Reproducible in:

slack-bolt==1.27.0
slack-sdk==3.38.0
Python 3.12.x

Steps to reproduce:

I'm currently using the new Slack chat_stream API to create an internal Slack bot for Airbnb employees. To repro this issue:

1. Create a Slack bot app with the necessary scopes (chat:write, chat:write.customize)
2. Send a message using chat.postMessage with username and icon_url parameters - observe that the message displays with the custom username and icon
3. Send a streaming message using chat_stream with the same username and icon_url parameters - observe that the message displays with the bot's default name/icon instead

Working call (chat.postMessage):

from slack_sdk.web.async_client import AsyncWebClient
client = AsyncWebClient(token="xoxb-...")
# This works - message displays as "Claude Code" with custom icon
response = await client.chat_postMessage(
channel="C1234567890",
thread_ts="1234567890.123456",
text="Hello from Claude Code!",
username="Claude Code",
icon_url="https://claude.ai/images/claude_app_icon.png",
)

Non-working call (chat_stream):

from slack_sdk.web.async_client import AsyncWebClient
client = AsyncWebClient(token="xoxb-...")
# This does NOT work - message displays with bot's default name, ignoring overrides
streamer = await client.chat_stream(
channel="C1234567890",
thread_ts="1234567890.123456",
username="Claude Code",
icon_url="https://claude.ai/images/claude_app_icon.png",
)
await streamer.append(markdown_text="Hello from Claude Code!")
await streamer.stop()

Expected result:

When passing username and icon_url (or icon_emoji) to chat_stream, the streaming message should display with the custom username and icon, just like chat.postMessage does.

The underlying chat.postMessage.stream API should support these parameters since regular chat.postMessage does.

Actual result:

The username and icon_url parameters are silently ignored when passed to chat_stream. The message displays with the bot's default identity (the name/icon configured in the Slack app settings) instead of the custom values.

This makes it impossible to use persona overrides (showing different display names/icons for different agents in a multi-agent bot) with the streaming API.