DevUI: Serialize workflow input as string to maintain conformance with OpenAI Responses format (#2021)

Co-authored-by: Victor Dibia <chuvidi2003@gmail.com>

dotnet/src/Microsoft.Agents.AI.Hosting.OpenAI/Responses/Models/ResponseInput.cs

@@ -182,7 +182,9 @@ internal sealed class ResponseInputJsonConverter : JsonConverter<ResponseInput>
             return messages is not null ? ResponseInput.FromMessages(messages) : null;
         }
 
-        throw new JsonException($"Unexpected token type for ResponseInput: {reader.TokenType}");
+        throw new JsonException(
+            "ResponseInput must be either a string or an array of messages. " +
+            $"Objects are not supported. Received token type: {reader.TokenType}");
     }
 
     /// <inheritdoc/>

dotnet/tests/Microsoft.Agents.AI.Hosting.OpenAI.UnitTests/OpenAIResponsesSerializationTests.cs

@@ -344,6 +344,20 @@ public sealed class OpenAIResponsesSerializationTests : ConformanceTestBase
         Assert.NotNull(request.Input);
     }
 
+    [Fact]
+    public void Deserialize_InvalidInputObject_ThrowsHelpfulException()
+    {
+        // Arrange
+        const string Json = "{\"model\":\"gpt-4o-mini\",\"input\":{\"input\":\"testing!\"},\"stream\":true}";
+
+        // Act & Assert
+        var exception = Assert.Throws<JsonException>(() =>
+            JsonSerializer.Deserialize(Json, OpenAIHostingJsonContext.Default.CreateResponse));
+
+        Assert.Contains("ResponseInput must be either a string or an array of messages", exception.Message);
+        Assert.Contains("Objects are not supported", exception.Message);
+    }
+
     [Fact]
     public void Deserialize_AllRequests_CanBeDeserialized()
     {

python/packages/devui/frontend/src/services/api.ts

@@ -747,7 +747,7 @@ class ApiClient {
     // Convert to OpenAI format - use metadata.entity_id for routing
     const openAIRequest: AgentFrameworkRequest = {
       metadata: { entity_id: workflowId }, // Entity ID in metadata for routing
-      input: request.input_data || "", // Send dict directly, no stringification needed
+      input: JSON.stringify(request.input_data || {}), // Serialize workflow input as JSON string
       stream: true,
       conversation: request.conversation_id, // Include conversation if present
       extra_body: request.checkpoint_id

python/packages/devui/tests/test_execution.py

@@ -238,6 +238,29 @@ def test_executor_parse_raw_falls_back_to_string():
     assert parsed == "hi there"
 
 
+def test_executor_parse_stringified_json_workflow_input():
+    """Stringified JSON workflow input (from frontend JSON.stringify) is correctly parsed."""
+    from pydantic import BaseModel
+
+    class WorkflowInput(BaseModel):
+        input: str
+        metadata: dict | None = None
+
+    executor = AgentFrameworkExecutor(EntityDiscovery(None), MessageMapper())
+    start_executor = _DummyStartExecutor(handlers={WorkflowInput: lambda *_: None})
+    workflow = _DummyWorkflow(start_executor)
+
+    # Simulate frontend sending JSON.stringify({"input": "testing!", "metadata": {"key": "value"}})
+    stringified_json = '{"input": "testing!", "metadata": {"key": "value"}}'
+
+    parsed = executor._parse_raw_workflow_input(workflow, stringified_json)
+
+    # Should parse into WorkflowInput object
+    assert isinstance(parsed, WorkflowInput)
+    assert parsed.input == "testing!"
+    assert parsed.metadata == {"key": "value"}
+
+
 async def test_executor_handles_non_streaming_agent():
     """Test executor can handle agents with only run() method (no run_stream)."""
     from agent_framework import AgentRunResponse, AgentThread, ChatMessage, Role, TextContent