Sneaky issue with `st.chat_input()` when dynamically updating `placeholder` argument

[spencer-thompson]

Checklist

• [X] I have searched the existing issues for similar issues.
• [X] I added a very descriptive title to this issue.
• [X] I have provided sufficient information below to help reproduce this issue.

Summary

When dynamically updating the placeholder inside st.chat_input() messages can not send (premature rerun?). For example randomly choosing a placeholder causes the message to be lost.

Reproducible Code Example

import streamlit as st
import random

if user_input := st.chat_input(
    random.choice(["say hi", "say hey", "say hello"]),
):

    st.chat_message("user").markdown(user_input)

    with st.chat_message("assistant"):
        st.write("testing")

Steps To Reproduce

1. pip install streamlit
2. Copy the code sample above
3. streamlit run app.py

Expected Behavior

It appears to be a premature rerun, the code should display a new chat message but does not. Upon running the code it will be clear.

Current Behavior

It appears to be a premature rerun. The message simply seems to disappear.

Is this a regression?

• [ ] Yes, this used to work in a previous version.

Debug info

• Streamlit version: 1.38
• Python version: 3.12.6
• Operating System: Arch Linux
• Browser: Firefox

Additional Information

Certainly not a critical bug, but given that streamlit is incredibly useful for LLM/Chat Apps it would be good to at least document.

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