gofiber/fiber · error

failed to decode session data: %w

Error message

failed to decode session data: %w

What it means

Raised by Session.decodeSessionData when gob cannot decode the raw bytes pulled from the session store back into the session's map[any]any (data.Data). Because the value type is a map keyed by any, every concrete value stored in the session must be registered with gob; the wrapped error is typically 'gob: type not registered for interface' or io.ErrUnexpectedEOF for truncated bytes.

Source

Thrown at middleware/session/session.go:588

// decodeSessionData decodes session data from raw bytes
//
// Parameters:
//   - rawData: The raw byte data to decode.
//
// Returns:
//   - error: An error if the decoding fails.
//
// Usage:
//
//	err := s.decodeSessionData(rawData)
func (s *Session) decodeSessionData(rawData []byte) error {
	byteBuffer := byteBufferPool.Get().(*bytes.Buffer) //nolint:forcetypeassert,errcheck // We store nothing else in the pool
	defer byteBufferPool.Put(byteBuffer)
	defer byteBuffer.Reset()
	_, _ = byteBuffer.Write(rawData)
	decCache := gob.NewDecoder(byteBuffer)
	if err := decCache.Decode(&s.data.Data); err != nil {
		return fmt.Errorf("failed to decode session data: %w", err)
	}
	return nil
}

// 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)

View on GitHub (pinned to 9a4c7e57fe)

Solutions

1. Register every custom type with store.RegisterType(MyType{}) at startup before serving requests.
2. Invalidate/clear stale sessions in the storage backend after changing any stored type's fields.
3. Confirm the storage backend is not truncating payloads (raise Redis max value size / memcache item size if needed).

Example fix

// before
store := session.NewStore()
sess, err := store.Get(c) // err: failed to decode session data: gob: type not registered for interface

// after
store := session.NewStore()
store.RegisterType(User{})
store.RegisterType(Permissions{})
sess, err := store.Get(c)

// Register every concrete type you will ever store, once at startup.
func registerSessionTypes(s *session.Store) {
    s.RegisterType(User{})
    s.RegisterType(time.Time{})
    // ...one call per concrete value type used in sess.Set
}

sess, err := store.Get(c)
if err != nil {
    // Treat unreadable session as corrupt: destroy and start fresh.
    _ = sess.Destroy()
    sess, err = store.Get(c) // regenerates a fresh session
    if err != nil {
        return fiber.ErrInternalServerError
    }
}

When it happens

Trigger: Calling code that reads an existing session whose stored payload holds a concrete Go type never passed to Store.RegisterType, or whose struct shape changed since it was written, or whose bytes were truncated by the storage backend (value-size cap).

Common situations: Deploying a new struct into the session without RegisterType; rolling out a schema change while old sessions remain in Redis/memstore; one replica registered a type and another did not; storage backend truncating large values.

Related errors

• failed to encode 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/2aa537ca5af1c6e6.json. Report an issue: GitHub.