fix(core): remove orphaned ToolMessages in trim_message

libs/core/langchain_core/messages/utils.py

@@ -1420,6 +1420,43 @@ def convert_to_openai_messages(
     return oai_messages
 
 
+def _remove_orphaned_tool_messages(messages: list[BaseMessage]) -> list[BaseMessage]:
+    """Remove ToolMessages that don't have a corresponding AIMessage with tool_calls.
+
+    When trimming messages, we may accidentally orphan ToolMessages by removing
+    the AIMessage that made the tool call. This function cleans up such orphans
+    to maintain valid message history.
+
+    Args:
+        messages: List of messages to clean.
+
+    Returns:
+        List of messages with orphaned ToolMessages removed.
+    """
+    if not messages:
+        return messages
+
+    # Build a set of valid tool_call_ids from AIMessages
+    valid_tool_call_ids: set[str] = set()
+    for msg in messages:
+        if isinstance(msg, AIMessage) and msg.tool_calls:
+            for tool_call in msg.tool_calls:
+                if tool_call_id := tool_call.get("id"):
+                    valid_tool_call_ids.add(tool_call_id)
+
+    # Filter out ToolMessages with invalid tool_call_ids
+    cleaned_messages: list[BaseMessage] = []
+    for msg in messages:
+        if isinstance(msg, ToolMessage):
+            if msg.tool_call_id in valid_tool_call_ids:
+                cleaned_messages.append(msg)
+            # else: skip orphaned ToolMessage
+        else:
+            cleaned_messages.append(msg)
+
+    return cleaned_messages
+
+
 def _first_max_tokens(
     messages: Sequence[BaseMessage],
     *,
@@ -1535,7 +1572,9 @@ def _first_max_tokens(
             else:
                 break
 
-    return messages[:idx]
+    trimmed = messages[:idx]
+    # Remove any orphaned ToolMessages that lost their corresponding AIMessage
+    return _remove_orphaned_tool_messages(trimmed)
 
