fix(context): keep _partition in bounds when nothing is kept

agent_core/runtime/loop/compact_llm.py

@@ -383,7 +383,7 @@ def _partition(
             forward += 1
         if forward < len(rest):
             split_idx = forward
-        else:
+        elif split_idx < len(rest):
             # The whole tail is tool results — a parallel tool-call turn that
             # emitted at least ``keep_recent`` of them. Walking forward runs off
             # the end and leaves the last result orphaned (the bug this guard
@@ -393,6 +393,10 @@ def _partition(
             # recent history survives, not a cap.
             while split_idx > 0 and is_tool_msg(rest[split_idx]):
                 split_idx -= 1
+        # ``split_idx == len(rest)`` (``keep_recent=0``: summarise everything) is
+        # neither case. The kept window is EMPTY, so no orphan is possible, and
+        # both walks are meaningless — the forward one has nothing to scan and
+        # the backward one would index one past the end. Leave the split alone.
 
         return sys_msgs, rest[:split_idx], rest[split_idx:]
 

tests/test_compact_llm.py

@@ -232,6 +232,33 @@ def test_partition_still_shrinks_the_window_when_it_can() -> None:
     assert middle == history[1:3]
 
 
+def test_partition_summarises_everything_when_nothing_is_kept() -> None:
+    """``keep_recent=0`` asks for the whole history to be summarised.
+
+    The kept window is then empty, so the orphan guard has nothing to protect:
+    walking forward runs off the end, and the backward walk added for a
+    tool-result tail indexed one past it — an ``IndexError`` that callers
+    summarising a finished session (``keep_recent=0`` is how they ask for it)
+    swallowed as "no summary available", silently losing the rollup.
+    """
+    history: list[Message] = [
+        system_msg("S"),
+        user_msg("q"),
+        {
+            "role": "assistant",
+            "content": "",
+            "tool_calls": [{"id": "a", "function": {"name": "web_search", "arguments": "{}"}}],
+        },
+        tool_msg("result a", "a"),
+    ]
+
+    system, middle, recent = LLMSummaryCompactor._partition(history, keep_recent=0)
+
+    assert system == [history[0]]
+    assert middle == history[1:]
+    assert recent == []
+
+
 def test_rate_limit_naming_a_retry_delay_is_still_transient() -> None:
     """A bare ``400`` inside a provider message is not a status code.