Failed to Upsert Data into Lakehouse Delta Table – Missing System.Linq.Async Assembly

[Yazdan]

Hi All,

While ingesting data from an on-premises SQL Server database table into a Lakehouse Delta table in Microsoft Fabric using the upsert method in copy activity, I encountered the following error:

ErrorCode=FailedToUpsertDataIntoDeltaTable, 'Type=Microsoft.DataTransfer.Common.Shared.HybridDeliveryException, Message=Hit an error when upsert data to table in Lakehouse. Error message: Could not load file or assembly 'System.Linq.Async, Version=6.0.0.0, Culture=neutral, PublicKeyToken=94bc3704cddfc263' or one of its dependencies. The system cannot find the file specified., Source=Microsoft.DataTransfer.Connectors.LakehouseTableConnector,''Type=System.IO.FileNotFoundException, Message=Could not load file or assembly 'System.Linq.Async, Version=6.0.0.0, Culture=neutral, PublicKeyToken=94bc3704cddfc263' or one of its dependencies. The system cannot find the file specified., Source=Microsoft.DataTransfer.Connectors.LakehouseTableConnector,'

Steps to reproduce:

1. Configure pipeline to copy data from on-prem SQL Server to Fabric Lakehouse Delta table using copy activity.
2. Use Upsert method.
3. Run the pipeline.
4. Error occurs during execution.

Expected behavior:
The pipeline should upsert data into the Lakehouse Delta table without assembly-related errors.

Actual behavior:
The pipeline fails with the missing assembly error for System.Linq.Async.

Environment details:

• Source: On-premises SQL Server
• Destination: Fabric Lakehouse Delta table
• Copy activity with Upsert enabled

Request:
Please investigate and advise on:

• Whether this is a known issue with the Fabric Lakehouse connector
• Any non-Notebook workarounds (e.g., configuration change, library update, alternative method)
• ETA for a fix if this is a product bug,

Thanks All,

[Thomaslleblanc]

create a support ticket for microsoft to investigate, if it is in product, put a high priority

[Yazdan · Accepted solution]

I was able to resolve this issue by turning on Enable staging in the Settings tab of the copy activity.

I believe if the staging step is required for this scenario, the error message should be clearer and point users towards enabling staging as the solution. The current error message is complex and not well defined, which makes troubleshooting harder than it needs to be.