Recently, I was developing an SSIS package that read from an Azure SQL DB table and wrote to an on-premises SQL Server 2025 database table. I encountered the following error:

[ADO NET Source [41]] Error: ADO NET Source has failed to acquire the connection {<ConnectionManagerGUID>} with the following error message: “Could not load file or assembly ‘Microsoft.Data.SqlClient, Version=6.0.0.0, Culture=neutral, PublicKeyToken=<PublicKeyToken>’ or one of its dependencies. The located assembly’s manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)”.

Back in the day, I would search for an answer using a search engine to locate forum posts or blogs by my fellow SSIS data engineers. But this fine morning, I checked the system time in the Windows 11 system tray and noted that it’s 2026 (already, Can you believe it? Time flies, I tell ya…).

I fired up a Claude Code session instead

At the time of this writing (12 Jun 2026), Anthropic has recently released Fable 5 – touting it as the first “Mythos-class model” – and is allowing users to tinker around with the model until 22 Jun 2026 (I have an Anthropic Max 20 subscription, so I’m not sure if all users get to tinker. Max 20 subscribers get to tinker, at least).

I opened cmd on a Dev virtual machine – a VM running Windows Server 2022, SQL Server 2025, and Visual Studio 2026 Insiders¹ – navigated to the local repo, and shared the error with Claude Code.

CC’s first pass identified an issue:

This is a classic SSIS assembly-binding problem: the package’s ADO.NET connection manager references Microsoft.Data.SqlClient 6.0.0.0, but the version installed/registered on the machine doesn’t match. Let me look at the package and what’s actually installed on this machine.

Fair enough. The model did a little exploring using PowerShell to check some registry settings and adjusted some SSIS connection manager settings, and then claimed all was well. The model wasn’t wrong. the original error was solved, but we had a new error (it had that new error smell):

[ADO NET Source [41]] Error: ADO NET Source has failed to acquire the connection
{CC2302DB-AC2E-4F3C-851D-A1D81BCAE0E0} with the following error message: “An error occurred while sending the request.”.

I was a little surprised when the first “fix” applied by Fable 5 didn’t solve the entire issue in one shot, but I may have had high expectations. Claude Code and I endeavored to persevere.

I confess, I found this impressive: Fable 5 next used PowerShell to fire up a network trace. It found my server was attempting to connect to the Azure SQL DB using TLS 1.0. In June 2026, Azure AD endpoints flat refuse to accept TLS 1.0 connections, with good reason. the recommend fix was:

The fix: Set Microsoft’s recommended strong-crypto registry values, which make all .NET Framework apps (including the SSIS hosts) use the OS default of TLS 1.2+. This needs an elevated shell, which I don’t have — please run this in PowerShell started as Administrator:

Set-ItemProperty ‘HKLM:\SOFTWARE\Microsoft\.NETFramework\v4.0.30319’ -Name SchUseStrongCrypto -Value 1 -Type DWord
Set-ItemProperty ‘HKLM:\SOFTWARE\Microsoft\.NETFramework\v4.0.30319’ -Name SystemDefaultTlsVersions -Value 1 -Type DWord
Set-ItemProperty ‘HKLM:\SOFTWARE\WOW6432Node\Microsoft\.NETFramework\v4.0.30319’ -Name SchUseStrongCrypto -Value 1 -Type DWord
Set-ItemProperty ‘HKLM:\SOFTWARE\WOW6432Node\Microsoft\.NETFramework\v4.0.30319’ -Name SystemDefaultTlsVersions -Value 1 -Type DWord

This is Microsoft’s documented TLS best practice for .NET Framework, it’s machine-wide, and no reboot is needed — though restart Visual Studio so DtsDebugHost picks it up. The only theoretical downside is if some other .NET Framework app on this server depends on TLS 1.0 to reach an ancient endpoint, which is unlikely and would already be on borrowed time.

I took note of “…an ancient endpoint…”. Was that attitude, Fable 5??

Like a good HIL (Human In the Loop), I did what I was told. And, it worked!

Conclusion

Comparing the amount of time it took to write this post to the amount of time it took to implement the fix using Claude Code and the Fable 5 model, it took more time to write this post.

Notes

¹You can get my AI-Assisted Data Engineering Development Playbook at the link for $49. It’s included in a paid subscription to my Engineer of Data Substack newsletter, which is less expensive.