What causes "failed to encode data: %w" in gofiber/fiber?

Inspect the wrapped error — gob names the unencodable type (e.g. gob: type not registered for interface: main.Foo).

failed to encode data

Error message

failed to encode data: %w

What it means

Thrown by saveSessionWithContext when the session's in-memory data map cannot be gob-encoded before being written to the Storage backend. The %w wraps the underlying gob.Encoder error, which names the offending type or value. This is the top-level error a caller of Save()/saveSessionWithContext observes; the sibling error 202 is the same root cause surfaced from the lower-level encodeSessionData.

Solutions

Inspect the wrapped error — gob names the unencodable type (e.g. gob: type not registered for interface: main.Foo).
Call gob.Register(YourType{}) in an init() for every custom struct stored in the session.
Avoid storing channels, funcs, complex numbers, or unregistered interface values; store primitive/serializable representations instead.
If you must store dynamic types, wrap them in a concrete registered struct.

Example fix

// before
sess.Set("user", User{Name: "x", Conn: someChan})
sess.Save() // -> failed to encode data: gob: unsupported type

// after (init)
func init() { gob.Register(User{}) }

Defensive patterns

Strategy: validation

Validate before calling

// Run before storing custom types in the session.
func registerSessionTypes() {
    gob.Register(MyStruct{})
    gob.Register(&MyStruct{})
    gob.Register(map[string]any{})
}
func isGobEncodable(v any) error {
    switch reflect.TypeOf(v).Kind() {
    case reflect.Chan, reflect.Func, reflect.Complex64, reflect.Complex128, reflect.UnsafePointer:
        return fmt.Errorf("type %T is not gob-encodable", v)
    }
    return nil
}

Type guard

func isStorableInSession(v any) bool {
    if v == nil { return true }
    switch reflect.TypeOf(v).Kind() {
    case reflect.Chan, reflect.Func, reflect.Complex64, reflect.Complex128:
        return false
    }
    return true
}

Try / catch

if err := sess.Save(); err != nil {
    // a failed encode usually means a bad value type; log and drop the value
    log.Printf("session save failed: %v", err)
    sess.Destroy()
}

Prevention

gob.Register every custom struct type at program init, before any session is touched.
Never store channels, funcs, or complex numbers in the session.
Prefer storing simple structs / primitives rather than map[string]any with dynamic concrete types.
Add a unit test that round-trips every value type you intend to store through gob encode+decode.

When it happens

Trigger: Calling session.Save() (or relying on the middleware's auto-save) while s.data.Data holds a value gob cannot encode: a channel, func, complex64/128, an interface{} whose concrete type was not registered with gob.Register, a struct containing such fields, or a value with a cycle gob refuses.

Common situations: Storing a custom struct in the session without calling gob.Register at init; storing map[string]any with arbitrarily-typed values whose concrete types gob does not know about; storing closures/callbacks; upgrading Fiber and storing newly-typed values without registering them.

Related errors

AI-assisted analysis of gofiber/fiber@a105acad6c (2026-08-11). Data as JSON: /api/errors/4ca824aa3fbf868a. Report an issue: GitHub.

Appendix: 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{}
	}

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