Sending to `ServiceBus` with `BrokerProperties` fails with `Bad Request`

[gaevoy]

When I run the following SQL script to send a TestMessage message to Azure Service Bus topic {ServiceBusTopicName}:

declare @headers nvarchar(max), @url nvarchar(max), @payload nvarchar(max), @response nvarchar(max);
set @headers = json_object(
        'Accept': 'application/xml',
        'Content-Type': 'application/json',
        'BrokerProperties': '{"Label": "TestMessage", "SessionId": "Session1"}'
	);
set @payload = json_object('MessageId': 1);
select @headers, @payload;
set @url = 'https://{ServiceBusName}.servicebus.windows.net/{ServiceBusTopicName}/messages'
exec sp_invoke_external_rest_endpoint
    @url = @url,
    @method = 'POST',
    @headers = @headers,
    @payload = @payload,
    @credential = [https://{ServiceBusName}.servicebus.windows.net],
    @response = @response output
select cast(@response as xml)
go

It fails with 400 Bad Request pointing to 'BrokerProperties' is invalid:

<output>
  <response>
    <status>
      <http code="400" description="Bad Request" />
    </status>
    <headers>
      <header key="Date" value="Thu, 05 Oct 2023 13:32:20 GMT" />
      <header key="Transfer-Encoding" value="chunked" />
      <header key="Content-Type" value="application/xml; charset=utf-8" />
      <header key="Server" value="Microsoft-HTTPAPI/2.0" />
      <header key="Strict-Transport-Security" value="max-age=31536000" />
    </headers>
  </response>
  <result>
    <Error>
      <Code>400</Code>
      <Detail>The value '{\"Label\": \"TestMessage\", \"SessionId\": \"Session1\"}' of the HTTP header 'BrokerProperties' is invalid. TrackingId:1030e3ab-5a69-4414-b28e-73af5a76888d_G58, SystemTracker:***, Timestamp:2023-10-05T13:32:19</Detail>
    </Error>
  </result>
</output>

However, once I remove BrokerProperties from the header, it works returning 201 Created:

<output>
  <response>
    <status>
      <http code="201" description="Created" />
    </status>
    <headers>
      <header key="Date" value="Thu, 05 Oct 2023 13:39:17 GMT" />
      <header key="Transfer-Encoding" value="chunked" />
      <header key="Content-Type" value="application/xml; charset=utf-8" />
      <header key="Server" value="Microsoft-HTTPAPI/2.0" />
      <header key="Strict-Transport-Security" value="max-age=31536000" />
    </headers>
  </response>
</output>

My best guess is sp_invoke_external_rest_endpoint does not correctly decode a value that contains encoded JSON

{"Accept":"application\/xml","Content-Type":"application\/json","BrokerProperties":"{\"Label\": \"TestMessage\", \"SessionId\": \"Session1\"}"}

What am I doing wrong? Do you have any workarounds? It there better place to file this bug?

[JetterMcTedder]

@gaevoy thank you for logging this. We do know of this issue and are working on a fix.

[gaevoy]

Perfect! Happy, you are working on it 👍