dotnet/aspnetcore · error · Error

Unknown transport: ${transport}.

Error message

Unknown transport: ${transport}.

What it means

Thrown at HttpConnection.ts:541 in the default branch of _constructTransport's switch. The switch handles WebSockets, ServerSentEvents, and LongPolling; any other numeric value (or a corrupted HttpTransportType) falls through to default. In practice this should be unreachable with the published enum, but it guards against bad input.

Source

Thrown at src/SignalR/clients/ts/signalr/src/HttpConnection.ts:541

    }

    private _constructTransport(transport: HttpTransportType): ITransport {
        switch (transport) {
            case HttpTransportType.WebSockets:
                if (!this._options.WebSocket) {
                    throw new Error("'WebSocket' is not supported in your environment.");
                }
                return new WebSocketTransport(this._httpClient, this._accessTokenFactory, this._logger, this._options.logMessageContent!,
                    this._options.WebSocket, this._options.headers || {});
            case HttpTransportType.ServerSentEvents:
                if (!this._options.EventSource) {
                    throw new Error("'EventSource' is not supported in your environment.");
                }
                return new ServerSentEventsTransport(this._httpClient, this._httpClient._accessToken, this._logger, this._options);
            case HttpTransportType.LongPolling:
                return new LongPollingTransport(this._httpClient, this._logger, this._options);
            default:
                throw new Error(`Unknown transport: ${transport}.`);
        }
    }

    private _startTransport(url: string, transferFormat: TransferFormat): Promise<void> {
        this.transport!.onreceive = this.onreceive;
        if (this.features.reconnect) {
            this.transport!.onclose = async (e) => {
                let callStop = false;
                if (this.features.reconnect) {
                    try {
                        this.features.disconnected();
                        await this.transport!.connect(url, transferFormat);
                        await this.features.resend();
                    } catch {
                        callStop = true;
                    }
                } else {
                    this._stopConnection(e);

View on GitHub (pinned to 294cab2f9b)

Solutions

1. Use a single concrete HttpTransportType value (WebSockets, ServerSentEvents, LongPolling) rather than a bitwise combination.
2. If you need 'auto', omit the transport option entirely and let negotiation select.
3. Ensure you are not passing a string like "WebSockets" — pass the enum member.

Example fix

// before (wrong: bitwise combo)
withUrl(url, { transport: HttpTransportType.WebSockets | HttpTransportType.LongPolling })

// after (auto-select)
withUrl(url)
// or explicit single transport
withUrl(url, { transport: HttpTransportType.WebSockets })

const VALID = [HttpTransportType.WebSockets, HttpTransportType.ServerSentEvents, HttpTransportType.LongPolling];
function assertSingleTransport(t) {
  if (!VALID.includes(t)) throw new Error(`Invalid transport: ${t}. Use one of ${VALID.join(', ')}`);
}
assertSingleTransport(options.transport);

function isSingleValidTransport(t: unknown): t is HttpTransportType {
  return t === HttpTransportType.WebSockets || t === HttpTransportType.ServerSentEvents || t === HttpTransportType.LongPolling;
}

try { await conn.start(transferFormat); }
catch (e) {
  if (/Unknown transport/.test(String(e))) {
    delete options.transport; // let negotiation auto-select
    return new HttpConnection(url, options).start(transferFormat);
  }
  throw e;
}

When it happens

Trigger: Passing a numeric transport value outside 1/2/4 (e.g. 3, 7, 0) via a cast or untyped JS; an enum that was renamed/reordered across versions; constructing a custom transport value with bitwise math that produces an invalid combo (e.g. WebSockets | ServerSentEvents = 3 is not a single transport).

Common situations: Calling `_constructTransport` directly in tests with a fabricated number; combining transport flags where only a single concrete transport is expected; a future version of the enum introducing a new value the old client doesn't handle.

Related errors

• Invalid message type.
• Unknown ${name} value: ${val}.
• The ComponentRenderMode field only exists on frames of type
• Invalid boolean value '${attributeValue}' for parameter '${p
• Invalid number value '${attributeValue}' for parameter '${pa

AI-assisted analysis of dotnet/aspnetcore@294cab2f9b (2026-08-06). Data as JSON: /api/errors/1d39ed5b4cc9bd5e. Report an issue: GitHub.