Go Error Handling Agent Skill

Go Error Handling

> Compatibility: errors.Is, errors.As, and %w wrapping require Go 1.13+; structured logging examples may use log/slog from Go 1.21+.

Resource Routing

• scripts/check-errors.sh - Run when checking string-based error matching, bare error propagation, and log-and-return patterns.
• scripts/check-errors-ast.go - Implementation helper invoked by check-errors.sh; patch this when changing error-flow analysis behavior.
• references/ERROR-FLOW.md - Read when deciding where to handle, wrap, log, or return errors.
• references/ERROR-TYPES.md - Read when choosing sentinel errors, typed errors, or opaque errors.
• references/WRAPPING.md - Read when choosing %w versus %v or crossing package boundaries.

In Go, errors are values — they are

created by code and consumed by code.

Choosing an Error Strategy

• System boundary (RPC, IPC, storage)? → Wrap with %v to avoid leaking internals
• Caller needs to match specific conditions? → Sentinel or typed error, wrap with %w
• Caller just needs debugging context? → fmt.Errorf("...: %w", err)
• Leaf function, no wrapping needed? → Return the error directly

Default: wrap with %w and place it at the end of the format string.

Core Rules

Never Return Concrete Error Types

Never return concrete error types from exported functions — a concrete nil

pointer can become a non-nil interface:

// Bad: Concrete type can cause subtle bugs
func Bad() *os.PathError { /*...*/ }

// Good: Always return the error interface
func Good() error { /*...*/ }

Error Strings

Error strings should not be capitalized and should not end with

punctuation. Exception: exported names, proper nouns, or acronyms.

// Bad
err := fmt.Errorf("Something bad happened.")

// Good
err := fmt.Errorf("something bad happened")

For displayed messages (logs, test failures, API responses), capitalization is

appropriate.

Return Values on Error

When a function returns an error, callers must treat all non-error return values

as unspecified unless explicitly documented.

Tip: Functions taking a context.Context should usually return an error

so callers can determine if the context was cancelled.

Handling Errors

When encountering an error, make a deliberate choice — do not discard

with _:

• Handle immediately — address the error and continue
• Return to caller — optionally wrapped with context
• In exceptional cases — log.Fatal or panic

To intentionally ignore: add a comment explaining why.

n, _ := b.Write(p) // never returns a non-nil error

For related concurrent operations, use

errgroup:

g, ctx := errgroup.WithContext(ctx)
g.Go(func() error { return task1(ctx) })
g.Go(func() error { return task2(ctx) })
if err := g.Wait(); err != nil { return err }

Avoid In-Band Errors

Don't return -1, nil, or empty string to signal errors. Use multiple

returns:

// Bad: In-band error value
func Lookup(key string) int  // returns -1 for missing

// Good: Explicit error or ok value
func Lookup(key string) (string, bool)

This prevents callers from writing Parse(Lookup(key)) — it causes a

compile-time error since Lookup(key) has 2 outputs.

Error Flow

Handle errors before normal code. Early returns keep the happy path unindented:

// Good: Error first, normal code unindented
if err != nil {
    return err
}
// normal code

Handle errors once — either log or return, never both:

Error encountered?
├─ Caller can act on it? → Return (with context via %w)
├─ Top of call chain? → Log and handle
└─ Neither? → Log at appropriate level, continue

Error Types

> Advisory: Recommended best practice.

| Caller needs to match? | Message type | Use |

|------------------------|--------------|-----|

| No | static | errors.New("message") |

| No | dynamic | fmt.Errorf("msg: %v", val) |

| Yes | static | var ErrFoo = errors.New("...") |

| Yes | dynamic | custom error type |

Default: Wrap with fmt.Errorf("...: %w", err). Escalate to sentinels for

errors.Is(), to custom types for errors.As().

Error Wrapping

> Advisory: Recommended best practice.

• Use %v: At system boundaries, for logging, to hide internal details
• Use %w: To preserve error chain for errors.Is/errors.As

Key rules: Place %w at the end. Add context callers don't have. If

annotation adds nothing, return err directly.

> Validation: After implementing error handling, run bash scripts/check-errors.sh to detect common anti-patterns. Then run go vet ./... to catch additional issues.

Related Skills

• Error naming: See go-naming when naming sentinel errors (ErrFoo) or custom error types
• Testing errors: See go-testing when testing error semantics with errors.Is/errors.As or writing error-checking helpers
• Panic handling: See go-defensive when deciding between panic and error returns, or writing recover guards
• Guard clauses: See go-control-flow when structuring early-return error flow or reducing nesting
• Logging decisions: See go-logging when choosing log levels, configuring structured logging, or deciding what context to include in log messages