gofiber/fiber · error

failed to decode session data: %w

Error message

failed to decode session data: %w

What it means

Returned by the public Store.Get(c) path: getSession pulls existing bytes from storage and decodeSessionData fails (store.go:174-179). Same root cause as the Session-level decode error but reached whenever you use the Store API directly or the middleware loads a session.

Source

Thrown at middleware/session/store.go:179

	sess := acquireSession()

	sess.mu.Lock()

	sess.ctx = c
	sess.config = s
	sess.id = id
	sess.isFresh = isFresh
	sess.extractor = selectedExtractor

	// Decode session data if found
	if rawData != nil {
		sess.data.Lock()
		err := sess.decodeSessionData(rawData)
		sess.data.Unlock()
		if err != nil {
			sess.mu.Unlock()
			sess.Release()
			return nil, fmt.Errorf("failed to decode session data: %w", err)
		}
	}

	sess.mu.Unlock()

	if isFresh && s.AbsoluteTimeout > 0 {
		sess.setAbsExpiration(time.Now().Add(s.AbsoluteTimeout))
	} else if sess.isAbsExpired() {
		if err := sess.Reset(); err != nil {
			return nil, fmt.Errorf("failed to reset session: %w", err)
		}
		sess.setAbsExpiration(time.Now().Add(s.AbsoluteTimeout))
	}

	return sess, nil
}

// getSessionID returns the session ID using the configured extractor.

View on GitHub (pinned to 9a4c7e57fe)

Solutions

1. Register every custom type with store.RegisterType at startup.
2. Clear stale sessions from the store after schema/type changes.
3. Check the storage backend for truncated/corrupt values and raise value-size limits if needed.

sess, err := store.Get(c)
if err != nil {
    // Corrupt or unreadable session -> regenerate.
    log.Warnf("session decode failed, resetting: %v", err)
    _ = sess.Destroy()
    sess, err = store.Get(c)
    if err != nil {
        return fiber.ErrInternalServerError
    }
}

When it happens

Trigger: A request arrives with a valid session ID whose stored bytes cannot gob-decode: unregistered type, schema drift, or truncated storage value, surfaced through store.Get(c)/getSession.

Common situations: Same as the middleware decode case: type-registration gaps across replicas, post-deploy stale sessions, storage truncation.

Related errors

• failed to decode session data: %w
• failed to encode 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/d3e0efc3a1f05f3c.json. Report an issue: GitHub.