DeltaToolCall only supports `function`, missing `custom` tool type

[JoongWonSeo]

Confirm this is an issue with the Python library and not an underlying OpenAI API

• This is an issue with the Python library

Describe the bug

Now that custom tools are supported, I can see that the return types have been updated to a discriminated union:

openai-python/src/openai/types/chat/chat_completion_message_tool_call.py

Lines 12 to 15 in e4ec91e

	ChatCompletionMessageToolCall: TypeAlias = Annotated[
	Union[ChatCompletionMessageFunctionToolCall, ChatCompletionMessageCustomToolCall],
	PropertyInfo(discriminator="type"),
	]

However, I noticed that in the streaming version, the delta tool chunk currently supports function calls:

openai-python/src/openai/types/chat/chat_completion_chunk.py

Lines 47 to 56 in e4ec91e

	class ChoiceDeltaToolCall(BaseModel):
	index: int
	id: Optional[str] = None
	"""The ID of the tool call."""
	function: Optional[ChoiceDeltaToolCallFunction] = None
	type: Optional[Literal["function"]] = None
	"""The type of the tool. Currently, only `function` is supported."""

Are streaming custom tool calls not supported yet or is this just an oversight?

OS

macOS

Python version

Python 3.13

Library version

openai v1.99.3