feat(core): add subagent_name attribute to BaseTool

Declares the subagent a tool dispatches to. Accepts a static string
or a Callable[[ToolCall], str] for dynamic resolution at dispatch time.

This is a pure attribute addition with no behavior. Downstream
consumers (langgraph ToolNode, langchain create_agent's typed
projection) will read this attribute to surface subagent identity on
streaming lifecycle events.

libs/core/langchain_core/tools/base.py

@@ -530,6 +530,20 @@ class ChildTool(BaseTool):
         ```
     """
 
+    subagent_name: str | Callable[[ToolCall], str] | None = None
+    """Declared name of the subagent this tool dispatches to, if any.
+
+    A static string declares a fixed dispatch target (one tool -> one subagent).
+    A callable resolves the name from the parsed `ToolCall` at dispatch time,
+    enabling registry-style dispatching (one tool -> many subagents based on
+    LLM-supplied args).
+
+    Read by `ToolNode` at dispatch time and surfaced on the wire as
+    `LifecyclePayload.graphName`. Purely declarative: the framework stamps the
+    resolved value into runtime config metadata; consumers project it onto
+    typed surfaces. Setting this does not change tool execution behavior.
+    """
+
     def __init__(self, **kwargs: Any) -> None:
         """Initialize the tool.
 

libs/core/tests/unit_tests/test_tools.py

@@ -3741,3 +3741,42 @@ def test_tool_invoke_returns_list_of_mixin() -> None:
     assert isinstance(result, list)
     assert len(result) == 3
     assert all(isinstance(m, ToolMessage) for m in result)
+
+
+# --- subagent_name attribute tests ---
+
+
+class _SubagentNameDummyTool(BaseTool):
+    """Minimal BaseTool subclass for subagent_name attribute tests."""
+
+    name: str = "dummy"
+    description: str = "A dummy tool used in subagent_name tests."
+
+    def _run(self, *args: Any, **kwargs: Any) -> str:
+        return "dummy result"
+
+
+def test_basetool_subagent_name_default_none() -> None:
+    tool = _SubagentNameDummyTool()
+    assert tool.subagent_name is None
+
+
+def test_basetool_subagent_name_accepts_static_string() -> None:
+    tool = _SubagentNameDummyTool(subagent_name="weather_agent")
+    assert tool.subagent_name == "weather_agent"
+
+
+def test_basetool_subagent_name_accepts_callable() -> None:
+    def resolver(call: ToolCall) -> str:
+        return str(call["args"]["subagent_type"])
+
+    tool = _SubagentNameDummyTool(subagent_name=resolver)
+    assert tool.subagent_name is resolver
+    fake_call: ToolCall = {
+        "name": "task",
+        "args": {"subagent_type": "researcher"},
+        "id": "id-1",
+        "type": "tool_call",
+    }
+    assert callable(tool.subagent_name)
+    assert tool.subagent_name(fake_call) == "researcher"