langgraph[patch]: remove stringify of tool msg contnt

libs/langgraph/langgraph/prebuilt/tool_node.py

@@ -1,4 +1,4 @@
 from __future__ import annotations
 
 import asyncio
 import json
@@ -43,9 +43,20 @@
 TOOL_CALL_ERROR_TEMPLATE = "Error: {error}\n Please fix your mistakes."
 
 
-def str_output(output: Any) -> str:
+def msg_content_output(output: Any) -> str | List[dict]:
+    recognized_content_block_types = ("image", "image_url", "text", "json")
     if isinstance(output, str):
         return output
+    elif all(
+        [
+            isinstance(x, dict) and x.get("type") in recognized_content_block_types
+            for x in output
+        ]
+    ):
+        return output
+    # Technically a list of strings is also valid message content but it's not currently
+    # well tested that all chat models support this. And for backwards compatibility
+    # we want to make sure we don't break any existing ToolNode usage.
     else:
         try:
             return json.dumps(output, ensure_ascii=False)
@@ -138,8 +149,9 @@
             tool_message: ToolMessage = self.tools_by_name[call["name"]].invoke(
                 input, config
             )
-            # TODO: handle this properly in core
-            tool_message.content = str_output(tool_message.content)
+            tool_message.content = cast(
+                Union[str, list], msg_content_output(tool_message.content)
+            )
             return tool_message
         except Exception as e:
             if not self.handle_tool_errors:
@@ -155,8 +167,9 @@
             tool_message: ToolMessage = await self.tools_by_name[call["name"]].ainvoke(
                 input, config
             )
-            # TODO: handle this properly in core
-            tool_message.content = str_output(tool_message.content)
+            tool_message.content = cast(
+                Union[str, list], msg_content_output(tool_message.content)
+            )
             return tool_message
         except Exception as e:
             if not self.handle_tool_errors:

libs/langgraph/tests/test_prebuilt.py

@@ -262,6 +262,12 @@ async def tool3(some_val: int, some_other_val: str) -> str:
             {"key_1": some_other_val, "key_2": "baz"},
         ]
 
+    async def tool4(some_val: int, some_other_val: str) -> str:
+        """Tool 4 docstring."""
+        return [
+            {"type": "image_url", "image_url": {"url": "abdc"}},
+        ]
+
     result = ToolNode([tool1]).invoke(
         {
             "messages": [
@@ -397,6 +403,28 @@ async def tool3(some_val: int, some_other_val: str) -> str:
     )
     assert tool_message.tool_call_id == "some 0"
 
+    # list of content blocks tool content
+    result4 = await ToolNode([tool4]).ainvoke(
+        {
+            "messages": [
+                AIMessage(
+                    "hi?",
+                    tool_calls=[
+                        {
+                            "name": "tool4",
+                            "args": {"some_val": 2, "some_other_val": "bar"},
+                            "id": "some 0",
+                        }
+                    ],
+                )
+            ]
+        }
+    )
+    tool_message: ToolMessage = result4["messages"][-1]
+    assert tool_message.type == "tool"
+    assert tool_message.content == [{"type": "image_url", "image_url": {"url": "abdc"}}]
+    assert tool_message.tool_call_id == "some 0"
+
 
 def my_function(some_val: int, some_other_val: str) -> str:
     return f"{some_val} - {some_other_val}"