dotnet/aspnetcore · error · Error

Invalid SequenceId for Ack message.

Error message

Invalid SequenceId for Ack message.

What it means

An Ack message (type 8) is part of the reliable-transport extension and carries a numeric `sequenceId` that the receiver acknowledges. A non-numeric sequenceId breaks the contiguous-sequence contract used to detect lost or duplicated messages.

Source

Thrown at src/SignalR/clients/ts/signalr/src/JsonHubProtocol.ts:123

            throw new Error("Invalid payload for StreamItem message.");
        }
    }

    private _isCompletionMessage(message: CompletionMessage): void {
        if (message.result && message.error) {
            throw new Error("Invalid payload for Completion message.");
        }

        if (!message.result && message.error) {
            this._assertNotEmptyString(message.error, "Invalid payload for Completion message.");
        }

        this._assertNotEmptyString(message.invocationId, "Invalid payload for Completion message.");
    }

    private _isAckMessage(message: AckMessage): void {
        if (typeof message.sequenceId !== 'number') {
            throw new Error("Invalid SequenceId for Ack message.");
        }
    }

    private _isSequenceMessage(message: SequenceMessage): void {
        if (typeof message.sequenceId !== 'number') {
            throw new Error("Invalid SequenceId for Sequence message.");
        }
    }

    private _assertNotEmptyString(value: any, errorMessage: string): void {
        if (typeof value !== "string" || value === "") {
            throw new Error(errorMessage);
        }
    }
}

View on GitHub (pinned to 294cab2f9b)

Solutions

1. Ensure both client and server run SignalR versions that agree on reliable-session support.
2. Confirm the server emits sequenceId as a JSON number, not a string.
3. Disable reliable sessions on the server if the client does not support them, or upgrade the client.
4. Inspect the raw ack message with logMessageContent:true.

Example fix

// before
{ "type": 8, "sequenceId": "5" }

// after
{ "type": 8, "sequenceId": 5 }

function validAck(m: any): boolean {
  return m?.type === 8 && typeof m.sequenceId === 'number';
}

function isAckMessage(m: any): m is { type: 8; sequenceId: number } {
  return m?.type === 8 && typeof m.sequenceId === 'number';
}

When it happens

Trigger: Server sends {"type":8,"sequenceId":"5"} (string) or omits sequenceId while reliable sessions are negotiated. Occurs when only one side enables the reliable protocol feature.

Common situations: Client/server version skew where one side supports reliable sessions (ack/sequence) and the other sends a malformed ack, or a hand-built server emitting a string sequenceId.

Related errors

• Invalid SequenceId for Sequence message.
• Invalid input for JSON hub protocol. Expected a string.
• Invalid payload.
• Invalid payload for StreamItem message.
• Invalid payload for Completion message.

AI-assisted analysis of dotnet/aspnetcore@294cab2f9b (2026-08-06). Data as JSON: /api/errors/a47bdbd021f637b9. Report an issue: GitHub.