What causes "Invalid payload for StreamItem message." in dotnet/aspnetcore?

Confirm client and server SignalR versions match, especially for streaming scenarios.

Invalid payload for StreamItem message.

Error message

Invalid payload for StreamItem message.

What it means

Thrown by _createStreamItemMessage when the decoded properties array has fewer than 4 elements. A StreamItem is [MessageType.StreamItem, headers, invocationId, item], so four is the minimum. This fires when a streaming result frame is missing its invocationId or item slot.

Source

Thrown at src/SignalR/clients/ts/signalr-protocol-msgpack/src/MessagePackHubProtocol.ts:218

                target: properties[3] as string,
                type: MessageType.Invocation,
            };
        } else {
            return {
                arguments: properties[4],
                headers,
                streamIds: [],
                target: properties[3],
                type: MessageType.Invocation,
            };
        }

    }

    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];

View on GitHub (pinned to 294cab2f9b)

Solutions

Confirm client and server SignalR versions match, especially for streaming scenarios.
Check that server-side streaming methods (IAsyncEnumerable / Channel) complete cleanly.
Catch the error and end the affected stream subscription / reconnect.
Inspect the failing hub method for exceptions during enumeration.

Example fix

// before
connection.stream('StreamData').subscribe({ next: x => {} });
// server emits short StreamItem -> throws inside parseMessages

// after
connection.stream('StreamData').subscribe({
  next: x => {},
  error: err => logger.log(LogLevel.Error, 'Stream failed: ' + err),
});

Defensive patterns

Strategy: try-catch

Try / catch

const subscription = connection.stream('StreamData').subscribe({
  next: x => handleItem(x),
  error: err => {
    if (/StreamItem message/.test(err?.message)) {
      logger.log(LogLevel.Error, 'Malformed StreamItem; resubscribing.');
      reconnect();
    }
  },
});

Prevention

Always provide an error handler to connection.stream(...).subscribe().
Keep streaming implementations on both ends at compatible SignalR versions.
Ensure server-side streaming methods (Channel / IAsyncEnumerable) don't throw mid-enumeration producing malformed frames.

When it happens

Trigger: Server sends a StreamItem frame missing the item element; corruption strips trailing bytes of a streaming result; the server is an older/newer version using a different StreamItem layout.

Common situations: Version mismatch between client and server streaming implementation; a server-side streaming hub method throwing mid-stream producing a malformed frame; network truncation of a large streamed item.

Related errors

AI-assisted analysis of dotnet/aspnetcore@294cab2f9b (2026-08-06). Data as JSON: /api/errors/97a24669f9422bef. Report an issue: GitHub.