I’m building a Slack bot using FastAPI that performs a long-running task and updates the original message in Slack after the task completes. However, instead of updating the original message, it sends a new one.

Expected Behavior:

• The bot should display an initial message (“Processing your request…”) and later update this same message with the final result.

Actual Behavior:

• The initial message is sent correctly, but when the task completes, a new message is posted instead of updating the original one.

Code Implementation:

FastAPI Endpoint:

@app.post("/greet")
async def greet_user(
    background_tasks: BackgroundTasks,
    user_name: str = Form(...),
    response_url: str = Form(...)  # Provided by Slack
):
    background_tasks.add_task(
        process_long_running_task,
        user_name=user_name,
        response_url=response_url
    )
    return JSONResponse(
        content={
            "response_type": "ephemeral",
            "text": "Processing your request..."
        }
    )

Background Task:

async def process_long_running_task(user_name: str, response_url: str):
    await asyncio.sleep(15)  # Simulate long-running task
    final_response = {
        "response_type": "in_channel",
        "text": f"Hii, {user_name}. How can I help you today?"
    }
    async with httpx.AsyncClient() as client:
        await client.post(response_url, json=final_response)

Question:

How can I ensure the bot updates the original message instead of posting a new one? Do I need to use additional parameters or a different API method?

Thank you for any help!