dotnet/aspnetcore · error

Invalid payload for Completion message.

Error message

Invalid payload for Completion message.

What it means

Thrown by _createCompletionMessage when the decoded properties array has fewer than 4 elements. A Completion frame is [MessageType.Completion, headers, invocationId, resultKind, ...] so it needs at least the type, headers, invocationId, and resultKind. This guard fires before resultKind is inspected.

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 294cab2f9b)

Solutions

1. Align SignalR client and server versions.
2. Verify the server isn't a custom/non-ASP.NET implementation producing non-standard Completion frames.
3. Catch and treat as a failed invocation; surface the error to the caller of connection.invoke.
4. Inspect server logs for the completing hub method.

Example fix

// before
const result = await connection.invoke('DoWork');
// server sends short Completion -> throws

// after
try {
  const result = await connection.invoke('DoWork');
} catch (e) {
  logger.log(LogLevel.Error, 'Invoke failed (possibly malformed Completion): ' + e.message);
  await reconnect();
}

try {
  const result = await connection.invoke('DoWork');
} catch (e) {
  if (/Completion message/.test(e?.message)) {
    logger.log(LogLevel.Error, 'Malformed Completion frame; reconnecting.');
    await connection.start();
  } else throw e;
}

When it happens

Trigger: Server sends a Completion frame missing the resultKind (properties[3]); frame corruption removes trailing elements; protocol version mismatch.

Common situations: Version skew; a server-side hub method returns and the Completion frame is serialized without the resultKind byte; network truncation.

Related errors

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

AI-assisted analysis of dotnet/aspnetcore@294cab2f9b (2026-08-06). Data as JSON: /api/errors/4bedeae609815013. Report an issue: GitHub.