Feature Request: Add username and icon_url support to chatStream API

[yamadashy]

Summary

The chatStream() API currently does not support customizing the bot's display name and icon, unlike chat.postMessage. This makes it difficult to implement multi-agent systems where different agents need distinct visual identities.

Current Behavior

// chat.postMessage supports username and icon customization
await client.chat.postMessage({
  channel: 'C123456',
  text: 'Hello from Agent A',
  username: 'Agent A',
  icon_url: 'https://example.com/agent-a-icon.png'
});

// chatStream does not support these parameters
const streamer = client.chatStream({
  channel: 'C123456',
  recipient_team_id: 'T123456',
  recipient_user_id: 'U123456',
  thread_ts: '1234567890.123456',
  // username: 'Agent A',  // Not available
  // icon_url: 'https://...'  // Not available
});

Desired Behavior

Add username and icon_url parameters to chatStream() to allow bot identity customization:

const streamer = client.chatStream({
  channel: 'C123456',
  recipient_team_id: 'T123456',
  recipient_user_id: 'U123456',
  thread_ts: '1234567890.123456',
  username: 'Agent A',
  icon_url: 'https://example.com/agent-a-icon.png'
});

Use Case

Building a multi-agent AI assistant where different agents (e.g., "Research Agent", "Code Agent", "Writing Agent") need to be visually distinguishable to users. Currently, all streaming messages appear from the same bot identity.

Additional Context

• chat.postMessage already supports these parameters via the Authorship type
• This feature would maintain consistency across Slack's messaging APIs
• Related to multi-agent and AI assistant use cases

Thank you for considering this feature request!

Requirements

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[zimeg]

Hey @yamadashy! 👋 Thanks for writing in about these agentic happenings! 👾

Right now we have slackapi/bolt-python#1406 open with a similar request and no updates to share at the moment, but we can keep this issue open in the meantime!