dotnet/aspnetcore · error · Error

Invalid payload for Completion message.

Error message

Invalid payload for Completion message.

What it means

Thrown by _createCompletionMessage (first check) when the decoded properties array has fewer than 4 elements. A Completion message is [MessageType.Completion, headers, invocationId, resultKind, result?]. resultKind (properties[3]) is mandatory — fewer than 4 elements means the result kind is absent.

Solutions

1. Verify the server's completion encoder includes the resultKind slot.
2. Match SignalR protocol versions between client and server.
3. Reconnect the HubConnection; the invocation whose completion was malformed will not resolve, so callers awaiting it should also be notified.

Example fix

// before
// server emits [3, {}, '42'] — missing resultKind → throws

// after — server must emit [3, {}, '42', resultKind, ...]
// e.g. void result: [3, {}, '42', 0]

function isValidCompletionProperties(p: unknown[]): boolean {
  return p.length >= 4;
}

function isCompletionPayload(p: unknown[]): p is [number, unknown, string, number, unknown?] {
  return p.length >= 4;
}

try { return protocol.parseMessages(buf, logger); }
catch (e) {
  if (/Invalid payload for Completion/.test((e as Error).message)) {
    logger.log(LogLevel.Error, 'Malformed Completion — reconnecting');
    await connection.stop(); await connection.start();
  } else throw e;
}

When it happens

Trigger: A Completion frame missing the resultKind byte; corrupt bytes decoding to a short array typed as Completion; a non-conforming server sending an incompletely-encoded completion.

Common situations: Server-side completion serialization bug; protocol version mismatch; wire corruption after an invocation.

Related errors

• Invalid payload for Close message.
• Invalid payload for Invocation message.
• Invalid payload.
• Invalid payload for Ping message.
• Invalid payload for StreamItem message.

AI-assisted analysis of dotnet/aspnetcore@3600ca084e (2026-08-11). Data as JSON: /api/errors/4bedeae609815013. Report an issue: GitHub.

Appendix: source

Thrown at src/SignalR/clients/ts/signalr-protocol-msgpack/src/MessagePackHubProtocol.ts:232

    private _createStreamItemMessage(headers: MessageHeaders, properties: any[]): StreamItemMessage {
        // check minimum length to allow protocol to add items to the end of objects in future releases
        if (properties.length < 4) {
            throw new Error("Invalid payload for StreamItem message.");
        }

        return {
            headers,
            invocationId: properties[2],
            item: properties[3],
            type: MessageType.StreamItem,
        } as StreamItemMessage;
    }

    private _createCompletionMessage(headers: MessageHeaders, properties: any[]): CompletionMessage {
        // check minimum length to allow protocol to add items to the end of objects in future releases
        if (properties.length < 4) {
            throw new Error("Invalid payload for Completion message.");
        }

        const resultKind = properties[3];

        if (resultKind !== this._voidResult && properties.length < 5) {
            throw new Error("Invalid payload for Completion message.");
        }

        let error: string | undefined;
        let result: any;

        switch (resultKind) {
            case this._errorResult:
                error = properties[4];
                break;
            case this._nonVoidResult:
                result = properties[4];
                break;

View on GitHub (pinned to 3600ca084e)