Outage in Azure Devops
VsTest errors with Microsoft.TestPlatform v18.5.0
Resolved Minor
April 22, 2026 - Started about 15 hours ago
- Lasted 44 minutes
Incident Report
Summary AI Generated
Azure DevOps customers experienced VsTest failures in distributed test flows due to a System.IO.FileNotFoundException related to the System.Collections.Immutable assembly when using Microsoft.TestPlatform version 18.5.0. The issue affected testing services globally across all regions for 44 minutes. The problem was resolved through coordination with the publishing team, with customers advised to either switch to single agent VsTest flow or revert to earlier Microsoft.TestPlatform versions as workarounds.
Some customers might encounter exceptions when using VsTest in distributed test flow, of the format "System.IO.FileNotFoundException: Could not load file or assembly 'System.Collections.Immutable, Version=8.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies." These errors are correlating with the 18.5.0 version of https://www.nuget.org/packages/Microsoft.TestPlatform/.
Impacted customers should either move to the single agent flow of VsTest or revert to earlier versions of Microsoft.TestPlatform. Azure DevOps engineers are coordinating with the publishing team to resolve the issue.
Components affected
Azure Devops Brazil Azure Devops India Azure Devops Canada Azure Devops Asia Pacific Azure Devops United Kingdom Azure Devops United States Azure Devops Europe Azure Devops Australia Azure Devops Other servicesTrusted by 1,000+ teams
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UPDATE
Our engineers are working to understand and mitigate the issue.
UPDATE
Some customers might encounter exceptions when using VsTest in distributed test flow, of the format "System.IO.FileNotFoundException: Could not load file or assembly 'System.Collections.Immutable, Version=8.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies." These errors are correlating with the 18.5.0 version of https://www.nuget.org/packages/Microsoft.TestPlatform/.
Impacted customers should either move to the single agent flow of VsTest or revert to earlier versions of Microsoft.TestPlatform. Azure DevOps engineers are coordinating with the publishing team to resolve the issue.
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