/golang-error-handling

name: golang-error-handling
description: Standards for error wrapping, checking, and definition in Golang. Use when wrapping errors, defining sentinel errors, or handling errors idiomatically in Go.
metadata:
  triggers:
    files:
    - 'fmt.Errorf'
    - 'errors.Is'
    - 'errors.As'
    keywords:
    - error wrapping
    - sentinel error
    - error handling

Golang Error Handling Standards

**Priority: P0 (CRITICAL)**

Principles

• **Errors Values**: Handle them like any other value.
• **Handle Once**: Log OR Return. Never Log AND Return (creates duplicate logs).
• **Add Context**: Don't return `err` bubble up. Wrap it with context: `fmt.Errorf("failed to open file: %w", err)`.
• **Use Standard Lib**: Go 1.13+ `errors` package (`Is`, `As`, `Unwrap`) sufficient. Avoid `pkg/errors` (deprecated).

Guidelines

• **Sentinel Errors**: Expoted, fixed errors (`io.EOF`, `sql.ErrNoRows`). Use `errors.Is(err, io.EOF)`.
• **Error Types**: Structs implementing `error`. Use `errors.As(err, &target)`.
• **Panic**: Only for unrecoverable startup errors.

Anti-Patterns

• **No bare return err**: Wrap with `fmt.Errorf("context: %w", err)` to preserve call chain.
• **No string error checks**: Use `errors.Is`/`errors.As`; string comparison brittle.
• **No swallowed errors**: Never assign errors to `_`; always handle or propagate.

References

• [Error Wrapping Patterns](references/error-wrapping.md)