 
 def _last_max_tokens(

libs/core/tests/unit_tests/messages/test_utils.py

@@ -1584,3 +1584,270 @@ def test_convert_to_openai_messages_reasoning_content() -> None:
         ],
     }
     assert mixed_result == expected_mixed
+
+
+def test_trim_messages_removes_orphaned_tool_messages() -> None:
+    """Test that trim_messages removes orphaned ToolMessages.
+
+    When the corresponding AIMessage with tool_calls is trimmed away, the
+    ToolMessage becomes orphaned and should be removed.
+
+    This is the exact scenario from the bug report:
+    https://github.com/langchain-ai/langchain/issues/xxxxx
+    """
+    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!"),
+    ]
+
+    # Trim to last 5 messages (by count)
+    trimmed_messages = trim_messages(
+        messages,
+        strategy="last",
+        token_counter=len,
+        max_tokens=5,
+    )
+
+    # Should NOT include the orphaned ToolMessage
+    # The AIMessage with tool_calls was removed, so ToolMessage should be removed too
+    assert len(trimmed_messages) == 4
+    assert isinstance(trimmed_messages[0], HumanMessage)
+    assert trimmed_messages[0].content == "I see"
+    assert isinstance(trimmed_messages[1], AIMessage)
+    assert trimmed_messages[1].content == "Do you want to know anything else?"
+    assert isinstance(trimmed_messages[2], HumanMessage)
+    assert trimmed_messages[2].content == "No, thanks"
+    assert isinstance(trimmed_messages[3], AIMessage)
+    assert trimmed_messages[3].content == "You're welcome! Have a great day!"
+
+    # Verify no ToolMessages in result
+    assert not any(isinstance(msg, ToolMessage) for msg in trimmed_messages)
+
+
+def test_trim_messages_preserves_valid_tool_calls() -> None:
+    """Test that valid tool call sequences are preserved when they fit in the budget."""
+    messages = [
+        HumanMessage("What's 2+2?"),
+        AIMessage(
+            "Let me calculate that",
+            tool_calls=[
+                {
+                    "name": "calculator",
+                    "args": {"expression": "2+2"},
+                    "id": "calc1",
+                    "type": "tool_call",
+                },
+            ],
+        ),
+        ToolMessage("4", name="calculator", tool_call_id="calc1"),
+        AIMessage("The answer is 4"),
+    ]
+
+    # Trim to include all messages
+    trimmed_messages = trim_messages(
+        messages,
+        strategy="last",
+        token_counter=len,
+        max_tokens=10,
+    )
+
+    # All messages should be preserved, including valid tool call sequence
+    assert len(trimmed_messages) == 4
+    assert isinstance(trimmed_messages[0], HumanMessage)
+    assert isinstance(trimmed_messages[1], AIMessage)
+    assert trimmed_messages[1].tool_calls[0]["id"] == "calc1"
+    assert isinstance(trimmed_messages[2], ToolMessage)
+    assert trimmed_messages[2].tool_call_id == "calc1"
+    assert isinstance(trimmed_messages[3], AIMessage)
+
+
+def test_trim_messages_multiple_tool_calls() -> None:
+    """Test handling of multiple tool calls in sequence."""
+    messages = [
+        HumanMessage("Get me weather and news"),
+        AIMessage(
+            "Fetching both...",
+            tool_calls=[
+                {
+                    "name": "get_weather",
+                    "args": {"location": "NYC"},
+                    "id": "tool1",
+                    "type": "tool_call",
+                },
+                {
+                    "name": "get_news",
+                    "args": {"topic": "tech"},
+                    "id": "tool2",
+                    "type": "tool_call",
+                },
+            ],
+        ),
+        ToolMessage("Sunny", name="get_weather", tool_call_id="tool1"),
+        ToolMessage("AI news update", name="get_news", tool_call_id="tool2"),
+        HumanMessage("Thanks"),
+        AIMessage("You're welcome!"),
+    ]
+
+    # Trim to last 2 messages (should remove all tool-related messages)
+    trimmed_messages = trim_messages(
+        messages,
+        strategy="last",
+        token_counter=len,
+        max_tokens=2,
+    )
+
+    assert len(trimmed_messages) == 2
+    assert isinstance(trimmed_messages[0], HumanMessage)
+    assert trimmed_messages[0].content == "Thanks"
+    assert isinstance(trimmed_messages[1], AIMessage)
+    assert trimmed_messages[1].content == "You're welcome!"
+    # No ToolMessages should remain
+    assert not any(isinstance(msg, ToolMessage) for msg in trimmed_messages)
+
+
+def test_trim_messages_partial_tool_orphaning() -> None:
+    """Test when some tool calls are preserved but others are orphaned."""
+    messages = [
+        HumanMessage("First question"),
+        AIMessage(
+            "Let me check",
+            tool_calls=[
+                {
+                    "name": "tool1",
+                    "args": {},
+                    "id": "old_tool",
+                    "type": "tool_call",
+                },
+            ],
+        ),
+        ToolMessage("Result 1", name="tool1", tool_call_id="old_tool"),
+        HumanMessage("Second question"),
+        AIMessage(
+            "Checking again",
+            tool_calls=[
+                {
+                    "name": "tool2",
+                    "args": {},
+                    "id": "new_tool",
+                    "type": "tool_call",
+                },
+            ],
+        ),
+        ToolMessage("Result 2", name="tool2", tool_call_id="new_tool"),
+        AIMessage("Done"),
+    ]
+
+    # Trim to last 4 messages - should keep only the second tool call sequence
+    trimmed_messages = trim_messages(
+        messages,
+        strategy="last",
+        token_counter=len,
+        max_tokens=4,
+    )
+
+    assert len(trimmed_messages) == 4
+    # Should have the second AIMessage with tool_calls
+    ai_msgs_with_tools = [
+        msg for msg in trimmed_messages if isinstance(msg, AIMessage) and msg.tool_calls
+    ]
+    assert len(ai_msgs_with_tools) == 1
+    assert ai_msgs_with_tools[0].tool_calls[0]["id"] == "new_tool"
+
+    # Should have matching ToolMessage
+    tool_msgs = [msg for msg in trimmed_messages if isinstance(msg, ToolMessage)]
+    assert len(tool_msgs) == 1
+    assert tool_msgs[0].tool_call_id == "new_tool"
+
+
+def test_trim_messages_strategy_first_removes_orphans() -> None:
+    """Test that strategy='first' also removes orphaned ToolMessages."""
+    messages = [
+        HumanMessage("Start"),
+        AIMessage("Response"),
+        HumanMessage("Question"),
+        AIMessage(
+            "Tool time",
+            tool_calls=[
+                {
+                    "name": "tool",
+                    "args": {},
+                    "id": "tool123",
+                    "type": "tool_call",
+                },
+            ],
+        ),
+        ToolMessage("Result", name="tool", tool_call_id="tool123"),
+        AIMessage("Final"),
+    ]
+
+    # Trim to first 4 messages - cuts off in the middle of tool call sequence
+    trimmed_messages = trim_messages(
+        messages,
+        strategy="first",
+        token_counter=len,
+        max_tokens=4,
+    )
+
+    # The AIMessage with tool_calls is included but ToolMessage is cut off
+    # However, since the ToolMessage isn't in the trimmed result, no orphaning occurs
+    assert len(trimmed_messages) == 4
+    assert isinstance(trimmed_messages[3], AIMessage)
+
+    # Now test with an orphaned ToolMessage at the beginning
+    # Trim first 3 messages which includes an orphan
+    messages_with_orphan = [
+        HumanMessage("A"),
+        ToolMessage("Orphaned", name="tool", tool_call_id="missing_id"),
+        HumanMessage("B"),
+        AIMessage(
+            "Using tool",
+            tool_calls=[
+                {
+                    "name": "valid_tool",
+                    "args": {},
+                    "id": "valid123",
+                    "type": "tool_call",
+                },
+            ],
+        ),
+        ToolMessage("Valid result", name="valid_tool", tool_call_id="valid123"),
+    ]
+
+    trimmed = trim_messages(
+        messages_with_orphan,
+        strategy="first",
+        token_counter=len,
+        max_tokens=3,
+    )
+
+    # Should remove the orphaned ToolMessage but keep valid messages
+    assert len(trimmed) == 2  # HumanMessage("A") and HumanMessage("B")
+    tool_messages = [msg for msg in trimmed if isinstance(msg, ToolMessage)]
+    assert len(tool_messages) == 0