Is streaming of new "custom" tool calls supported using Chat Completions?

[AmevinLS]

Context:

In the documentation here, an example is given for passing "custom" tools to models using the Chat Completions API.

However, in the chat completion chunk object described here, the choices.delta.tool_calls only seems to support type="function" with no mention of "custom". The same is reflected in the openai python SDK, currently leading to among other things typing issues in downstream custom code.

Question:

So does this mean that streaming from Chat Completions while passing "custom" tools isn't supported? Or perhaps is the API definition incomplete?

Thanks in advance.

[fadighomrani77-byte]

هاي

[antoninche]

Hi!

At the moment, streaming tool calls with Chat Completions isn’t fully supported for “custom” tool types in the official API or SDK. The spec and current SDK implementations only explicitly list support for "function" tool calls in the streaming chunks, and streaming support for other kinds (e.g., custom tool call objects) isn’t documented or surfaced in the typical chat completions streaming responses yet.

In short:

• The API does support streaming text responses (stream: true).
• Tool calls appear in chunk deltas (e.g., choices.delta.tool_calls), but only for type: "function". There’s no official indication that custom tool call chunks are streamed today.
• That means if you send custom tools in tools=[...], the model may produce custom tool call entries in the final response, but these aren’t currently streamed chunk by chunk in chat.completions streaming.

If you need streaming of tool calls or fine-grained streaming of custom tool invocations, one option is to use the Responses API instead — it has richer streaming events and better tooling support for advanced tool interactions, including custom tools. The Responses API is the newer recommended interface for complex agent-style streaming.

Here’s a minimal streaming snippet showing what is supported with function tools:

from openai import OpenAI

client = OpenAI()

stream = client.chat.completions.create(
    model="gpt-5",
    messages=[{"role": "user", "content": "Hello"}],
    tools=[
        {"type": "function", "function": {"name": "my_tool"}}
    ],
    stream=True,
)

for chunk in stream:
    # text arrives chunk by chunk
    print(chunk.choices[0].delta)