core[patch]: fix arbitrary config keys (#18827)

libs/core/langchain_core/runnables/base.py

@@ -4021,6 +4021,10 @@ class RunnableBindingBase(RunnableSerializable[Input, Output]):
             custom_output_type=custom_output_type,
             **other_kwargs,
         )
+        # if we don't explicitly set config to the TypedDict here,
+        # the pydantic init above will strip out any of the "extra"
+        # fields even though total=False on the typed dict.
+        self.config = config or {}
 
     def get_name(
         self, suffix: Optional[str] = None, *, name: Optional[str] = None

libs/core/tests/unit_tests/runnables/test_config.py

@@ -1,6 +1,9 @@
+from typing import Any, cast
+
 from langchain_core.callbacks.manager import CallbackManager
 from langchain_core.callbacks.stdout import StdOutCallbackHandler
 from langchain_core.callbacks.streaming_stdout import StreamingStdOutCallbackHandler
+from langchain_core.runnables import RunnableBinding, RunnablePassthrough
 from langchain_core.runnables.config import RunnableConfig, merge_configs
 from langchain_core.tracers.stdout import ConsoleCallbackHandler
 
@@ -32,3 +35,11 @@ def test_merge_config_callbacks() -> None:
     assert len(merged) == 2
     assert isinstance(merged[0], ConsoleCallbackHandler)
     assert isinstance(merged[1], StreamingStdOutCallbackHandler)
+
+
+def test_config_arbitrary_keys() -> None:
+    base: RunnablePassthrough[Any] = RunnablePassthrough()
+    bound = base.with_config(my_custom_key="my custom value")
+    config = cast(RunnableBinding, bound).config
+
+    assert config.get("my_custom_key") == "my custom value"