.NET: [Bug]: YAML validation is weak in some cases. Declarative.dll errors misleading or not self explainable

[dsslight]

Description

For the Declarative workflow documentation
YAML validation is weak in some cases. For example, I can set an invalid value on SendActivity instead of text (like prompt.test, not prompt.text) and only get runtime errors about missing variables instead of YAML validation errors.
If I set a variable such as Topic.ToolResponse in YAML, even when the namespace is wrong, validation does not complain. Fixing this problem need to be done carefully, cos we have undocumented behavior about namespaces Topic vs Local here

In general I can only infer what went wrong from limited runtime errors produced by the declarative.DLL that executes the YAML instructions.
Dll errors quite limited and do not provide useful info, so I need to guess what's going on.
If declarative.dll code debuggable, would be great to have instruction on how to do it/turn it on.

Code Sample

Error Messages / Stack Traces

Package Versions

Microsoft.Agents.AI.Workflows.Declarative

.NET Version

.NET 10.0.300

Additional Context

documentation related issue

[github-actions]

🤖 Automated triage reproduction notes (agent-authored — trust but verify)

Agent analysis

DeclarativeWorkflowBuilder.cs::ReadWorkflow at line 98 accepts a SendActivity expression containing the invalid known member System.LastMessage.Test; WorkflowActionVisitor.cs::Visit(SendActivity) then routes it to runtime evaluation in SendActivityExecutor.cs::ExecuteAsync. Minimal repro: build an otherwise valid workflow with activity: =System.LastMessage.Test and assert that DeclarativeModelException is raised during Build<string>.

• Failing test: dotnet/tests/Microsoft.Agents.AI.Workflows.Declarative.UnitTests/DeclarativeWorkflowTest.cs
• Files examined: dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/DeclarativeWorkflowBuilder.cs, dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/Interpreter/WorkflowActionVisitor.cs, dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/ObjectModel/SendActivityExecutor.cs, dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/PowerFx/WorkflowFormulaState.cs, dotnet/tests/Microsoft.Agents.AI.Workflows.Declarative.UnitTests/DeclarativeWorkflowTest.cs, dotnet/tests/Microsoft.Agents.AI.Workflows.Declarative.UnitTests/ObjectModel/SendActivityExecutorTest.cs, dotnet/tests/Microsoft.Agents.AI.Workflows.Declarative.UnitTests/Workflows/SendActivity.yaml, dotnet/nuget/nuget-package.props
• Tests run: DeclarativeWorkflowTest.UndefinedSendActivityVariableIsRejected (failed), DeclarativeWorkflowTest.InvalidWorkflowAsync (3 passed)
• Current version: 1.20.0