[DOCS] Incomplete `Message` Union Type in Python SDK Reference

Documentation Type

Missing documentation (feature not documented)

Documentation Location

https://platform.claude.com/docs/en/agent-sdk/python

Section/Topic

The Message Types section, specifically the Message union type definition.

Current Documentation

### Message Union type of all possible messages. ``python Message = UserMessage | AssistantMessage | SystemMessage | ResultMessage ``

What's Wrong or Missing?

The documentation for the Message union type is incomplete. It omits the StreamEvent type, which is a valid member of the union in the actual SDK implementation.

When a developer enables the include_partial_messages option in ClaudeAgentOptions, the iterator yields StreamEvent objects. Because this type is missing from the documented union, it causes type-checking failures (e.g., with MyPy) and leads to unhandled cases in code that uses isinstance checks or pattern matching to process the message stream.

Suggested Improvement

Update the Message union type definition to include StreamEvent:

Message

Union type of all possible messages.

Message = UserMessage | AssistantMessage | SystemMessage | ResultMessage | StreamEvent

Additionally, ensure the StreamEvent dataclass is listed under the Message Types section (it appears to be present in the types list but missing from this specific core union definition).

Impact

High - Prevents users from using a feature

Additional Context

• Source Code Verification: In the SDK source code (at src/claude_agent_sdk/types.py), the type is explicitly defined as:

Message = UserMessage | AssistantMessage | SystemMessage | ResultMessage | StreamEvent

• Functional Impact: Developers relying on the documentation for type-safe message handling will miss StreamEvent logic, which is critical when utilizing partial message streaming features.
• Related Documentation: This discrepancy exists on the main Python SDK reference page at https://platform.claude.com/docs/en/agent-sdk/python.