fix(api): preserve workflow failure chaining

Author: Aitomates

src/cas_reference_product/app.py

@@ -13,6 +13,10 @@
 _tracer = trace.get_tracer(__name__)
 
 
+def _raise_workflow_backend_failure(error: WorkflowAgentServiceError) -> None:
+    raise HTTPException(status_code=502, detail="Workflow backend request failed") from error
+
+
 def create_app(settings: Settings | None = None) -> FastAPI:
     app_settings = settings or get_settings()
 
@@ -55,7 +59,7 @@ def execute(envelope: PromptEnvelope, request: Request) -> WorkflowResult:
             orchestrator = WorkflowOrchestrator(service, app_settings.repository)
             try:
                 result = orchestrator.execute(envelope)
-            except WorkflowAgentServiceError:
+            except WorkflowAgentServiceError as error:
                 span.add_event(
                     "workflow.failed",
                     attributes={
@@ -64,9 +68,7 @@ def execute(envelope: PromptEnvelope, request: Request) -> WorkflowResult:
                         "error": True,
                     },
                 )
-                raise HTTPException(
-                    status_code=502, detail="Workflow backend request failed"
-                ) from None
+                _raise_workflow_backend_failure(error)
             span.add_event(
                 "workflow.completed",
                 attributes={

tests/test_api.py

@@ -1,9 +1,11 @@
 from typing import Any
 from unittest.mock import patch
 
+import pytest
+from fastapi import HTTPException
 from fastapi.testclient import TestClient
 
-from cas_reference_product.app import create_app
+from cas_reference_product.app import _raise_workflow_backend_failure, create_app
 from cas_reference_product.config import Settings
 from cas_reference_product.workflow import WorkflowAgentServiceError
 
@@ -69,3 +71,23 @@ def test_workflow_api_sanitizes_external_service_failures(envelope: "Any") -> No
     assert response.status_code == 502
     assert response.json() == {"detail": "Workflow backend request failed"}
     assert "sensitive" not in response.text
+
+
+def test_workflow_backend_failure_preserves_exception_chain() -> None:
+    with pytest.raises(HTTPException) as caught:
+        try:
+            raise RuntimeError("sensitive provider detail")
+        except RuntimeError as provider_error:
+            try:
+                raise WorkflowAgentServiceError("Foundry workflow invocation failed") from (
+                    provider_error
+                )
+            except WorkflowAgentServiceError as workflow_error:
+                _raise_workflow_backend_failure(workflow_error)
+
+    assert caught.value.status_code == 502
+    assert caught.value.detail == "Workflow backend request failed"
+    assert isinstance(caught.value.__cause__, WorkflowAgentServiceError)
+    assert str(caught.value.__cause__) == "Foundry workflow invocation failed"
+    assert isinstance(caught.value.__cause__.__cause__, RuntimeError)
+    assert str(caught.value.__cause__.__cause__) == "sensitive provider detail"