fix(api): preserve compaction summaries at any history position during Responses auto-truncation

The gateway /compress path can force a user-leading layout that leaves
the compaction summary after a retained system head, so scanning only
the leading block misses it. Preserve marker-carrying messages wherever
they sit, filling the remaining budget with the most recent other
messages, and cover the non-leading position with a test.

gateway/platforms/api_server.py

@@ -200,28 +200,37 @@ def _auto_truncate_response_history(
     *,
     limit: int = RESPONSES_AUTO_TRUNCATION_HISTORY_LIMIT,
 ) -> List[Dict[str, Any]]:
-    """Keep recent Responses history without dropping the compaction handoff."""
+    """Keep recent Responses history without dropping the compaction handoff.
+
+    Compaction summaries are preserved wherever they sit in the history —
+    the gateway /compress path can leave them after a retained system head
+    (see ``context_compressor`` force-user-leading handling), so a
+    leading-block-only scan would silently drop them.
+    """
     if limit <= 0 or len(conversation_history) <= limit:
         return conversation_history
 
-    leading_summaries: List[Dict[str, Any]] = []
-    first_conversation_index = 0
-    for message in conversation_history:
-        if not _is_compressed_summary_message(message):
-            break
-        leading_summaries.append(message)
-        first_conversation_index += 1
-
-    if not leading_summaries:
+    summary_indices = [
+        index
+        for index, message in enumerate(conversation_history)
+        if _is_compressed_summary_message(message)
+    ]
+    if not summary_indices:
         return conversation_history[-limit:]
 
-    preserved = leading_summaries[:limit]
-    remaining = limit - len(preserved)
-    if remaining <= 0:
-        return preserved
+    kept_indices = set(summary_indices[:limit])
+    remaining = limit - len(kept_indices)
+    if remaining > 0:
+        summary_index_set = set(summary_indices)
+        for index in range(len(conversation_history) - 1, -1, -1):
+            if index in summary_index_set:
+                continue
+            kept_indices.add(index)
+            remaining -= 1
+            if remaining <= 0:
+                break
 
-    recent_tail = conversation_history[first_conversation_index:][-remaining:]
-    return preserved + recent_tail
+    return [conversation_history[index] for index in sorted(kept_indices)]
 
 
 def _normalize_chat_content(

tests/gateway/test_api_server.py

@@ -3580,6 +3580,53 @@ class TestTruncation:
         assert history[1]["content"] == "msg 50"
         assert history[-1]["content"] == "msg 148"
 
+    @pytest.mark.asyncio
+    async def test_truncation_auto_preserves_non_leading_compaction_summary(self, adapter):
+        """A summary sitting after a retained system head must survive too.
+
+        The gateway /compress path can force a user-leading layout that
+        leaves the compaction summary after a kept system message, so the
+        preservation predicate must not assume the summary is at index 0.
+        """
+        mock_result = {"final_response": "OK", "messages": [], "api_calls": 1}
+
+        system_head = {"role": "system", "content": "You are a helpful agent."}
+        summary = {
+            "role": "user",
+            "content": "[CONTEXT COMPACTION — REFERENCE ONLY]\nEarlier work.",
+            "_compressed_summary": True,
+        }
+        long_history = [system_head, summary] + [
+            {"role": "user", "content": f"msg {i}"}
+            for i in range(148)
+        ]
+        adapter._response_store.put("resp_summary_mid", {
+            "response": {"id": "resp_summary_mid", "object": "response"},
+            "conversation_history": long_history,
+            "instructions": None,
+        })
+
+        app = _create_app(adapter)
+        async with TestClient(TestServer(app)) as cli:
+            with patch.object(adapter, "_run_agent", new_callable=AsyncMock) as mock_run:
+                mock_run.return_value = (mock_result, {"input_tokens": 0, "output_tokens": 0, "total_tokens": 0})
+                resp = await cli.post(
+                    "/v1/responses",
+                    json={
+                        "model": "hermes-agent",
+                        "input": "follow up",
+                        "previous_response_id": "resp_summary_mid",
+                        "truncation": "auto",
+                    },
+                )
+
+        assert resp.status == 200
+        history = mock_run.call_args.kwargs["conversation_history"]
+        assert len(history) == 100
+        assert history[0] == summary
+        assert history[1]["content"] == "msg 49"
+        assert history[-1]["content"] == "msg 147"
+
     @pytest.mark.asyncio
     async def test_no_truncation_keeps_full_history(self, adapter):
         """Without truncation=auto, long history is passed as-is."""