From 6fb2297f6d3df56de8f8c398a8ed29205e09d033 Mon Sep 17 00:00:00 2001
From: Navpreet Singh <ns.2004@outlook.com>
Date: Sat, 4 Oct 2025 18:41:46 +0100
Subject: [PATCH] fix(core): drop orphaned tool messages when trimming

---
 libs/core/langchain_core/messages/utils.py    | 45 +++++++++--
 .../tests/unit_tests/messages/test_utils.py   | 78 +++++++++++++++++++
 2 files changed, 117 insertions(+), 6 deletions(-)

diff --git a/libs/core/langchain_core/messages/utils.py b/libs/core/langchain_core/messages/utils.py
index 7e2bd238c1b69..6814ff4077374 100644
--- a/libs/core/langchain_core/messages/utils.py
+++ b/libs/core/langchain_core/messages/utils.py
@@ -1440,11 +1440,10 @@ def _first_max_tokens(
         # When all messages fit, only apply end_on filtering if needed
         if end_on:
             for _ in range(len(messages)):
-                if not _is_message_type(messages[-1], end_on):
-                    messages.pop()
-                else:
+                if not messages or _is_message_type(messages[-1], end_on):
                     break
-        return messages
+                messages.pop()
+        return _remove_orphaned_tool_messages(messages)
 
     # Use binary search to find the maximum number of messages within token limit
     left, right = 0, len(messages)
@@ -1535,7 +1534,7 @@ def _first_max_tokens(
             else:
                 break
 
-    return messages[:idx]
+    return _remove_orphaned_tool_messages(messages[:idx])
 
 
 def _last_max_tokens(
@@ -1594,7 +1593,41 @@ def _last_max_tokens(
     if system_message:
         result = [system_message, *result]
 
-    return result
+    return _remove_orphaned_tool_messages(result)
+
+
+def _remove_orphaned_tool_messages(
+    messages: Sequence[BaseMessage],
+) -> list[BaseMessage]:
+    """Drop tool messages whose corresponding tool calls are absent."""
+    if not messages:
+        return []
+
+    valid_tool_call_ids: set[str] = set()
+    for message in messages:
+        if isinstance(message, AIMessage):
+            if message.tool_calls:
+                for tool_call in message.tool_calls:
+                    tool_call_id = tool_call.get("id")
+                    if tool_call_id:
+                        valid_tool_call_ids.add(tool_call_id)
+            if isinstance(message.content, list):
+                for block in message.content:
+                    if (
+                        isinstance(block, dict)
+                        and block.get("type") == "tool_use"
+                        and block.get("id")
+                    ):
+                        valid_tool_call_ids.add(block["id"])
+
+    cleaned_messages: list[BaseMessage] = []
+    for message in messages:
+        if isinstance(message, ToolMessage) and (
+            not valid_tool_call_ids or message.tool_call_id not in valid_tool_call_ids
+        ):
+            continue
+        cleaned_messages.append(message)
+    return cleaned_messages
 
 
 _MSG_CHUNK_MAP: dict[type[BaseMessage], type[BaseMessageChunk]] = {
diff --git a/libs/core/tests/unit_tests/messages/test_utils.py b/libs/core/tests/unit_tests/messages/test_utils.py
index 32151132b3d4c..93e8e0fc9c9c1 100644
--- a/libs/core/tests/unit_tests/messages/test_utils.py
+++ b/libs/core/tests/unit_tests/messages/test_utils.py
@@ -393,6 +393,84 @@ def test_trim_messages_last_30_include_system_allow_partial_end_on_human() -> No
     assert _MESSAGES_TO_TRIM == _MESSAGES_TO_TRIM_COPY
 
 
+def test_trim_messages_last_removes_orphaned_tool_message() -> None:
+    messages = [
+        HumanMessage("What's the weather in Florida?"),
+        AIMessage(
+            [
+                {"type": "text", "text": "Let's check the weather in Florida"},
+                {
+                    "type": "tool_use",
+                    "id": "abc123",
+                    "name": "get_weather",
+                    "input": {"location": "Florida"},
+                },
+            ],
+            tool_calls=[
+                {
+                    "name": "get_weather",
+                    "args": {"location": "Florida"},
+                    "id": "abc123",
+                    "type": "tool_call",
+                }
+            ],
+        ),
+        ToolMessage("It's sunny.", name="get_weather", tool_call_id="abc123"),
+        HumanMessage("I see"),
+        AIMessage("Do you want to know anything else?"),
+        HumanMessage("No, thanks"),
+        AIMessage("You're welcome! Have a great day!"),
+    ]
+
+    trimmed = trim_messages(
+        messages,
+        strategy="last",
+        token_counter=len,
+        max_tokens=5,
+    )
+
+    expected = [
+        HumanMessage("I see"),
+        AIMessage("Do you want to know anything else?"),
+        HumanMessage("No, thanks"),
+        AIMessage("You're welcome! Have a great day!"),
+    ]
+
+    assert trimmed == expected
+    assert _MESSAGES_TO_TRIM == _MESSAGES_TO_TRIM_COPY
+
+
+def test_trim_messages_last_preserves_tool_message_when_call_present() -> None:
+    messages = [
+        HumanMessage("Start"),
+        AIMessage(
+            "Sure, let me check",
+            tool_calls=[
+                {
+                    "name": "search",
+                    "args": {"query": "status"},
+                    "id": "tool-1",
+                    "type": "tool_call",
+                }
+            ],
+        ),
+        ToolMessage("All systems operational", tool_call_id="tool-1"),
+        HumanMessage("Thanks"),
+    ]
+
+    trimmed = trim_messages(
+        messages,
+        strategy="last",
+        token_counter=len,
+        max_tokens=3,
+    )
+
+    expected = messages[1:]
+
+    assert trimmed == expected
+    assert _MESSAGES_TO_TRIM == _MESSAGES_TO_TRIM_COPY
+
+
 def test_trim_messages_last_40_include_system_allow_partial_start_on_human() -> None:
     expected = [
         SystemMessage("This is a 4 token text."),