stapp: "A fragment tried to write to a container created outside the fragment" on streaming (Streamlit 1.60)

[zhangPinkdolphin]

Describe the bug

When the agent starts streaming output in the Streamlit frontend (frontends/stapp.py), Streamlit raises:

A fragment tried to write to a container created outside the fragment. This is not allowed. Streamlit may not be able to correctly render the container ... Could not reserve a stable position.

Environment

• GenericAgent commit: d426d45 (main, 2026-08-08)
• Streamlit: 1.60.0 (declared requirement is streamlit>=1.28)
• Platform: macOS, Python 3.12

Root cause

_tick() is a @st.fragment(run_every=...) that writes into _stream_fh, a container created outside the fragment (module scope). Streamlit only allows a fragment to write to an external container if that container has already been claimed during a full app run; otherwise the first write raises the error above. The current code never claims the container before writing expanders into it.

Steps to reproduce

1. streamlit run frontends/stapp.py
2. Send any prompt that produces streaming output (LLM Running ...)
3. Observe the fragment error once the first step expander is folded into _stream_fh

Suggested fix

Claim the external container before streaming into it, e.g. inside _tick:

_stream_claimed = False  # module scope

# inside _tick(), before the first expander write:
if frozen == 0 and not _stream_claimed:
    with _stream_fh:
        st.markdown("")   # claim: reserve a stable position in the external container
    _stream_claimed = True

This works on Streamlit 1.60 and avoids the error entirely.

Related: #574 (fragment rerun scope fallback) fixed a different fragment issue but not this one.