What causes "fiber: failed to %s shared state %s value: %w" in gofiber/fiber?

Replace the panicking codec with a real implementation (msgpack/cbor/json) — the message indicates which format/direction panicked.

gofiber/fiber · error

fiber: failed to %s shared state %s value: %w

Error message

fiber: failed to %s shared state %s value: %w

What it means

Returned by sharedStateCodecPanicError (shared_state.go:410) when a configured shared-state encoder/decoder PANICS during operation and the recovered value implements the error interface. Fiber installs a recover() around the codec call (shared_state.go:357-359, 387-389) and converts the panic into a wrapped error so callers get a normal error instead of a crash. The message names the operation (encode/decode), the format, and wraps the recovered error.

Source

Thrown at shared_state.go:410

	}()

	if recovered != nil {
		return sharedStateCodecPanicError("decode", format, recovered)
	}
	if err != nil {
		return fmt.Errorf("fiber: failed to decode shared state %s value: %w", format, err)
	}

	return nil
}

func sharedStateCodecNotConfiguredError(format, direction string) error {
	return fmt.Errorf("fiber: shared state %s %s is not configured", format, direction)
}

func sharedStateCodecPanicError(operation, format string, recovered any) error {
	if err, ok := recovered.(error); ok {
		return fmt.Errorf("fiber: failed to %s shared state %s value: %w", operation, format, err)
	}

	return fmt.Errorf("fiber: failed to %s shared state %s value: %v", operation, format, recovered)
}

func (s *SharedState) storageKey(key string) (string, bool) {
	if key == "" {
		return "", false
	}

	return s.prefix + hex.EncodeToString(utils.UnsafeBytes(key)), true
}

View on GitHub (pinned to 9a4c7e57fe)

Solutions

Replace the panicking codec with a real implementation (msgpack/cbor/json) — the message indicates which format/direction panicked.
If using binder.Unimplemented* placeholders, wire the actual encoder/decoder in fiber.Config.
Audit your custom codec to return errors instead of panicking.
Read the wrapped error to find the panic message and offending codec.

Example fix

// before: placeholder codec that panics
cfg.MsgPackEncoder = binder.UnimplementedMsgPackMarshal

// after: real codec
import "github.com/vmihailenco/msgpack/v5"
cfg.MsgPackEncoder = msgpack.Marshal
cfg.MsgPackDecoder = msgpack.Unmarshal

Defensive patterns

Strategy: try-catch

Try / catch

if err := state.SetMsgPack(ctx, "k", v, ttl); err != nil {
    if strings.Contains(err.Error(), "failed to") && strings.Contains(err.Error(), "shared state") {
        // codec panicked — replace with a real implementation
    }
}

Prevention

Never use binder.Unimplemented* codecs in production.
Replace placeholder codecs with real implementations before going live.
Audit custom codecs to ensure they return errors, not panics.

When it happens

Trigger: A custom or binder.Unimplemented* codec panics with an error value while serializing/deserializing shared state. The recover in encodeSharedStateValue/decodeSharedStateValue catches it and sharedStateCodecPanicError formats it because recovered.(error) succeeds.

Common situations: Using Fiber's binder.Unimplemented* placeholders that panic to signal 'not implemented'; a buggy custom encoder that panics on edge-case types; a codec library that panics on nil inputs instead of returning an error.

Related errors

AI-assisted analysis of gofiber/fiber@9a4c7e57fe (2026-08-04). Data as JSON: /data/errors/eb3c6a8550d45ded.json. Report an issue: GitHub.