Two-pronged fix for the WebUI "context compaction block in place of
last assistant response" regression.
Agent layer (the real fix). ``_find_tail_cut_by_tokens`` already had
``_ensure_last_user_message_in_tail`` to keep the most recent user
request out of the compressed middle (#10896), but no symmetric
anchor for the assistant side. When the conversation has an
oversized recent tool result or a long stretch of tool-call/result
pairs *after* the assistant's last visible reply, the token-budget
walk can stop with the previously-visible reply on the wrong side
of ``cut_idx``. The summariser then rolls it into the single
``[CONTEXT COMPACTION — REFERENCE ONLY]`` block persisted as
``role="user"`` or ``role="assistant"``, and from the operator's
perspective the WebUI session viewer
(``web/src/pages/SessionsPage.tsx``) and the TUI chat panel both
suddenly show the opaque "Context compaction" block in the slot
where they were just reading the actual answer:
User: "i cant see the output of the last message you sent,
i did see it previously, however now see 'context
compaction'"
Added ``_ensure_last_assistant_message_in_tail`` mirror of the
user-side anchor. It looks for the most recent assistant message
with non-empty text content (skipping tool-call-only assistant
"stubs" which the UI renders as small "calling tool X" indicators
rather than a readable bubble) and walks ``cut_idx`` back through
the standard ``_align_boundary_backward`` so we don't split a
tool_call/result group that immediately precedes it. The two
anchors are chained — each only walks ``cut_idx`` backward, so
the tail can only grow.
Falls back to "most recent assistant of any kind" only when no
content-bearing reply exists in the compressible region (fresh
multi-step tool sequence with no prior reply) — in that case the
agent-side fix is effectively a no-op and the existing
user-message anchor carries the load.
WebUI layer (clarity). Added ``isCompactionMessage`` detector that
recognises the ``[CONTEXT COMPACTION — REFERENCE ONLY]`` (current)
and ``[CONTEXT SUMMARY]:`` (legacy) prefixes from
``agent/context_compressor.py``, and a new ``compaction`` entry
in ``MessageBubble``'s ``ROLE_STYLES`` map. Compaction blocks
now render as muted, italicised system-style rows labelled
``Context handoff`` — clearly metadata, not the assistant's
actual reply — so an operator scrolling back through a long
session can't mistake the summary for a real answer.
Keeping the detected prefixes inline (rather than importing them)
because the WebUI bundle has no Python interop. A guardrail comment
points readers at the source-of-truth constants in
``agent/context_compressor.py``.
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