[APO-2194] Raise error on json serialization errors for outputs

src/vellum/utils/json_encoder.py

@@ -1,3 +1,4 @@
+import base64
 from dataclasses import asdict, is_dataclass
 from datetime import datetime
 import enum
@@ -48,6 +49,12 @@ def default(self, obj: Any) -> Any:
         if hasattr(obj, "__vellum_encode__") and callable(getattr(obj, "__vellum_encode__")):
             return obj.__vellum_encode__()
 
+        if isinstance(obj, (bytes, bytearray)):
+            try:
+                return obj.decode("utf-8")
+            except UnicodeDecodeError:
+                return base64.b64encode(obj).decode("ascii")
+
         if isinstance(obj, UUID):
             return str(obj)
 

src/vellum/workflows/nodes/bases/tests/test_base_node.py

@@ -6,6 +6,7 @@
 from vellum.client.types.string_vellum_value_request import StringVellumValueRequest
 from vellum.workflows.constants import undefined
 from vellum.workflows.descriptors.tests.test_utils import FixtureState
+from vellum.workflows.events.types import default_serializer
 from vellum.workflows.inputs.base import BaseInputs
 from vellum.workflows.nodes import FinalOutputNode
 from vellum.workflows.nodes.bases.base import BaseNode
@@ -15,6 +16,7 @@
 from vellum.workflows.references.node import NodeReference
 from vellum.workflows.references.output import OutputReference
 from vellum.workflows.state.base import BaseState, StateMeta
+from vellum.workflows.workflows.base import BaseWorkflow
 
 
 def test_base_node__node_resolution__unset_pydantic_fields():
@@ -379,3 +381,37 @@ class Ports(MyNode.Ports):
     # Potentially in the future, we support inheriting ports from multiple parents.
     # For now, we take only the declared ports, so that not all nodes have the default port.
     assert ports == ["bar"]
+
+
+def test_base_node__bytes_output_converts_to_string():
+    """Test that returning bytes in node outputs automatically converts to string."""
+
+    # GIVEN a node that returns bytes
+    class BytesOutputNode(BaseNode):
+        class Outputs(BaseNode.Outputs):
+            result: str
+
+        def run(self) -> "BytesOutputNode.Outputs":
+            b = b"hello"
+            return self.Outputs(result=b)  # type: ignore[arg-type]
+
+    class OutputNode(FinalOutputNode):
+        class Outputs(FinalOutputNode.Outputs):
+            result = BytesOutputNode.Outputs.result
+
+    class BytesWorkflow(BaseWorkflow):
+        graph = BytesOutputNode >> OutputNode
+
+        class Outputs(BaseWorkflow.Outputs):
+            result = OutputNode.Outputs.result
+
+    workflow = BytesWorkflow()
+
+    # WHEN we run the workflow
+    result = workflow.run()
+
+    # THEN the execution is fulfilled successfully
+    assert result.name == "workflow.execution.fulfilled"
+
+    # AND the bytes are converted to a UTF-8 string when serialized
+    assert default_serializer(result.outputs.result) == "hello"

src/vellum/workflows/runner/runner.py

@@ -49,7 +49,14 @@
     NodeExecutionRejectedBody,
     NodeExecutionStreamingBody,
 )
-from vellum.workflows.events.types import BaseEvent, NodeParentContext, ParentContext, SpanLink, WorkflowParentContext
+from vellum.workflows.events.types import (
+    BaseEvent,
+    NodeParentContext,
+    ParentContext,
+    SpanLink,
+    WorkflowParentContext,
+    default_serializer,
+)
 from vellum.workflows.events.workflow import (
     WorkflowEventStream,
     WorkflowExecutionFulfilledBody,
@@ -599,6 +606,20 @@ def initiate_node_streaming_output(
                                 parent=execution.parent_context,
                             )
 
+            for descriptor, output_value in outputs:
+                if output_value is undefined:
+                    continue
+                try:
+                    default_serializer(output_value)
+                except (TypeError, ValueError) as exc:
+                    raise NodeException(
+                        message=(
+                            f"Node {node.__class__.__name__} produced output '{descriptor.name}' "
+                            f"that could not be serialized to JSON: {exc}"
+                        ),
+                        code=WorkflowErrorCode.INVALID_OUTPUTS,
+                    ) from exc
+
             node.state.meta.node_execution_cache.fulfill_node_execution(node.__class__, span_id)
 
             with execution_context(parent_context=updated_parent_context, trace_id=execution.trace_id):