core[major]: only use function description (#21622)

Do not prefix function signature

---

* Reason for this is that information is already present with tool
calling models.
* This will save on tokens for those models, and makes it more obvious
what the description is!
* The @tool can get more parameters to allow a user to re-introduce the
the signature if we want

libs/core/tests/unit_tests/test_tools.py

@@ -332,9 +332,8 @@ def test_structured_tool_from_function_docstring() -> None:
         "required": ["bar", "baz"],
     }
 
-    prefix = "foo(bar: int, baz: str) -> str - "
     assert foo.__doc__ is not None
-    assert structured_tool.description == prefix + textwrap.dedent(foo.__doc__.strip())
+    assert structured_tool.description == textwrap.dedent(foo.__doc__.strip())
 
 
 def test_structured_tool_from_function_docstring_complex_args() -> None:
@@ -365,9 +364,8 @@ def test_structured_tool_from_function_docstring_complex_args() -> None:
         "required": ["bar", "baz"],
     }
 
-    prefix = "foo(bar: int, baz: List[str]) -> str - "
     assert foo.__doc__ is not None
-    assert structured_tool.description == prefix + textwrap.dedent(foo.__doc__).strip()
+    assert structured_tool.description == textwrap.dedent(foo.__doc__).strip()
 
 
 def test_structured_tool_lambda_multi_args_schema() -> None:
@@ -700,9 +698,8 @@ def test_structured_tool_from_function() -> None:
         "required": ["bar", "baz"],
     }
 
-    prefix = "foo(bar: int, baz: str) -> str - "
     assert foo.__doc__ is not None
-    assert structured_tool.description == prefix + textwrap.dedent(foo.__doc__.strip())
+    assert structured_tool.description == textwrap.dedent(foo.__doc__.strip())
 
 
 def test_validation_error_handling_bool() -> None:
@@ -906,3 +903,15 @@ async def test_async_tool_pass_context() -> None:
     assert (
         await foo.ainvoke({"bar": "baz"}, {"configurable": {"foo": "not-bar"}}) == "baz"  # type: ignore
     )
+
+
+def test_tool_description() -> None:
+    def foo(bar: str) -> str:
+        """The foo."""
+        return bar
+
+    foo1 = tool(foo)
+    assert foo1.description == "The foo."  # type: ignore
+
+    foo2 = StructuredTool.from_function(foo)
+    assert foo2.description == "The foo."