fix(core): Support no-args tools by defaulting args to empty dict (#32530)

Supersedes #32408

Description:  
This PR ensures that tool calls without explicitly provided `args` will
default to an empty dictionary (`{}`), allowing tools with no parameters
(e.g. `def foo() -> str`) to be registered and invoked without
validation errors. This change improves compatibility with agent
frameworks that may omit the `args` field when generating tool calls.

Issue:  
See
[langgraph#5722](https://github.com/langchain-ai/langgraph/issues/5722)
–
LangGraph currently emits tool calls without `args`, which leads to
validation errors
when tools with no parameters are invoked. This PR ensures compatibility
by defaulting
`args` to `{}` when missing.

Dependencies:  
None

---------

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---------

Signed-off-by: jitokim <pigberger70@gmail.com>
Co-authored-by: jito <pigberger70@gmail.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

libs/core/langchain_core/messages/ai.py

@@ -358,7 +358,10 @@ class AIMessageChunk(AIMessage, BaseMessageChunk):
 
         for chunk in self.tool_call_chunks:
             try:
-                args_ = parse_partial_json(chunk["args"]) if chunk["args"] != "" else {}  # type: ignore[arg-type]
+                if chunk["args"] is not None and chunk["args"] != "":
+                    args_ = parse_partial_json(chunk["args"])
+                else:
+                    args_ = {}
                 if isinstance(args_, dict):
                     tool_calls.append(
                         create_tool_call(

libs/core/tests/unit_tests/test_messages.py

@@ -455,9 +455,9 @@ def test_message_chunk_to_message() -> None:
         tool_calls=[
             create_tool_call(name="tool1", args={"a": 1}, id="1"),
             create_tool_call(name="tool2", args={}, id="2"),
+            create_tool_call(name="tool3", args={}, id="3"),
         ],
         invalid_tool_calls=[
-            create_invalid_tool_call(name="tool3", args=None, id="3", error=None),
             create_invalid_tool_call(name="tool4", args="abc", id="4", error=None),
         ],
     )