diff --git a/libs/partners/anthropic/langchain_anthropic/chat_models.py b/libs/partners/anthropic/langchain_anthropic/chat_models.py
index a23a05463e7..266d60bdce8 100644
--- a/libs/partners/anthropic/langchain_anthropic/chat_models.py
+++ b/libs/partners/anthropic/langchain_anthropic/chat_models.py
@@ -643,6 +643,17 @@ def _format_messages(
                 _lc_tool_calls_to_anthropic_tool_use_blocks(missing_tool_calls),
             )
 
+        if role == "assistant" and _i == len(merged_messages) - 1:
+            if isinstance(content, str):
+                content = content.rstrip()
+            elif (
+                isinstance(content, list)
+                and content
+                and isinstance(content[-1], dict)
+                and content[-1].get("type") == "text"
+            ):
+                content[-1]["text"] = content[-1]["text"].rstrip()
+
         if not content and role == "assistant" and _i < len(merged_messages) - 1:
             # anthropic.BadRequestError: Error code: 400: all messages must have
             # non-empty content except for the optional final assistant message
diff --git a/libs/partners/anthropic/tests/unit_tests/test_chat_models.py b/libs/partners/anthropic/tests/unit_tests/test_chat_models.py
index 42bb97e2944..e669b0655c0 100644
--- a/libs/partners/anthropic/tests/unit_tests/test_chat_models.py
+++ b/libs/partners/anthropic/tests/unit_tests/test_chat_models.py
@@ -2319,3 +2319,23 @@ def test_extras_with_multiple_fields() -> None:
     assert tool_def.get("defer_loading") is True
     assert tool_def.get("cache_control") == {"type": "ephemeral"}
     assert "input_examples" in tool_def
+
+
+def test__format_messages_trailing_whitespace() -> None:
+    """Test that trailing whitespace is trimmed from the final assistant message."""
+    human = HumanMessage("foo")  # type: ignore[misc]
+
+    # Test string content
+    ai_string = AIMessage("thought ")  # type: ignore[misc]
+    _, anthropic_messages = _format_messages([human, ai_string])
+    assert anthropic_messages[-1]["content"] == "thought"
+
+    # Test list content
+    ai_list = AIMessage([{"type": "text", "text": "thought "}])  # type: ignore[misc]
+    _, anthropic_messages = _format_messages([human, ai_list])
+    assert anthropic_messages[-1]["content"][0]["text"] == "thought"  # type: ignore[index]
+
+    # Test that intermediate messages are NOT trimmed
+    ai_intermediate = AIMessage("thought ")  # type: ignore[misc]
+    _, anthropic_messages = _format_messages([human, ai_intermediate, human])
+    assert anthropic_messages[1]["content"] == "thought "