Support message trimming on single messages

libs/core/langchain_core/messages/utils.py

@@ -876,13 +876,14 @@ def _first_max_tokens(
     ] = None,
 ) -> list[BaseMessage]:
     messages = list(messages)
+    if not messages:
+        return messages
     idx = 0
     for i in range(len(messages)):
         if token_counter(messages[:-i] if i else messages) <= max_tokens:
             idx = len(messages) - i
             break
-
-    if idx < len(messages) - 1 and partial_strategy:
+    if partial_strategy and (idx < len(messages) - 1 or idx == 0):
         included_partial = False
         if isinstance(messages[idx].content, list):
             excluded = messages[idx].model_copy(deep=True)

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

@@ -299,6 +299,24 @@ def test_trim_messages_last_40_include_system_allow_partial_start_on_human() ->
     assert _MESSAGES_TO_TRIM == _MESSAGES_TO_TRIM_COPY
 
 
+def test_trim_messages_allow_partial_one_message() -> None:
+    expected = [
+        HumanMessage("Th", id="third"),
+    ]
+
+    actual = trim_messages(
+        [HumanMessage("This is a 4 token text.", id="third")],
+        max_tokens=2,
+        token_counter=lambda messages: sum(len(m.content) for m in messages),
+        text_splitter=lambda x: list(x),
+        strategy="first",
+        allow_partial=True,
+    )
+
+    assert actual == expected
+    assert _MESSAGES_TO_TRIM == _MESSAGES_TO_TRIM_COPY
+
+
 def test_trim_messages_allow_partial_text_splitter() -> None:
     expected = [
         HumanMessage("a 4 token text.", id="third"),

libs/partners/anthropic/langchain_anthropic/chat_models.py

@@ -1,3 +1,4 @@
+import copy
 import re
 import warnings
 from operator import itemgetter
@@ -116,7 +117,6 @@ def _merge_messages(
     """Merge runs of human/tool messages into single human messages with content blocks."""  # noqa: E501
     merged: list = []
     for curr in messages:
-        curr = curr.model_copy(deep=True)
         if isinstance(curr, ToolMessage):
             if isinstance(curr.content, list) and all(
                 isinstance(block, dict) and block.get("type") == "tool_result"
@@ -139,12 +139,12 @@ def _merge_messages(
             if isinstance(last.content, str):
                 new_content: List = [{"type": "text", "text": last.content}]
             else:
-                new_content = last.content
+                new_content = copy.copy(last.content)
             if isinstance(curr.content, str):
                 new_content.append({"type": "text", "text": curr.content})
             else:
                 new_content.extend(curr.content)
-            last.content = new_content
+            merged[-1] = curr.model_copy(update={"content": new_content}, deep=False)
         else:
             merged.append(curr)
     return merged
@@ -174,9 +174,11 @@ def _format_messages(
                 raise ValueError("System message must be at beginning of message list.")
             if isinstance(message.content, list):
                 system = [
-                    block
-                    if isinstance(block, dict)
-                    else {"type": "text", "text": "block"}
+                    (
+                        block
+                        if isinstance(block, dict)
+                        else {"type": "text", "text": "block"}
+                    )
                     for block in message.content
                 ]
             else: