fix(session): keep auto compact suffix on user turn

tests/agent/test_auto_compact.py

@@ -45,6 +45,33 @@ def _add_turns(session, turns: int, *, prefix: str = "msg") -> None:
         session.add_message("assistant", f"{prefix} assistant {i}")
 
 
+def _add_tool_turn(session, prefix: str, idx: int) -> None:
+    call_id = f"{prefix}_{idx}"
+    session.messages.append(
+        {
+            "role": "assistant",
+            "content": "",
+            "tool_calls": [
+                {
+                    "id": call_id,
+                    "type": "function",
+                    "function": {"name": "exec", "arguments": "{}"},
+                }
+            ],
+            "timestamp": datetime.now().isoformat(),
+        }
+    )
+    session.messages.append(
+        {
+            "role": "tool",
+            "tool_call_id": call_id,
+            "name": "exec",
+            "content": "ok",
+            "timestamp": datetime.now().isoformat(),
+        }
+    )
+
+
 def _make_fake_compact(
     loop: AgentLoop,
     *,
@@ -76,7 +103,10 @@ def _make_fake_compact(
             metadata={},
             last_consolidated=0,
         )
-        dropped, already_consolidated = probe.retain_recent_legal_suffix(max_suffix)
+        dropped, already_consolidated = probe.retain_recent_legal_suffix(
+            max_suffix,
+            extend_to_user=True,
+        )
         kept = probe.messages
         archive_msgs = dropped[already_consolidated:]
 
@@ -305,6 +335,35 @@ class TestAutoCompact:
         assert session_after.messages[-1]["content"] == "msg assistant 5"
         await loop.close_mcp()
 
+    @pytest.mark.asyncio
+    async def test_auto_compact_extends_recent_suffix_to_user_turn(self, tmp_path):
+        loop = _make_loop(tmp_path, session_ttl_minutes=15)
+        session = loop.sessions.get_or_create("cli:test")
+        _add_turns(session, 2, prefix="old")
+        session.add_message("user", "record this")
+        for i in range(8):
+            _add_tool_turn(session, "recent", i)
+        session.add_message("assistant", "done")
+        loop.sessions.save(session)
+
+        await loop.auto_compact._archive("cli:test")
+
+        session_after = loop.sessions.get_or_create("cli:test")
+        assert len(session_after.messages) > loop.auto_compact._RECENT_SUFFIX_MESSAGES
+        assert session_after.messages[0]["content"] == "record this"
+        assert session_after.messages[-1]["content"] == "done"
+        tool_results = {
+            m.get("tool_call_id")
+            for m in session_after.messages
+            if m.get("role") == "tool"
+        }
+        assert all(
+            tc["id"] in tool_results
+            for m in session_after.messages
+            for tc in (m.get("tool_calls") or [])
+        )
+        await loop.close_mcp()
+
     @pytest.mark.asyncio
     async def test_auto_compact_stores_summary(self, tmp_path):
         """_archive should store the summary in _summaries."""

tests/agent/test_consolidator.py

@@ -561,8 +561,8 @@ class TestCompactIdleSession:
         real_consolidator,
         mock_provider,
     ):
-        """Assistant-only tails retain a non-contiguous slice, so archive the
-        actual dropped messages rather than a computed prefix."""
+        """Assistant-only tails extend back to the latest user turn, so archive
+        the actual dropped messages rather than a computed prefix."""
         mock_provider.chat_with_retry.return_value = MagicMock(
             content="Tail summary.", finish_reason="stop"
         )
@@ -585,12 +585,16 @@ class TestCompactIdleSession:
             "assistant-02",
             "assistant-03",
             "assistant-04",
+            "assistant-05",
+            "assistant-06",
+            "assistant-07",
+            "assistant-08",
+            "assistant-09",
         ]
 
         # #4264: idle compaction now summarizes the full unconsolidated tail, so
-        # the dropped head (user-00), the non-contiguous dropped tail
-        # (assistant-09), and the retained suffix (user-14) are all summarized.
-        # Retention above still proves the non-contiguous suffix is handled.
+        # the dropped head (user-00) and retained suffix (user-14 through
+        # assistant-09) are all summarized.
         archived_call = mock_provider.chat_with_retry.call_args
         user_content = archived_call.kwargs["messages"][1]["content"]
         assert "user-00" in user_content

tests/agent/test_session_manager_history.py

@@ -623,6 +623,24 @@ def test_retain_recent_legal_suffix_hard_cap_with_long_non_user_chain():
     assert len(session.messages) <= 6
 
 
+def test_retain_recent_legal_suffix_can_extend_to_user_for_long_recent_turn():
+    session = Session(key="test:extend-to-user")
+    session.messages.append({"role": "user", "content": "old"})
+    session.messages.append({"role": "assistant", "content": "old answer"})
+    session.messages.append({"role": "user", "content": "record this"})
+    for i in range(4):
+        session.messages.extend(_tool_turn("recent", i))
+    session.messages.append({"role": "assistant", "content": "done"})
+
+    session.retain_recent_legal_suffix(8, extend_to_user=True)
+
+    assert len(session.messages) > 8
+    assert session.messages[0]["content"] == "record this"
+    assert session.messages[-1]["content"] == "done"
+    history = session.get_history(max_messages=500)
+    _assert_no_orphans(history)
+
+
 # --- enforce_file_cap archive correctness (issue #4128) ---