.NET: [Bug]: Exception in workflow, that is added as agent, results in json serialisation error message instead of exception

[hansmbakker]

Description

Have a workflow that throws an exception. Have the workflow registered as an agent.

Call the agent. Instead of the actual exception being logged, it results in a JSON serialisation error towards the user.

This was an issue in Semantic Kernel already: microsoft/semantic-kernel#10708

Code Sample

var workflowAsAgent = builder
    .AddWorkflow("super-agent", CreateWorkflowDelegate)
    .AddAsAIAgent();  // Now the workflow can be used as an agent

Error Messages / Stack Traces

Internal (swallowed) exception:

Exception has occurred: CLR/System.InvalidOperationException
Exception thrown: 'System.InvalidOperationException' in Microsoft.Agents.AI.Workflows.dll: 'Executor 'AggregatorExecutor' cannot send messages of type 'MySolution.Workflow.RawAggregate'.'
   at Microsoft.Agents.AI.Workflows.InProc.InProcessRunnerContext.<SendMessageAsync>d__30.MoveNext() in /home/vsts/work/1/s/agent-framework/dotnet/src/Microsoft.Agents.AI.Workflows/InProc/InProcessRunnerContext.cs:line 235

What the user sees:

Serialization and deserialization of 'System.Reflection.MethodBase' instances is not supported. Path: $.TargetSite. (Code: execution_error)

Package Versions

    <PackageVersion Include="Microsoft.Agents.AI" Version="1.20.0" />
    <PackageVersion Include="Microsoft.Agents.AI.A2A" Version="1.20.0-preview.260831.1" />
    <PackageVersion Include="Microsoft.Agents.AI.DevUI" Version="1.20.0-preview.260831.1" />
    <PackageVersion Include="Microsoft.Agents.AI.Hosting.A2A" Version="1.20.0-preview.260831.1" />
    <PackageVersion Include="Microsoft.Agents.AI.Hosting.A2A.AspNetCore" Version="1.20.0-preview.260831.1" />
    <PackageVersion Include="Microsoft.Agents.AI.Hosting.AGUI.AspNetCore" Version="1.20.0-preview.260831.1" />
    <PackageVersion Include="Microsoft.Agents.AI.Hosting.AspNetCore" Version="1.20.0-preview.260831.1" />
    <PackageVersion Include="Microsoft.Agents.AI.Hosting.OpenAI" Version="1.20.0-alpha.260831.1" />
    <PackageVersion Include="Microsoft.Agents.AI.Workflows" Version="1.20.0" />
    <PackageVersion Include="Microsoft.Agents.AI.Workflows.Generators" Version="1.20.0" />

.NET Version

.Net 10.0

Additional Context

No response

[github-actions]

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

Agent analysis

The bug is in dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowSession.cs::InvokeStageAsync at lines 538-552, where a WorkflowErrorEvent containing the original Exception is assigned to AgentResponseUpdate.RawRepresentation. It is triggered when a workflow-as-agent throws and a host serializes that raw representation. Minimal repro: run a throwing workflow through AsAIAgent(), select the emitted WorkflowErrorEvent raw representation, and serialize it with System.Text.Json.

• Failing test: dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/WorkflowHostSmokeTests.cs
• Files examined: dotnet/src/Microsoft.Agents.AI.Workflows/InProc/InProcessRunnerContext.cs, dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowSession.cs, dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowHostAgent.cs, dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowsJsonUtilities.cs, dotnet/src/Microsoft.Agents.AI.Hosting.OpenAI/Responses/InMemoryResponsesService.cs, dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/WorkflowHostSmokeTests.cs
• Tests run: WorkflowHostSmokeTests.Test_AsAgent_ErrorContentStreamedOutAsync, WorkflowHostSmokeTests.Test_AsAgent_WorkflowExceptionUpdateCanBeSerializedAsync
• Reported version: 1.20.0