gofiber/fiber · error

failed to encode session data: %w

Error message

failed to encode session data: %w

What it means

Raised by Session.encodeSessionData when gob cannot serialize data.Data (map[any]any) to bytes, which happens on sess.Save(). The encoder must describe each concrete value through the any interface, so an unregistered type or a gob-incompatible value (channels, funcs, unexported-field structs) makes Encode fail.

Source

Thrown at middleware/session/session.go:610

// encodeSessionData encodes session data to raw bytes
//
// Parameters:
//   - rawData: The raw byte data to encode.
//
// Returns:
//   - error: An error if the encoding fails.
//
// Usage:
//
//	err := s.encodeSessionData(rawData)
func (s *Session) encodeSessionData() ([]byte, error) {
	byteBuffer := byteBufferPool.Get().(*bytes.Buffer) //nolint:forcetypeassert,errcheck // We store nothing else in the pool
	defer byteBufferPool.Put(byteBuffer)
	defer byteBuffer.Reset()
	encCache := gob.NewEncoder(byteBuffer)
	if err := encCache.Encode(&s.data.Data); err != nil {
		return nil, fmt.Errorf("failed to encode session data: %w", err)
	}
	// Copy the bytes
	// Copy the data in buffer
	encodedBytes := make([]byte, byteBuffer.Len())
	copy(encodedBytes, byteBuffer.Bytes())

	return encodedBytes, nil
}

// absExpiration returns the session absolute expiration time or a zero time if not set.
//
// Returns:
//   - time.Time: The session absolute expiration time. Zero time if not set.
//
// Usage:
//
//	expiration := s.absExpiration()
func (s *Session) absExpiration() time.Time {

View on GitHub (pinned to 9a4c7e57fe)

Solutions

1. Register all custom types via store.RegisterType at startup.
2. Store only gob-compatible values: exported fields, no chan/func/unsafe.Pointer.
3. If you must store an exotic type, store a registered plain struct or []byte representation instead.

Example fix

// before
store := session.NewStore()
sess, _ := store.Get(c)
sess.Set("profile", Profile{})
err := sess.Save() // failed to encode session data

// after
store := session.NewStore()
store.RegisterType(Profile{})
sess, _ := store.Get(c)
sess.Set("profile", Profile{})
err := sess.Save()

var registeredTypes = []any{User{}, Profile{}, time.Time{}}
func registerSessionTypes(s *session.Store) {
    for _, t := range registeredTypes {
        s.RegisterType(t)
    }
}

// typeGuard: only allow gob-encodable, pre-registered kinds into the session.
func isGobSafe(v any) bool {
    if v == nil { return true }
    switch reflect.TypeOf(v).Kind() {
    case reflect.Chan, reflect.Func, reflect.UnsafePointer, reflect.Complex64, reflect.Complex128:
        return false
    }
    return true
}

if err := sess.Save(); err != nil {
    log.Errorf("session save failed: %v", err)
    return fiber.ErrInternalServerError
}

When it happens

Trigger: Calling sess.Set("user", User{}) on an unregistered User, then sess.Save() triggers encodeSessionData and gob.Encode returns an error; or storing a value gob cannot represent.

Common situations: Introducing new session data with custom struct types without RegisterType; storing non-gob-encodable values (chan/func/complex); structs with unexported fields gob cannot reach.

Related errors

• failed to decode session data: %w
• failed to decode session data: %w
• failed to encode data: %w
• cache: failed to unmarshal key %q: %w
• cache: failed to marshal key %q: %w

AI-assisted analysis of gofiber/fiber@9a4c7e57fe (2026-08-04). Data as JSON: /data/errors/4484f007876b2f63.json. Report an issue: GitHub.