fix(langchain): ensure parent AI messages are preserved during summarization

libs/langchain_v1/langchain/agents/middleware/summarization.py

@@ -7,6 +7,7 @@
 from typing import Any, Literal, cast
 
 from langchain_core.messages import (
+    AIMessage,
     AnyMessage,
     MessageLikeRepresentation,
     RemoveMessage,
@@ -469,7 +470,41 @@ def _find_safe_cutoff(self, messages: list[AnyMessage], messages_to_keep: int) -
             return 0
 
         target_cutoff = len(messages) - messages_to_keep
-        return self._find_safe_cutoff_point(messages, target_cutoff)
+        cutoff = self._find_safe_cutoff_point(messages, target_cutoff)
+
+        # Scan preserved messages to find any orphaned ToolCalls
+        ids_to_find = set()
+        for i in range(cutoff, len(messages)):
+            msg = messages[i]
+            if isinstance(msg, ToolMessage):
+                ids_to_find.add(msg.tool_call_id)
+            # If we find the parent in preserved, we are good for that ID
+            if isinstance(msg, AIMessage):
+                for tc in msg.tool_calls:
+                    if tc["id"] in ids_to_find:
+                        ids_to_find.remove(tc["id"])
+
+        # If ids_to_find is not empty, it means we have orphans.
+        # We must look BACKWARDS from cutoff to find the parents.
+        if ids_to_find:
+            # We iterate backwards from just before the cutoff
+            for i in range(cutoff - 1, -1, -1):
+                msg = messages[i]
+                if isinstance(msg, AIMessage):
+                    found_any = False
+                    for tc in msg.tool_calls:
+                        if tc["id"] in ids_to_find:
+                            ids_to_find.remove(tc["id"])
+                            found_any = True
+
+                    if found_any:
+                        # We found a parent! We must move the cutoff here.
+                        cutoff = i
+
+                if not ids_to_find:
+                    break
+
+        return cutoff
 
     def _find_safe_cutoff_point(self, messages: list[AnyMessage], cutoff_index: int) -> int:
         """Find a safe cutoff point that doesn't split AI/Tool message pairs.

libs/langchain_v1/tests/unit_tests/agents/middleware/implementations/test_summarization.py

@@ -887,3 +887,39 @@ def test_summarization_middleware_cutoff_at_start_of_tool_sequence() -> None:
     # Index 2 is an AIMessage (safe cutoff point), so no adjustment needed
     cutoff = middleware._find_safe_cutoff(messages, messages_to_keep=4)
     assert cutoff == 2
+
+
+def test_summarization_middleware_orphaned_tool_prevention() -> None:
+    """Test that middleware prevents orphaned ToolMessages by preserving parent AIMessages."""
+    middleware = SummarizationMiddleware(
+        model=MockChatModel(), trigger=("messages", 2), keep=("messages", 2)
+    )
+
+    # Scenario: AIMessage -> HumanMessage -> ToolMessage
+    # Standard summarization keeping 1 message would only keep the ToolMessage,
+    # causing an orphan. Fix should force keeping the AIMessage too.
+    messages: list[AnyMessage] = [
+        HumanMessage(content="context"),
+        AIMessage(content="call", tool_calls=[{"name": "tool", "args": {}, "id": "call-1"}]),
+        HumanMessage(content="interruption"),
+        ToolMessage(content="result", tool_call_id="call-1"),
+    ]
+
+    state = {"messages": messages}
+    result = middleware.before_model(state, None)  # type: ignore[arg-type]
+
+    assert result is not None
+    assert "messages" in result
+
+    # We expect 3 preserved messages: AI, Human, Tool
+    # Plus the RemoveMessage and the summary HumanMessage
+    # Total messages in result: Remove + Summary + [AI, Human, Tool] = 5
+
+    preserved_messages = result["messages"][2:]
+    assert len(preserved_messages) == 3
+    assert isinstance(preserved_messages[0], AIMessage)
+    assert isinstance(preserved_messages[1], HumanMessage)
+    assert isinstance(preserved_messages[2], ToolMessage)
+
+    assert preserved_messages[0].tool_calls[0]["id"] == "call-1"
+    assert preserved_messages[2].tool_call_id == "call-1"