Follow-up to the END-MARKER reorder: moving the summary prefix after the
[PRIOR CONTEXT] wrapper meant _is_context_summary_content (prefix-at-start)
no longer recognized a merged-tail summary. That silently broke three
consumers — the last-real-user anchor (would pick the merged summary as a
real user turn, causing active-task loss), the carry-forward summary find,
and the auto-focus skip. _strip_summary_prefix would also carry the wrapper
+ stale tail content forward as the next summary body.
Extract the two delimiter strings into _MERGED_PRIOR_CONTEXT_HEADER /
_MERGED_SUMMARY_DELIMITER constants (writer + detector stay in sync), teach
_is_context_summary_content and _strip_summary_prefix to look past the
delimiter, and add a regression test. Standalone summaries unchanged.
A context-compaction handoff banner is inserted with role="user" when the
protected head ends in an assistant/tool message. On a resumed or
multi-compaction session, _find_last_user_message_idx would return that
banner as the latest user turn, so _ensure_last_user_message_in_tail anchored
the tail to the summary and rolled the genuine last user message into the
next compaction — the exact active-task loss the anchor exists to prevent
(#10896/#22523).
Reuse the existing _is_context_summary_content helper to skip summary banners
when locating the last real user message.
Salvaged from #36626 by Frank Song (issue #36624). The PR's other two changes
(demoting completed tool results inside the protected tail; a preflight
compression_exhausted result) are superseded on current main by the min_tail
floor (#39170), the no-op compression counting (#40803), and the existing
413/disabled terminal-error paths.
21 cases pinning the new ``_ensure_last_assistant_message_in_tail``
anchor and its interaction with the existing tail-cut path:
* ``TestFindLastAssistantMessageIdx`` — helper contract: prefers a
content-bearing assistant message, skips ``tool_calls``-only
stubs, multimodal text-block content counts, falls back to
"any assistant" when no content-bearing reply exists, honours
``head_end``, returns -1 when there's none.
* ``TestEnsureLastAssistantMessageInTail`` — direct: no-op when
already in the tail, walks ``cut_idx`` back when the reply is
in the compressed middle, never crosses into the head region,
re-aligns through a preceding ``tool_call`` / ``tool_result``
group instead of orphaning it.
* ``TestFindTailCutByTokensAnchorsAssistant`` — integration:
reporter repro (long tool-output run after the visible reply)
now preserves the reply; user and assistant anchors compose
in a single tail-cut call; a soft-ceiling-overrunning oversized
tool result no longer strands the prior reply.
* ``TestCompactionRollupReproduction`` — end-to-end through
``compress()`` with a stubbed ``_generate_summary``: the
visible reply text survives either as its own standalone
assistant message (normal path) or concatenated onto the
merged summary tail (double-collision path the WebUI then
re-splits). The standalone-summary case is asserted strictly
(exactly one summary row, exactly one separate assistant
row carrying the reply) — that's the dominant path and any
drift there reintroduces the original bug.
* ``TestSourceGuardrail`` — static asserts on
``agent/context_compressor.py``: the helper exists, the
anchor is wired into ``_find_tail_cut_by_tokens`` AFTER the
user-message anchor (so chaining is monotonic), the
content-bearing preference is preserved, and the issue
number is referenced so future bisects can find this fix.
kshitijk4poor
Frank Song
xxxigm