.NET: [Flaky test] DevUIIntegrationTests.TestServerWithDevUI_ResolvesMixedAgentsAndWorkflows_AllRegistrationsAsync

Summary

Microsoft.Agents.AI.DevUI.UnitTests.DevUIIntegrationTests.TestServerWithDevUI_ResolvesMixedAgentsAndWorkflows_AllRegistrationsAsync is failing intermittently in the merge_group across PRs that do not touch DevUI, blocking the queue.

Failure signature

System.NullReferenceException : Object reference not set to an instance of an object.
   at Microsoft.Agents.AI.DevUI.UnitTests.DevUIIntegrationTests.TestServerWithDevUI_ResolvesMixedAgentsAndWorkflows_AllRegistrationsAsync()
   in dotnet/tests/Microsoft.Agents.AI.DevUI.UnitTests/DevUIIntegrationTests.cs:273

Line 273 dereferences discoveryResponse.Entities.Count after an HTTP GET on /v1/entities, so either discoveryResponse or discoveryResponse.Entities is null on the failed runs.

Affected merge_group runs (last 30, 5 occurrences)

4 of 5 occurrences are on PRs that do not modify DevUI source. The one DevUI-touching PR (#5831) only changes logging via source generator, which is unlikely to cause an NRE on the discovery response.

Suspected cause

Race between app.StartAsync() and the test's GetTestClient().GetAsync(""/v1/entities"") discovery enumeration, possibly exposed after the polymorphic registration changes from #3792 layered with the DevUI access controls from #5739. The test registers six entities (3 agents + 3 workflows) using a mix of keyed and default registrations and asserts Entities.Count == 6.

Mitigation

A follow-up PR will quarantine this test with [Fact(Skip = ""..."")] and link this issue. Owner of the DevUI integration surface should investigate the race and re-enable.

Next steps

• Reproduce locally with repeated runs under Release on Linux.
• Verify discovery completes deterministically before the first GET, or guard the test with a poll/wait.
• Re-enable the test once the underlying race is fixed.