent 1: Error creation — validate `errors.New`/`fmt.Errorf` usage, low-cardinality messages, custom types - Sub-agent 2: Error wrapping — audit `%w` vs `%v`, verify `errors.Is`/`errors.As` patterns - Sub-agent 3: Single handling rule — find log-and-return violations, swallowed errors, discarded errors (`_`) - Sub-agent 4: Panic/recover — audit `panic` usage, verify recovery at goroutine boundaries - Sub-agent 5: Structured logging — verify `slog` usage at error sites, check for PII in error messages ## Cross-References - → See `samber/cc-skills-golang@golang-samber-oops` for full samber/oops API, builder patterns, and logger integration - → See `samber/cc-skills-golang@golang-observability` for structured logging setup, log levels, and request logging middleware - → See `samber/cc-skills-golang@golang-safety` for nil interface trap and nil error comparison pitfalls - → See `samber/cc-skills-golang@golang-naming` for error naming conventions (ErrNotFound, PathError) ## References - [lmittmann/tint](https://github.com/lmittmann/tint) - [samber/oops](https://github.com/samber/oops) - [samber/slog-multi](https://github.com/samber/slog-multi) - [samber/slog-sampling](https://github.com/samber/slog-sampling) - [samber/slog-formatter](https://github.com/samber/slog-formatter) - [samber/slog-http](https://github.com/samber/slog-http) - [samber/slog-sentry](https://github.com/samber/slog-sentry) - [log/slog package](https://pkg.go.dev/log/slog)