When the in-flight request survived in the protected head, the replay now
removes that original (one active row per message_uid). But the summary
role was chosen against a head that still ended on that user row, so the
compacted window then opened system -> assistant(tool_calls) -> tool ->
assistant summary: no user turn before the carrier, which strict
alternation templates (#58753 class) reject.
When the removal leaves the assistant carrier with no preceding visible
user, switch to the _force_user_leading layout: carrier role=user with the
request merged after _SUMMARY_END_MARKER (the existing merge path records
the uid as absorbed, so it stays single).
Co-authored-by: Yuan Li <dskwelmcy@163.com>
7b3fc99ce8