gofiber/fiber · error

failed to encode data: %w

Error message

failed to encode data: %w

What it means

Returned by Session.saveSessionWithContext when encodeSessionData fails. Session data is serialized with encoding/gob; this error wraps a gob encoder failure while persisting the session on Save/SaveWithContext (or the automatic save at handler return).

Source

Thrown at middleware/session/session.go:399

	}

	s.mu.Lock()
	defer s.mu.Unlock()

	// Set idleTimeout if not already set
	if s.idleTimeout <= 0 {
		s.idleTimeout = s.config.IdleTimeout
	}

	// Update client cookie
	s.setSession()

	// Encode session data
	s.data.RLock()
	encodedBytes, err := s.encodeSessionData()
	s.data.RUnlock()
	if err != nil {
		return fmt.Errorf("failed to encode data: %w", err)
	}

	// Pass copied bytes with session id to provider
	return s.config.Storage.SetWithContext(ctx, s.id, encodedBytes, s.idleTimeout)
}

// Keys retrieves all keys in the current session.
//
// Returns:
//   - []any: A slice of all keys in the session.
//
// Usage:
//
//	keys := s.Keys()
func (s *Session) Keys() []any {
	if s.data == nil {
		return []any{}
	}

View on GitHub (pinned to 9a4c7e57fe)

Solutions

1. Register all custom concrete types you store in the session: gob.Register(MyType{}) in init().
2. Avoid storing funcs, channels, or unexported-field structs in session data.
3. Prefer primitive types (strings, ints, []byte) in session values.
4. Add a unit test that calls Save to surface encoding errors early.

Example fix

// before: storing an unregistered custom type
type Cart struct{ Items []string }
sess.Set("cart", Cart{Items: []string{"a"}})
sess.Save() // gob encode fails

// after: register the type once at startup
func init() {
    gob.Register(Cart{})
}

// Register every custom type stored in the session at startup.
func init() {
    gob.Register(MyType{})
    gob.Register(map[string]any{})
    gob.Register([]string{})
}

// isGobEncodable approximates which values gob can encode; use it to
// filter session values before Set.
func isGobEncodable(v any) bool {
    switch v.(type) {
    case chan struct{}, func():
        return false
    }
    return true
}

if err := sess.Save(); err != nil {
    if strings.Contains(err.Error(), "failed to encode data") {
        log.Error().Err(err).Msg("session gob encode failed")
        // keep the request alive without persisting the unencodable value
        return c.SendStatus(fiber.StatusAccepted)
    }
    return err
}

When it happens

Trigger: encCache.Encode(&s.data.Data) fails at session.go:609. gob fails when a value's type was not registered (for interface fields), when a type is not gob-encodable (e.g. channels, funcs, certain maps), or on cyclic structures.

Common situations: Storing a custom struct in the session without gob.Register; storing a map/struct containing non-encodable fields; storing an interface value whose concrete type gob doesn't know; storing time.Time or nested types that need registration.

Related errors

• failed to decode session data: %w
• failed to encode session data: %w
• failed to decode session data: %w
• remote address cannot be empty
• decode SHA256 password: invalid length

AI-assisted analysis of gofiber/fiber@9a4c7e57fe (2026-08-04). Data as JSON: /data/errors/4ca824aa3fbf868a.json. Report an issue: GitHub.