Enum values in TaskStatus/TaskState cause "not JSON serializable" error

[TheWhiteWord]

Hi,
I encountered a serialization error when using the A2A server and client stack. When a task status update or similar object containing a TaskState enum is serialized (e.g., for logging or output), the following error occurs:

TypeError: Object of type TaskState is not JSON serializable

Details:

• The error occurs when code calls json.dumps(task.model_dump(by_alias=True), indent=2) (or similar) on a Pydantic model that includes a TaskState enum.
• This happens both in the server and the CLI, depending on where the serialization is triggered.
• The root cause is that the standard json module does not know how to serialize Python Enums, and Pydantic’s model_dump() does not convert Enums to their .value unless mode=\"json\" is used (Pydantic v2), or unless a custom default is provided to json.dumps.

Workaround:
I patched my local code to use:

json.dumps(task.model_dump(by_alias=True), indent=2, default=lambda o: o.value if isinstance(o, Enum) else str(o))

or, for Pydantic v2:

json.dumps(task.model_dump(by_alias=True, mode=\"json\"), indent=2)

But this is not consistent across the codebase.

Suggestion:

• Ensure all serialization of Pydantic models containing Enums uses either:
  • .model_dump(mode=\"json\") (Pydantic v2)
  • or a custom default function in json.dumps
• Optionally, set use_enum_values=True in the Pydantic model config for affected models.

References:

• The error is triggered by models like TaskStatus, TaskStatusUpdateEvent, etc., which use TaskState enums.

Thank you for your work on this project!