What causes "invalid ws proto - %s" in slimtoolkit/slim?

Pass "ws" or "wss" as proto (correct case, scheme only, no trailing ://).

invalid ws proto - %s

Error message

invalid ws proto - %s

What it means

NewWebsocketClient validates the websocket protocol string before constructing the client address. Only "ws" (and "wss", per IsValidWSProto) are accepted; anything else — http, https, empty-but-invalid overrides — makes construction fail with this error. This is a fail-fast guard against forming an unusable URL scheme.

Source

Thrown at pkg/app/master/probe/http/wsclient.go:41

	PongCount acounter.Type
	PingCount acounter.Type
	Addr      string
	pongCh    chan string
	doneCh    chan struct{}
}

type WebsocketMessage struct {
	Type int
	Data []byte
}

func NewWebsocketClient(proto, host, port string) (*WebsocketClient, error) {
	if proto == "" {
		proto = ProtoWS
	}

	if !IsValidWSProto(proto) {
		return nil, fmt.Errorf("invalid ws proto - %s", proto)
	}

	wsclient := &WebsocketClient{
		Addr:   fmt.Sprintf("%s://%s:%s", proto, host, port),
		doneCh: make(chan struct{}),
		pongCh: make(chan string, 10),
	}

	return wsclient, nil
}

func IsValidWSProto(proto string) bool {
	switch proto {
	case ProtoWS, ProtoWSS:
		return true
	default:
		return false
	}

View on GitHub (pinned to 81940d17fa)

Solutions

Pass "ws" or "wss" as proto (correct case, scheme only, no trailing ://).
Convert a full URL: extract the scheme and map https->wss, http->ws.
Leave proto empty to get the default (ws) instead of passing an invalid scheme.
Normalize case before calling: strings.ToLower(proto).

Example fix

// before
c, err := NewWebsocketClient("https", host, port)
// after
proto := "wss"
if r.URL.Scheme == "http" { proto = "ws" }
c, err := NewWebsocketClient(proto, host, port)

Defensive patterns

Strategy: validation

Validate before calling

func validWSProto(p string) bool { return p == "" || p == "ws" || p == "wss" }
if !validWSProto(cfg.WSProto) {
    log.Fatalf("ws proto must be ws or wss, got %q", cfg.WSProto)
}

Type guard

func IsValidWSProto(proto string) bool {
    return proto == "ws" || proto == "wss"
}

Try / catch

c, err := NewWebsocketClient(proto, host, port)
if err != nil && strings.Contains(err.Error(), "invalid ws proto") {
    return fmt.Errorf("configure scheme as ws or wss (map http->ws, https->wss): %w", err)
}

Prevention

Store only scheme values (ws/wss) in config, never full URLs.
Map http->ws and https->wss when deriving from an HTTP URL.
Rely on the empty-string default (ws) unless TLS is required (wss).

When it happens

Trigger: Calling NewWebsocketClient with a proto value that fails IsValidWSProto, e.g. passing "http", "https", "HTTP" (case sensitivity), or a full URL instead of a scheme.

Common situations: Configuring the HTTP probe's websocket endpoint from an environment/config value that stores a base URL (https://host) instead of a scheme-only value; copy-pasting from browser devtools; assuming http/https work for websockets.

Related errors

AI-assisted analysis of slimtoolkit/slim@81940d17fa (2026-08-31). Data as JSON: /api/errors/1a6ab67b7beaa267. Report an issue: GitHub.