How to combine multiple sarif files when using the errorlog

[ilmax]

I was looking into the Roslyn SARIF support and I successfully produced the output while compiling a project using the ErrorLog MsBuild property. Uploading it to GitHub though generates a warning due to multiple files being created when compiling multiple projects (i.e. one SARIF file for each project) as can be seen in this workflow run

The upload SARIF action gives a warning and says that support for uploading multiple SARIF files for a single category will be discontinued in June 2025 as per this GitHub announcement.

The suggestion from the announcement is to upload using multiple categories but this is not easily achievable unless I build one project at a time using a matrix.

I also tried to output all the SARIF files to a single output to check if Roslyn merges them, but found it not to be the case.

I created a small repository with some test here using an aspire starter template and configured some code analysis to produce some results + added an intentional warning.

To my understanding I can simply merge the results array of the SARIS file and upload a single file.

Am I missing something obvious here? What's the suggested way forward?

Thank you