subscriber %v.%v has wrong number of outs: %v require signat

Error message

subscriber %v.%v has wrong number of outs: %v require signature %s

What it means

A valid gRPC subscriber handler must return exactly one value of type error, matching the signature `func(ctx context.Context, msg T) error`. validateSubscriber rejects any method whose number of return values is not exactly 1, reporting the actual count via this error.

Source

Thrown at server/grpc/subscriber.go:156

		hdlr := reflect.ValueOf(sub.Subscriber())
		name := reflect.Indirect(hdlr).Type().Name()

		for m := 0; m < typ.NumMethod(); m++ {
			method := typ.Method(m)

			switch method.Type.NumIn() {
			case 3:
				argType = method.Type.In(2)
			default:
				return fmt.Errorf("subscriber %v.%v takes wrong number of args: %v required signature %s",
					name, method.Name, method.Type.NumIn(), subSig)
			}

			if !isExportedOrBuiltinType(argType) {
				return fmt.Errorf("%v argument type not exported: %v", name, argType)
			}
			if method.Type.NumOut() != 1 {
				return fmt.Errorf(
					"subscriber %v.%v has wrong number of outs: %v require signature %s",
					name, method.Name, method.Type.NumOut(), subSig)
			}
			if returnType := method.Type.Out(0); returnType != typeOfError {
				return fmt.Errorf("subscriber %v.%v returns %v not error", name, method.Name, returnType.String())
			}
		}
	}

	return nil
}

func (g *grpcServer) createSubHandler(sb *subscriber, opts server.Options) broker.Handler {
	return func(p broker.Event) (err error) {
		defer func() {
			if r := recover(); r != nil {
				g.opts.Logger.Log(logger.ErrorLevel, "panic recovered: ", r)
				g.opts.Logger.Log(logger.ErrorLevel, string(debug.Stack()))

View on GitHub (pinned to 24529f1404)

Solutions

Change the handler to return exactly one value of type error: `func (h *h) Handle(ctx context.Context, evt *Event) error`
If you need to convey results, encode them into the message or use a published response topic instead of extra return values

Example fix

// before
func (h *handler) Handle(ctx context.Context, e *Event) (string, error) { ... }
// after
func (h *handler) Handle(ctx context.Context, e *Event) error { ... }

Defensive patterns

Strategy: validation

Validate before calling

func validHandlerOuts(fn interface{}) bool {
	return reflect.TypeOf(fn).NumOut() == 1
}

Prevention

Follow the canonical subscriber signature: func(ctx context.Context, msg *T) error
Never add extra return values to subscriber handlers; return data via messages or topics
Validate all handlers at startup by registering them immediately rather than lazily

When it happens

Trigger: Registering a subscriber whose method returns zero values (`func (h *h) Handle(evt *Event)`), two values (`(result, error)`), or more than one output.

Common situations: Copying a request/reply service handler style that returns `(resp, error)`; refactoring handlers to also return a value; forgetting the error return entirely.

Related errors

AI-assisted analysis of micro/go-micro@24529f1404 (2026-09-01). Data as JSON: /api/errors/4d68b91d910cc2ef. Report an issue: GitHub.